Error in dna replication

DNA Replication and Causes of Mutation

Errors Are a Natural Part of DNA Replication

After James Watson and Francis Crick published their model of the double-helix structure of DNA in 1953, biologists initially speculated that most replication errors were caused by what are called tautomeric shifts. Both the purine and pyrimidine bases in DNA exist in different chemical forms, or tautomers, in which the protons occupy different positions in the molecule (Figure 1). The Watson-Crick model required that the nucleotide bases be in their more common «keto» form (Watson & Crick, 1953). Scientists believed that if and when a nucleotide base shifted into its rarer tautomeric form (the «imino» or «enol» form), a likely result would be base-pair mismatching. But evidence for these types of tautomeric shifts remains sparse.

Today, scientists suspect that most DNA replication errors are caused by mispairings of a different nature: either between different but nontautomeric chemical forms of bases (e.g., bases with an extra proton, which can still bind but often with a mismatched nucleotide, such as an A with a G instead of a T) or between «normal» bases that nonetheless bond inappropriately (e.g., again, an A with a G instead of a T) because of a slight shift in position of the nucleotides in space (Figure 2). This type of mispairing is known as wobble . It occurs because the DNA double helix is flexible and able to accommodate slightly misshaped pairings (Crick, 1966).

Replication errors can also involve insertions or deletions of nucleotide bases that occur during a process called strand slippage . Sometimes, a newly synthesized strand loops out a bit, resulting in the addition of an extra nucleotide base (Figure 3). Other times, the template strand loops out a bit, resulting in the omission, or deletion, of a nucleotide base in the newly synthesized, or primer , strand. Regions of DNA containing many copies of small repeated sequences are particularly prone to this type of error.

Fixing Mistakes in DNA Replication

DNA polymerase enzymes are amazingly particular with respect to their choice of nucleotides during DNA synthesis, ensuring that the bases added to a growing strand are correctly paired with their complements on the template strand (i.e., A’s with T’s, and C’s with G’s). Nonetheless, these enzymes do make mistakes at a rate of about 1 per every 100,000 nucleotides. That might not seem like much, until you consider how much DNA a cell has. In humans, with our 6 billion base pairs in each diploid cell, that would amount to about 120,000 mistakes every time a cell divides!

Fortunately, cells have evolved highly sophisticated means of fixing most, but not all, of those mistakes. Some of the mistakes are corrected immediately during replication through a process known as proofreading , and some are corrected after replication in a process called mismatch repair . When an incorrect nucleotide is added to the growing strand, replication is stalled by the fact that the nucleotide’s exposed 3′-OH group is in the «wrong» position. (Recall that new nucleotides are added to the growing strand during replication by means of their 5′-phosphate group binding to the 3′-OH group of the previous nucleotide on the strand.) During proofreading, DNA polymerase enzymes recognize this and replace the incorrectly inserted nucleotide so that replication can continue. Proofreading fixes about 99% of these types of errors, but that’s still not good enough for normal cell functioning.

After replication, mismatch repair reduces the final error rate even further. Incorrectly paired nucleotides cause deformities in the secondary structure of the final DNA molecule. During mismatch repair, enzymes recognize and fix these deformities by removing the incorrectly paired nucleotide and replacing it with the correct nucleotide.

When Replication Errors Become Mutations

Incorrectly paired nucleotides that still remain following mismatch repair become permanent mutations after the next cell division . This is because once such mistakes are established, the cell no longer recognizes them as errors. Consider the case of wobble-induced replication errors. When these mistakes are not corrected, the incorrectly sequenced DNA strand serves as a template for future replication events, causing all the base-pairings thereafter to be wrong. For instance, in the lower half of Figure 2, the original strand had a C-G pair; then, during replication, cytosine (C) is incorrectly matched to adenine (A) because of wobble. In this example, wobble occurs because A has an extra hydrogen atom. In the next round of cell division, the double strand with the C-A pairing would separate during replication, each strand serving as a template for synthesis of a new DNA molecule. At that particular spot, C would pair with G, forming a double helix with the same sequence as its original (i.e., before the wobble occurred), but A would pair with T, forming a new DNA molecule with an A-T pair in place of the original C-G pair. This type of mutation is known as a base, or base-pair, substitution. Base substitutions involving replacement of one purine for another or one pyrimidine for another (e.g., a mismatched A-A pair, instead of A-T) are known as transitions; the replacement of a purine by a pyrimidine, or vice versa, is called a transversion .

Likewise, when strand-slippage replication errors are not corrected, they become insertion and deletion mutations. Much of the early research on strand-slippage mutations was conducted by George Streisinger in the 1970s. Streisinger, a professor at the University of Oregon and a fish hobbyist, is known by some as the «founding father of zebrafish research.» However, he is also known for his work with phage T4, a bacterial virus . Streisinger used this virus to show that most nucleotide insertion and deletion mutations occur in areas of DNA that contain many repeated sequences (also called tandem repeats), and he formulated the strand-slippage hypothesis to explain why this was the case (Streisinger et al., 1966). (In Figure 3, notice the series of repeat T’s on the template strand where the slippage has occurred.) When slippage takes place, the presence of nearby duplicate bases stabilizes the slippage so that replication can proceed. During the next round of replication, when the two strands separate, the insertion or deletion on either the template or primer strand, respectively, will be perpetuated as a permanent mutation . Scientists have collected enough evidence to confirm Streisinger’s strand-slippage hypothesis, and this type of mutagenesis remains an active field of scientific research.

Although most mutations are believed to be caused by replication errors, they can also be caused by various environmentally induced and spontaneous changes to DNA that occur prior to replication but are perpetuated in the same way as unfixed replication errors. As with replication errors, most environmentally induced DNA damage is repaired, resulting in fewer than 1 out of every 1,000 chemically induced lesions actually becoming permanent mutations. The same is true of so-called spontaneous mutations. «Spontaneous» refers to the fact that the changes occur in the absence of chemical, radiation , or other environmental damage. Rather, they are usually caused by normal chemical reactions that go on in cells, such as hydrolysis. These types of errors include depurination , which occurs when the bond connecting a purine to its deoxyribose sugar is broken by a molecule of water, resulting in a purine-free nucleotide that can’t act as a template during DNA replication, and deamination , which results in the loss of an amino group from a nucleotide, again by reaction with water. Again, most of these spontaneous errors are corrected by DNA repair processes. But if this does not occur, a nucleotide that is added to the newly synthesized strand can become a permanent mutation.

Even Low Mutation Rates Can Be Cause for Concern

Mutation rates vary substantially among taxa, and even among different parts of the genome in a single organism . Scientists have reported mutation rates as low as 1 mistake per 100 million (10 -8 ) to 1 billion (10 -9 ) nucleotides, mostly in bacteria , and as high as 1 mistake per 100 (10 -2 ) to 1,000 (10 -3 ) nucleotides, the latter in a group of error-prone polymerase genes in humans (Johnson et al., 2000).

Even mutation rates as low as 10 -10 can accumulate quickly over time, particularly in rapidly reproducing organisms like bacteria. This is one reason why antibiotic resistance is such an important public health problem; after all, mutations that accumulate in a population of bacteria provide ample genetic variation with which to adapt (or respond) to the natural selection pressures imposed by antibacterial drugs (Smolinski et al., 2003). Take E. coli, for example. The genome of this common intestinal bacterium has about 4.2 million base pairs, or 8.4 million bases. Assuming a mutation rate of 10 -9 (i.e., midway between reported estimates of 10 -8 and 10 -10 ), every time E. coli divides, each daughter cell will have, on average, 0.0084 new mutations. Or, another way to think about it is like this: Approximately 1% of bacterial cells will contain a new mutation. That may not seem like much. However, because bacteria can divide as rapidly as twice per hour, a single bacterium can grow into a colony of 1 million cells in only about 10 hours (2 20 = 1,048,576). At that point, approximately 10,000 of these bacteria will have accumulated at least one mutation. As the number of bacteria carrying different mutations increases, so too does the likelihood that at least one of them will develop a drug-resistant phenotype .

Likewise, in eukaryotes, cells accumulate mutations as they divide. In humans, if enough somatic mutations (i.e., mutations in body cells rather than sperm or egg cells) accumulate over the course of a person’s lifetime, the end result could be cancer. Or, less frequently, some cancer mutations are inherited from one or both parents; these are often referred to as germ-line mutations. One of the first cancer-associated somatic mutations was discovered in 1982, when researchers found that a mutated HRAS gene was associated with bladder cancer (Reddy et al., 1982). HRAS encodes for a protein that helps regulate cell division. Since then, scientists have identified several hundred additional «cancer genes.» Some of them, like the handful of germ-line mutations associated with a form of colorectal cancer known as hereditary nonpolyposis colorectal cancer (HNPCC), play crucial roles in DNA repair (Wijnen et al., 1998).

Of course, not all mutations are «bad.» But, because so many mutations can cause cancer, DNA repair is obviously a crucially important property of eukaryotic cells. However, too much of a good thing can be dangerous. If DNA repair were perfect and no mutations ever accumulated, there would be no genetic variation—and this variation serves as the raw material for evolution . Successful organisms have thus evolved the means to repair their DNA efficiently but not too efficiently, leaving just enough genetic variability for evolution to continue.

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Ошибки в репликации ДНК: 13 фактов, о которых не знает большинство новичков

Репликация ДНК иногда может быть нарушена из-за добавления или удаления новых нуклеотидных оснований. Давайте посмотрим, как возникают ошибки при репликации ДНК.

Ошибки в Репликация ДНК известны как проскальзывание цепей, когда новые нуклеотидные основания добавляются или удаляются в результате мутации или постоянных изменений последовательности и т. д. ДНК. Свежевыделенные петли прядей немного выходят наружу. Результатом этого изменения является добавление или удаление дополнительного нуклеотидного основания.

Есть в основном три типа ошибок в репликации ДНК. Это базовые замены, удаления и вставки. Если ошибку не исправить, она может вызвать рак. Здесь механизм репарации ДНК исправляет все ошибки.

Давайте обсудим, является ли репликация ДНК точной, каковы все ошибки в репликации ДНК, причины и последствия ошибок в репликации ДНК и многие другие связанные темы в этой статье.

Является ли репликация ДНК точной?

Частота ошибок при репликации ДНК очень мала. Давайте посмотрим, является ли репликация ДНК точной или нет.

Репликация ДНК настолько точна, что частота ошибок в репликации ДНК может быть незначительной. Это формируется таким образом, что геном стабильность зависит от точности репликации ДНК. Но дефектные геномы могут быть фатальными для животного организма из-за мутации всего генофонда.

Точность репликации ДНК хорошо определена благодаря трем факторам. Они есть нуклеотидная избирательность, Редактированиеи Несоответствие ремонта. Эти факторы являются основной причиной меньшего количества ошибок в репликации ДНК.

Что такое ошибки репликации ДНК?

Ошибки репликации ДНК в основном связаны с депуринацией всего геномного пула. Давайте обсудим, каковы все возможные ошибки в репликации ДНК.

Ошибки репликации ДНК делятся на три типа: Ошибки репликации, депуринизация ДНК и повреждение ДНК за счет образования активных форм кислорода.

Давайте посмотрим больше информации об ошибках в репликации ДНК ниже:

• Разрыв вызван молекулой воды, в результате чего образуется нуклеотид, не содержащий пуринов. Это нельзя использовать в репликации.
• Свободные пурины не могут функционировать во время репликации ДНК в качестве матрицы.
• Потеря аминогруппы из нуклеотида также вызвана дезаминированием для того, чтобы не функционировать в качестве матрицы во время ошибок в репликации ДНК по водной реакции.
• Эти непреднамеренные причины снова исправляются в процессе восстановления ДНК.
• Но затем добавляется новый нуклеотид, который становится постоянной мутацией.
• Это происходит во время синтеза новой цепи.
• Накопленные мутации или постоянные изменения последовательности являются причиной такого плохого поведения клеточной ДНК.

Причины ошибок в репликации ДНК

Существует несколько причин возникновения ошибок в репликации ДНК. Давайте посмотрим, что они из себя представляют в деталях.

Ошибки репликации ДНК в основном вызваны разной природой пар оснований. Они могут быть как различной природы, так и не таутомерными по химическим формам.

Три основные причины ошибок в репликации ДНК: подробно обсуждается ниже:

1. Делеция. Делеция — одна из основных ошибок в репликации ДНК. Это вызывает сдвиг структуры всего генофонда. Он изменяет последовательность ДНК, разрушая один (как минимум) или несколько нуклеотидов.

2. Вставка. Вставка — еще одна основная ошибка в репликации ДНК. Это вызывает сдвиг структуры всего генофонда. Он изменяет последовательность ДНК, добавляя одну (как минимум) или более пар нуклеотидных оснований.

3. Замена оснований. Замена оснований также является одной из важных ошибок в репликации ДНК. Он заменяет нужный нуклеотид любым другим нуклеотидом, что меняет весь генофонд. Он также может заменить одну аминокислоту на другую.

Частота ошибок репликации ДНК

Как обсуждалось ранее, существует небольшая вероятность того, что при репликации ДНК могут возникнуть ошибки. Давайте посмотрим на скорость, с которой происходят ошибки в ДНК.

Частота ошибок репликации ДНК составляет один на 10^10 нуклеотидов при синтезе ДНК. Это так меньше, но последствия могут быть фатальными. Это от 10 ^ -9 до 10 ^ -11 ошибок в репликации ДНК на пару оснований. Высокая точность процесса очень важна для поддержания точности генетической идентичности.

Например, E. палочки делает только одну ошибку на миллиард копий нуклеотидов. Он заканчивает свою репликацию в течение 60 минут и может воспроизводить 2000 нуклеотидов в секунду. По сравнению с человеческим телом количество ошибок невелико.

Последствия ошибок в репликации ДНК

Последствия ошибок в репликации ДНК в основном фатальные. Давайте подробно рассмотрим последствия ошибок в репликации ДНК.

Ошибки в репликации ДНК могут привести к опухолям, раку и т. д. Ошибки могут привести к мутациям, которые в дальнейшем приводят к опухолям и, наконец, вызывают рак.

Некоторые последствия ошибок в репликации ДНК:

1. Мутация зародышевой линии

2. Хромосомные изменения

3. Мутация сдвига рамки считывания

4. Точечная мутация

Если ошибки в ДНК не исправляются вовремя корректурным чтением, происходят мутации. Некоторое влияние ошибок в репликации ДНК: серповидноклеточная анемия, одна из форм бета-талассемия, кистозный фиброз, И т.д.

Могут ли ошибки в репликации ДНК привести к мутациям?

Механизм восстановления исправляет все ошибки, возникающие во время репликации ДНК. Давайте обсудим, приводят ли ошибки в репликации ДНК к мутациям.

Ошибки в репликации ДНК могут привести к таким мутациям, как постоянная мутация. В механизме репарации ДНК ферменты репарации находят возникающие ошибки и устраняют их. Впоследствии они рекрутируют нужный нуклеотид на место. Но некоторые ошибки репликации пропускают эти процессы и происходят мутации.

Например, некоторые мутации замещения оснований являются точечными мутациями, такими как мутации молчания, миссенс и нонсенс. Помимо некоторых мутаций, таких как мутация сдвига рамки, зародышевая или соматическая мутация. Основными видами мутаций являются делеция, инверсия, вставка, дупликация, транслокация, амплификация гена и др.

Как ошибки в репликации ДНК могут привести к мутациям?

Ошибки в репликации ДНК могут привести к мутации даже при постоянной мутации. Так что это играет очень важную роль. Формированию некоторых фигур помогает ведение. Давайте объясним это.

В приведенном ниже списке подробно показано, как ошибки в репликации ДНК могут привести к мутациям:

• Ошибки в репликации ДНК могут привести к мутациям, особенно в случае экспансии TNR (тринуклеотидный повтор).
• Этот повтор способствует проскальзыванию ДНК-полимеразы во время репликации.
• Вторичные структуры формируются как шпильки Intra strand.
• Такие вторичные структуры образуются за счет повторяющейся последовательности тринуклеотидных расширений.
• Для этого ферменты возвращаются и копируют прежнюю часть.
• В результате репарация не происходит. Так как репарация ошибок репликации ДНК не происходит, она идет по предыдущему пути.
• Для этого продолжаются ошибки репликации ДНК, и в результате происходит мутация, в частности, постоянная мутация.

Какие типы мутаций вызваны случайными ошибками в репликации ДНК?

Есть три типа мутаций, происходящих во время ошибок в репликации ДНК. Но типов под удаление, вставку, подстановку базы больше. Давайте обсудим их.

Типы мутаций, такие как точковая мутация, хромосомная мутация, зародышевая или соматическая мутация, а также мутация сдвига рамки считывания, вызваны случайными ошибками в репликации ДНК. Под точечной мутацией типов больше. Кроме того, эти мутации могут быть вызваны различными причинами.

Мутация вызывается веществом, называемым мутагеном. Это может быть радиация, химические вещества, токсичные материалы или что-то еще. Они очень спонтанны в окружающей среде.

Точечная мутация

Точечная мутация — это тип мутации, при котором изменяется, сдвигается или заменяется только один нуклеотид. Существует три типа точечных мутаций при ошибках репликации ДНК. Это немые, миссенс и нонсенс-мутации.

Хромосомная мутация

В случае хромосомной мутации структура хромосомы изменяется при ошибках репликации ДНК. Они могут быть изменены или изменены ядром.

Мутация сдвига рамки

При мутации со сдвигом рамки нуклеотиды могут быть добавлены или удалены из-за ошибок в репликации ДНК. Для этого изменяется смещение всего каркаса пула ДНК. Этот сдвиг может выполняться одним или несколькими нуклеотидами.

Индуцированные мутации инициируются ошибками в репликации ДНК.

Мутация изменяет всю последовательность ДНК конкретного организма. Давайте узнаем больше об индуцированных мутациях, которые инициируются ошибками в репликации ДНК.

Индуцированные мутации инициируются ошибками в репликации ДНК, так как при делении клеток в ДНК взрываются мутагены, которые в процессе репликации, привести их к мутации.

Затем индуцированная мутация приводит к постоянной мутации. Генная мутация может быть вызвана многими генами или может быть причиной потери одного или нескольких генов. Он может изменять нуклеотиды ДНК (один или несколько).

Как исправляются ошибки репликации ДНК?

Ошибки в репликации ДНК исправляются с помощью некоторых надежных процессов. Давайте посмотрим, что они из себя представляют в деталях.

Ошибки репликации ДНК исправляются в основном двумя процедурами: корректурой и исправлением несоответствия. Корректура — это то, где ошибки в репликации ДНК исправляются во время репликации ДНК, а исправление несоответствия — это то, где ошибки исправляются после репликации ДНК.

Редактирование

Вычитка создает структуру, которая приглашает другие белки исправить ошибку, потому что белки в ней способны сдерживать ошибки в репликации ДНК.

Когда происходит корректура, полимеразная форма ДНК выявляет ошибки в репликации ДНК. Затем он заменяет неправильно вставленный или удаленный нуклеотид. После исправления репликация продолжается своим потоком.

Несоответствие ремонта

Репарация несоответствия — это когда ошибки исправляются после того, как образование вилки не репарируется во время репликации ДНК. Но он специфичен для отдельных прядей. Ошибки в репликации ДНК исправляются после процесса, поскольку это окончательное исправление.

При репарации несоответствия существуют определенные гены, которые помогают предотвратить ошибки в репликации ДНК после завершения репликации. Гены PMS2, MLH1, MSH2, MSH6.

Почему ошибки репликации ДНК более значимы, чем ошибки транскрипции?

Репликация — более важный процесс, чем транскрипция. Кроме того, ошибки транскрипции РНК не так важны, как ошибки репликации ДНК. Давайте обсудим это.

Ошибки репликации ДНК более серьезны, чем ошибки транскрипции РНК, поскольку они не передаются по наследству, как ошибки репликации ДНК. Кроме того, при транскрипции изменяется очень небольшое количество белков. Изменение не очень вредно, как репликация, и его можно вылечить.

Например, частота ошибок в транскрипции составляет от 2.3*10^-5 в мРНК до 5.2*10^-5 в рРНК на нуклеотид для определенного вида бактерий. Но она не передается следующему поколению, как ошибки в репликации ДНК.

Может ли клетка исправить ошибку репликации ДНК?

Клетка может исправлять некоторые ошибки репликации ДНК. Очень хорошо, что клетки обладают превосходным качеством, позволяющим исправлять ошибки. Давайте исследовать больше.

В некоторых случаях клетка может исправлять ошибки репликации ДНК. Клетки обладают особыми свойствами для борьбы с некоторыми ошибками репликации ДНК. Клетки также могут фиксировать их на определенный процент. В течение клеточного цикла, путем корректурного чтения и устранения несоответствий, клетки могут управлять ими.

Некоторые из механизмов репарации для борьбы и исправления ошибок в репликации ДНК во время клеточного цикла — это прямое обращение повреждения, эксцизионная репарация, пострепликационная репарация, BER (иссечение основания), NER (эксцизионная репарация нуклеотидов), MMR (репарация несоответствия), HR (гомологичная рекомбинация) и NHEJ (негомологичное соединение концов).

Почему ошибки в репликации ДНК так редки?

Ошибки в репликации ДНК настолько редки, что при копировании приходится одна ошибка на миллиарды нуклеотидов. Давайте поймем причину этого.

Ошибки в репликации ДНК настолько редки из-за вычитка и исправление несоответствий механизмы исправления ошибок. Иногда это происходит, когда полимераза ДНК вставляет неправильные нуклеотиды. Если это не так, они могут привести к мутации, которая может привести к раку..

Вычитка устраняет все ошибки перед репликацией, а несоответствие устраняет ошибки после репликации. В результате частота ошибок составляет одну на каждые 10^4-10^5 нуклеотидов в синтезе. Даже если есть какие-то неисправности, скорость составляет менее 0.001%.

CАКЛЮЧЕНИЕ

В конце статьи доказано, что ошибки в репликации ДНК очень редки, и если ошибка остается после механизмов репарации, это может привести к постоянной мутации (хотя и множеству небольших мутаций), которая затем приводит к раку или другим фатальным последствиям. условия здоровья и для следующих поколений. Поскольку репликация ДНК является очень важной и важной процедурой во время клеточного цикла, это высокозащищенная система.

Привет . Я Ахели Дей, я получил степень магистра зоологии. Моя специализация – паразитология и иммунология. Я с большим энтузиазмом изучаю новые вещи. Я предпочитаю тяжелую и умную работу. Подключаемся через LinkedIn-https://www.linkedin.com/in/aheli-dey-793555249

Последние посты

Paramecium — одноклеточные эукариоты, принадлежащие к царству Protista. Обычно они встречаются в водной среде обитания. Сократительные вакуоли в парамециях — это органеллы, участвующие в.

Пищевые вакуоли представляют собой мешкообразные структуры, состоящие из однослойных мембран. Они содержат несколько ферментов для преобразования больших молекул в более мелкие. Пищевые вакуоли у простейших или других.

О НАС

Мы являемся группой профессионалов отрасли из различных областей образования, таких как наука, инженерия, английская литература, и создаем универсальное образовательное решение, основанное на знаниях.

Источник

Problems occur when the molecular machinery responsible for copying the DNA encounters an obstacle. This can cause things to slow down or briefly pause, which creates a narrow window of opportunity for mistakes to be made.

“The pausing of DNA replication leads to the accumulation of fragile, single-stranded DNA that is prone to base damage, slippage and double-strand breaks,” explains Nieduszynski. “Therefore, fork pausing is a major source of replicative errors, including point mutations, expansion/contraction of repeats, deletions and translocations.”

Although several checks and balances mean these obstacles normally get spotted and fixed during replication when things do go wrong the consequences can be catastrophic for the cell.

These rare but serious events are difficult to detect since most DNA replication is regular. Spotting them is something of a ‘needle in a haystack’ problem for researchers.

Nieduszynski’s Group has developed a high-throughput DNA sequencing technology that enables them to study the kinetics of DNA replication ‘in vivo’ on single molecules.

“This technology allows us to rapidly search for the ‘needle in the haystack’ and identify key molecular events, such as the slowing down or pausing of the DNA replication machinery,” he explains.

The Group is now applying this approach to determine what DNA sequences create obstacles to the DNA replication machinery, which protein factors assist in overcoming these hurdles, and how exactly pauses link to errors during the copying process.

“These approaches allow us to identify and characterise rare DNA replication mistakes, prioritising what determines and causes these mistakes and their resulting consequences,” says Nieduszynski.

“This is so important because a single DNA replication error on one chromosome in a single cell division has the potential to be harmless, lead to a subtle effect, or potentially be detrimental to the organism.”

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome.^[1] In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day.^[2] Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell’s ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell’s genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur, including double-strand breaks and DNA crosslinkages (interstrand crosslinks or ICLs).^[3][4] This can eventually lead to malignant tumors, or cancer as per the two hit hypothesis.

The rate of DNA repair is dependent on many factors, including the cell type, the age of the cell, and the extracellular environment. A cell that has accumulated a large amount of DNA damage, or one that no longer effectively repairs damage incurred to its DNA, can enter one of three possible states:

1. an irreversible state of dormancy, known as senescence
2. cell suicide, also known as apoptosis or programmed cell death
3. unregulated cell division, which can lead to the formation of a tumor that is cancerous

The DNA repair ability of a cell is vital to the integrity of its genome and thus to the normal functionality of that organism. Many genes that were initially shown to influence life span have turned out to be involved in DNA damage repair and protection.^[5]

The 2015 Nobel Prize in Chemistry was awarded to Tomas Lindahl, Paul Modrich, and Aziz Sancar for their work on the molecular mechanisms of DNA repair processes.^[6][7]

DNA damage[edit]

DNA damage, due to environmental factors and normal metabolic processes inside the cell, occurs at a rate of 10,000 to 1,000,000 molecular lesions per cell per day.^[2] While this constitutes only 0.000165% of the human genome’s approximately 6 billion bases, unrepaired lesions in critical genes (such as tumor suppressor genes) can impede a cell’s ability to carry out its function and appreciably increase the likelihood of tumor formation and contribute to tumour heterogeneity.

The vast majority of DNA damage affects the primary structure of the double helix; that is, the bases themselves are chemically modified. These modifications can in turn disrupt the molecules’ regular helical structure by introducing non-native chemical bonds or bulky adducts that do not fit in the standard double helix. Unlike proteins and RNA, DNA usually lacks tertiary structure and therefore damage or disturbance does not occur at that level. DNA is, however, supercoiled and wound around «packaging» proteins called histones (in eukaryotes), and both superstructures are vulnerable to the effects of DNA damage.

Sources[edit]

DNA damage can be subdivided into two main types:

1. endogenous damage such as attack by reactive oxygen species produced from normal metabolic byproducts (spontaneous mutation), especially the process of oxidative deamination
   1. also includes replication errors
2. exogenous damage caused by external agents such as
   1. ultraviolet [UV 200–400 nm] radiation from the sun or other artificial light sources
   2. other radiation frequencies, including x-rays and gamma rays
   3. hydrolysis or thermal disruption
   4. certain plant toxins
   5. human-made mutagenic chemicals, especially aromatic compounds that act as DNA intercalating agents
   6. viruses^[8]

The replication of damaged DNA before cell division can lead to the incorporation of wrong bases opposite damaged ones. Daughter cells that inherit these wrong bases carry mutations from which the original DNA sequence is unrecoverable (except in the rare case of a back mutation, for example, through gene conversion).

Types[edit]

There are several types of damage to DNA due to endogenous cellular processes:

1. oxidation of bases [e.g. 8-oxo-7,8-dihydroguanine (8-oxoG)] and generation of DNA strand interruptions from reactive oxygen species,
2. alkylation of bases (usually methylation), such as formation of 7-methylguanosine, 1-methyladenine, 6-O-Methylguanine
3. hydrolysis of bases, such as deamination, depurination, and depyrimidination.
4. «bulky adduct formation» (e.g., benzo[a]pyrene diol epoxide-dG adduct, aristolactam I-dA adduct)
5. mismatch of bases, due to errors in DNA replication, in which the wrong DNA base is stitched into place in a newly forming DNA strand, or a DNA base is skipped over or mistakenly inserted.
6. Monoadduct damage cause by change in single nitrogenous base of DNA
7. Diadduct damage

Damage caused by exogenous agents comes in many forms. Some examples are:

1. UV-B light causes crosslinking between adjacent cytosine and thymine bases creating pyrimidine dimers. This is called direct DNA damage.
2. UV-A light creates mostly free radicals. The damage caused by free radicals is called indirect DNA damage.
3. Ionizing radiation such as that created by radioactive decay or in cosmic rays causes breaks in DNA strands. Intermediate-level ionizing radiation may induce irreparable DNA damage (leading to replicational and transcriptional errors needed for neoplasia or may trigger viral interactions) leading to pre-mature aging and cancer.
4. Thermal disruption at elevated temperature increases the rate of depurination (loss of purine bases from the DNA backbone) and single-strand breaks. For example, hydrolytic depurination is seen in the thermophilic bacteria, which grow in hot springs at 40–80 °C.^[9][10] The rate of depurination (300 purine residues per genome per generation) is too high in these species to be repaired by normal repair machinery, hence a possibility of an adaptive response cannot be ruled out.
5. Industrial chemicals such as vinyl chloride and hydrogen peroxide, and environmental chemicals such as polycyclic aromatic hydrocarbons found in smoke, soot and tar create a huge diversity of DNA adducts- ethenobases, oxidized bases, alkylated phosphotriesters and crosslinking of DNA, just to name a few.

UV damage, alkylation/methylation, X-ray damage and oxidative damage are examples of induced damage. Spontaneous damage can include the loss of a base, deamination, sugar ring puckering and tautomeric shift. Constitutive (spontaneous) DNA damage caused by endogenous oxidants can be detected as a low level of histone H2AX phosphorylation in untreated cells.^[11]

Nuclear versus mitochondrial[edit]

In human cells, and eukaryotic cells in general, DNA is found in two cellular locations – inside the nucleus and inside the mitochondria. Nuclear DNA (nDNA) exists as chromatin during non-replicative stages of the cell cycle and is condensed into aggregate structures known as chromosomes during cell division. In either state the DNA is highly compacted and wound up around bead-like proteins called histones. Whenever a cell needs to express the genetic information encoded in its nDNA the required chromosomal region is unravelled, genes located therein are expressed, and then the region is condensed back to its resting conformation. Mitochondrial DNA (mtDNA) is located inside mitochondria organelles, exists in multiple copies, and is also tightly associated with a number of proteins to form a complex known as the nucleoid. Inside mitochondria, reactive oxygen species (ROS), or free radicals, byproducts of the constant production of adenosine triphosphate (ATP) via oxidative phosphorylation, create a highly oxidative environment that is known to damage mtDNA. A critical enzyme in counteracting the toxicity of these species is superoxide dismutase, which is present in both the mitochondria and cytoplasm of eukaryotic cells.

Senescence and apoptosis[edit]

Senescence, an irreversible process in which the cell no longer divides, is a protective response to the shortening of the chromosome ends, called telomeres. The telomeres are long regions of repetitive noncoding DNA that cap chromosomes and undergo partial degradation each time a cell undergoes division (see Hayflick limit).^[12] In contrast, quiescence is a reversible state of cellular dormancy that is unrelated to genome damage (see cell cycle). Senescence in cells may serve as a functional alternative to apoptosis in cases where the physical presence of a cell for spatial reasons is required by the organism,^[13] which serves as a «last resort» mechanism to prevent a cell with damaged DNA from replicating inappropriately in the absence of pro-growth cellular signaling. Unregulated cell division can lead to the formation of a tumor (see cancer), which is potentially lethal to an organism. Therefore, the induction of senescence and apoptosis is considered to be part of a strategy of protection against cancer.^[14]

Mutation[edit]

It is important to distinguish between DNA damage and mutation, the two major types of error in DNA. DNA damage and mutation are fundamentally different. Damage results in physical abnormalities in the DNA, such as single- and double-strand breaks, 8-hydroxydeoxyguanosine residues, and polycyclic aromatic hydrocarbon adducts. DNA damage can be recognized by enzymes, and thus can be correctly repaired if redundant information, such as the undamaged sequence in the complementary DNA strand or in a homologous chromosome, is available for copying. If a cell retains DNA damage, transcription of a gene can be prevented, and thus translation into a protein will also be blocked. Replication may also be blocked or the cell may die.

In contrast to DNA damage, a mutation is a change in the base sequence of the DNA. A mutation cannot be recognized by enzymes once the base change is present in both DNA strands, and thus a mutation cannot be repaired. At the cellular level, mutations can cause alterations in protein function and regulation. Mutations are replicated when the cell replicates. In a population of cells, mutant cells will increase or decrease in frequency according to the effects of the mutation on the ability of the cell to survive and reproduce.

Although distinctly different from each other, DNA damage and mutation are related because DNA damage often causes errors of DNA synthesis during replication or repair; these errors are a major source of mutation.

Given these properties of DNA damage and mutation, it can be seen that DNA damage is a special problem in non-dividing or slowly-dividing cells, where unrepaired damage will tend to accumulate over time. On the other hand, in rapidly dividing cells, unrepaired DNA damage that does not kill the cell by blocking replication will tend to cause replication errors and thus mutation. The great majority of mutations that are not neutral in their effect are deleterious to a cell’s survival. Thus, in a population of cells composing a tissue with replicating cells, mutant cells will tend to be lost. However, infrequent mutations that provide a survival advantage will tend to clonally expand at the expense of neighboring cells in the tissue. This advantage to the cell is disadvantageous to the whole organism because such mutant cells can give rise to cancer. Thus, DNA damage in frequently dividing cells, because it gives rise to mutations, is a prominent cause of cancer. In contrast, DNA damage in infrequently-dividing cells is likely a prominent cause of aging.^[15]

Mechanisms[edit]

Cells cannot function if DNA damage corrupts the integrity and accessibility of essential information in the genome (but cells remain superficially functional when non-essential genes are missing or damaged). Depending on the type of damage inflicted on the DNA’s double helical structure, a variety of repair strategies have evolved to restore lost information. If possible, cells use the unmodified complementary strand of the DNA or the sister chromatid as a template to recover the original information. Without access to a template, cells use an error-prone recovery mechanism known as translesion synthesis as a last resort.

Damage to DNA alters the spatial configuration of the helix, and such alterations can be detected by the cell. Once damage is localized, specific DNA repair molecules bind at or near the site of damage, inducing other molecules to bind and form a complex that enables the actual repair to take place.

Direct reversal[edit]

Cells are known to eliminate three types of damage to their DNA by chemically reversing it. These mechanisms do not require a template, since the types of damage they counteract can occur in only one of the four bases. Such direct reversal mechanisms are specific to the type of damage incurred and do not involve breakage of the phosphodiester backbone. The formation of pyrimidine dimers upon irradiation with UV light results in an abnormal covalent bond between adjacent pyrimidine bases. The photoreactivation process directly reverses this damage by the action of the enzyme photolyase, whose activation is obligately dependent on energy absorbed from blue/UV light (300–500 nm wavelength) to promote catalysis.^[16] Photolyase, an old enzyme present in bacteria, fungi, and most animals no longer functions in humans,^[17] who instead use nucleotide excision repair to repair damage from UV irradiation. Another type of damage, methylation of guanine bases, is directly reversed by the enzyme methyl guanine methyl transferase (MGMT), the bacterial equivalent of which is called ogt. This is an expensive process because each MGMT molecule can be used only once; that is, the reaction is stoichiometric rather than catalytic.^[18] A generalized response to methylating agents in bacteria is known as the adaptive response and confers a level of resistance to alkylating agents upon sustained exposure by upregulation of alkylation repair enzymes.^[19] The third type of DNA damage reversed by cells is certain methylation of the bases cytosine and adenine.

Single-strand damage[edit]

When only one of the two strands of a double helix has a defect, the other strand can be used as a template to guide the correction of the damaged strand. In order to repair damage to one of the two paired molecules of DNA, there exist a number of excision repair mechanisms that remove the damaged nucleotide and replace it with an undamaged nucleotide complementary to that found in the undamaged DNA strand.^[18]

1. Base excision repair (BER): damaged single bases or nucleotides are most commonly repaired by removing the base or the nucleotide involved and then inserting the correct base or nucleotide. In base excision repair, a glycosylase^[20] enzyme removes the damaged base from the DNA by cleaving the bond between the base and the deoxyribose. These enzymes remove a single base to create an apurinic or apyrimidinic site (AP site).^[20] Enzymes called AP endonucleases nick the damaged DNA backbone at the AP site. DNA polymerase then removes the damaged region using its 5’ to 3’ exonuclease activity and correctly synthesizes the new strand using the complementary strand as a template.^[20] The gap is then sealed by enzyme DNA ligase.^[21]
2. Nucleotide excision repair (NER): bulky, helix-distorting damage, such as pyrimidine dimerization caused by UV light is usually repaired by a three-step process. First the damage is recognized, then 12-24 nucleotide-long strands of DNA are removed both upstream and downstream of the damage site by endonucleases, and the removed DNA region is then resynthesized.^[22] NER is a highly evolutionarily conserved repair mechanism and is used in nearly all eukaryotic and prokaryotic cells.^[22] In prokaryotes, NER is mediated by Uvr proteins.^[22] In eukaryotes, many more proteins are involved, although the general strategy is the same.^[22]
3. Mismatch repair systems are present in essentially all cells to correct errors that are not corrected by proofreading. These systems consist of at least two proteins. One detects the mismatch, and the other recruits an endonuclease that cleaves the newly synthesized DNA strand close to the region of damage. In E. coli , the proteins involved are the Mut class proteins: MutS, MutL, and MutH. In most Eukaryotes, the analog for MutS is MSH and the analog for MutL is MLH. MutH is only present in bacteria. This is followed by removal of damaged region by an exonuclease, resynthesis by DNA polymerase, and nick sealing by DNA ligase.^[23]

Double-strand breaks[edit]

Double-strand breaks, in which both strands in the double helix are severed, are particularly hazardous to the cell because they can lead to genome rearrangements. In fact, when a double-strand break is accompanied by a cross-linkage joining the two strands at the same point, neither strand can be used as a template for the repair mechanisms, so that the cell will not be able to complete mitosis when it next divides, and will either die or, in rare cases, undergo a mutation.^[3][4] Three mechanisms exist to repair double-strand breaks (DSBs): non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homologous recombination (HR):^[18][24]

1. In NHEJ, DNA Ligase IV, a specialized DNA ligase that forms a complex with the cofactor XRCC4, directly joins the two ends.^[25] To guide accurate repair, NHEJ relies on short homologous sequences called microhomologies present on the single-stranded tails of the DNA ends to be joined. If these overhangs are compatible, repair is usually accurate.^{[26][27][28][29]} NHEJ can also introduce mutations during repair. Loss of damaged nucleotides at the break site can lead to deletions, and joining of nonmatching termini forms insertions or translocations. NHEJ is especially important before the cell has replicated its DNA, since there is no template available for repair by homologous recombination. There are «backup» NHEJ pathways in higher eukaryotes.^[30] Besides its role as a genome caretaker, NHEJ is required for joining hairpin-capped double-strand breaks induced during V(D)J recombination, the process that generates diversity in B-cell and T-cell receptors in the vertebrate immune system.^[31]
2. MMEJ starts with short-range end resection by MRE11 nuclease on either side of a double-strand break to reveal microhomology regions.^[32] In further steps,^[33] Poly (ADP-ribose) polymerase 1 (PARP1) is required and may be an early step in MMEJ. There is pairing of microhomology regions followed by recruitment of flap structure-specific endonuclease 1 (FEN1) to remove overhanging flaps. This is followed by recruitment of XRCC1–LIG3 to the site for ligating the DNA ends, leading to an intact DNA. MMEJ is always accompanied by a deletion, so that MMEJ is a mutagenic pathway for DNA repair.^[34]
3. HR requires the presence of an identical or nearly identical sequence to be used as a template for repair of the break. The enzymatic machinery responsible for this repair process is nearly identical to the machinery responsible for chromosomal crossover during meiosis. This pathway allows a damaged chromosome to be repaired using a sister chromatid (available in G2 after DNA replication) or a homologous chromosome as a template. DSBs caused by the replication machinery attempting to synthesize across a single-strand break or unrepaired lesion cause collapse of the replication fork and are typically repaired by recombination.

In an in vitro system, MMEJ occurred in mammalian cells at the levels of 10–20% of HR when both HR and NHEJ mechanisms were also available.^[32]

The extremophile Deinococcus radiodurans has a remarkable ability to survive DNA damage from ionizing radiation and other sources. At least two copies of the genome, with random DNA breaks, can form DNA fragments through annealing. Partially overlapping fragments are then used for synthesis of homologous regions through a moving D-loop that can continue extension until complementary partner strands are found. In the final step, there is crossover by means of RecA-dependent homologous recombination.^[35]

Topoisomerases introduce both single- and double-strand breaks in the course of changing the DNA’s state of supercoiling, which is especially common in regions near an open replication fork. Such breaks are not considered DNA damage because they are a natural intermediate in the topoisomerase biochemical mechanism and are immediately repaired by the enzymes that created them.

Another type of DNA double-strand breaks originates from the DNA heat-sensitive or heat-labile sites. These DNA sites are not initial DSBs. However, they convert to DSB after treating with elevated temperature. Ionizing irradiation can induces a highly complex form of DNA damage as clustered damage. It consists of different types of DNA lesions in various locations of the DNA helix. Some of these closely located lesions can probably convert to DSB by exposure to high temperatures. But the exact nature of these lesions and their interactions is not yet known^[36]

Translesion synthesis[edit]

Translesion synthesis (TLS) is a DNA damage tolerance process that allows the DNA replication machinery to replicate past DNA lesions such as thymine dimers or AP sites.^[37] It involves switching out regular DNA polymerases for specialized translesion polymerases (i.e. DNA polymerase IV or V, from the Y Polymerase family), often with larger active sites that can facilitate the insertion of bases opposite damaged nucleotides. The polymerase switching is thought to be mediated by, among other factors, the post-translational modification of the replication processivity factor PCNA. Translesion synthesis polymerases often have low fidelity (high propensity to insert wrong bases) on undamaged templates relative to regular polymerases. However, many are extremely efficient at inserting correct bases opposite specific types of damage. For example, Pol η mediates error-free bypass of lesions induced by UV irradiation, whereas Pol ι introduces mutations at these sites. Pol η is known to add the first adenine across the T^T photodimer using Watson-Crick base pairing and the second adenine will be added in its syn conformation using Hoogsteen base pairing. From a cellular perspective, risking the introduction of point mutations during translesion synthesis may be preferable to resorting to more drastic mechanisms of DNA repair, which may cause gross chromosomal aberrations or cell death. In short, the process involves specialized polymerases either bypassing or repairing lesions at locations of stalled DNA replication. For example, Human DNA polymerase eta can bypass complex DNA lesions like guanine-thymine intra-strand crosslink, G[8,5-Me]T, although it can cause targeted and semi-targeted mutations.^[38] Paromita Raychaudhury and Ashis Basu^[39] studied the toxicity and mutagenesis of the same lesion in Escherichia coli by replicating a G[8,5-Me]T-modified plasmid in E. coli with specific DNA polymerase knockouts. Viability was very low in a strain lacking pol II, pol IV, and pol V, the three SOS-inducible DNA polymerases, indicating that translesion synthesis is conducted primarily by these specialized DNA polymerases.
A bypass platform is provided to these polymerases by Proliferating cell nuclear antigen (PCNA). Under normal circumstances, PCNA bound to polymerases replicates the DNA. At a site of lesion, PCNA is ubiquitinated, or modified, by the RAD6/RAD18 proteins to provide a platform for the specialized polymerases to bypass the lesion and resume DNA replication.^[40][41] After translesion synthesis, extension is required. This extension can be carried out by a replicative polymerase if the TLS is error-free, as in the case of Pol η, yet if TLS results in a mismatch, a specialized polymerase is needed to extend it; Pol ζ. Pol ζ is unique in that it can extend terminal mismatches, whereas more processive polymerases cannot. So when a lesion is encountered, the replication fork will stall, PCNA will switch from a processive polymerase to a TLS polymerase such as Pol ι to fix the lesion, then PCNA may switch to Pol ζ to extend the mismatch, and last PCNA will switch to the processive polymerase to continue replication.

Global response to DNA damage[edit]

Cells exposed to ionizing radiation, ultraviolet light or chemicals are prone to acquire multiple sites of bulky DNA lesions and double-strand breaks. Moreover, DNA damaging agents can damage other biomolecules such as proteins, carbohydrates, lipids, and RNA. The accumulation of damage, to be specific, double-strand breaks or adducts stalling the replication forks, are among known stimulation signals for a global response to DNA damage.^[42] The global response to damage is an act directed toward the cells’ own preservation and triggers multiple pathways of macromolecular repair, lesion bypass, tolerance, or apoptosis. The common features of global response are induction of multiple genes, cell cycle arrest, and inhibition of cell division.

Initial steps[edit]

The packaging of eukaryotic DNA into chromatin presents a barrier to all DNA-based processes that require recruitment of enzymes to their sites of action. To allow DNA repair, the chromatin must be remodeled. In eukaryotes, ATP dependent chromatin remodeling complexes and histone-modifying enzymes are two predominant factors employed to accomplish this remodeling process.^[43]

Chromatin relaxation occurs rapidly at the site of a DNA damage.^[44][45] In one of the earliest steps, the stress-activated protein kinase, c-Jun N-terminal kinase (JNK), phosphorylates SIRT6 on serine 10 in response to double-strand breaks or other DNA damage.^[46] This post-translational modification facilitates the mobilization of SIRT6 to DNA damage sites, and is required for efficient recruitment of poly (ADP-ribose) polymerase 1 (PARP1) to DNA break sites and for efficient repair of DSBs.^[46] PARP1 protein starts to appear at DNA damage sites in less than a second, with half maximum accumulation within 1.6 seconds after the damage occurs.^[47] PARP1 synthesizes polymeric adenosine diphosphate ribose (poly (ADP-ribose) or PAR) chains on itself. Next the chromatin remodeler ALC1 quickly attaches to the product of PARP1 action, a poly-ADP ribose chain, and ALC1 completes arrival at the DNA damage within 10 seconds of the occurrence of the damage.^[45] About half of the maximum chromatin relaxation, presumably due to action of ALC1, occurs by 10 seconds.^[45] This then allows recruitment of the DNA repair enzyme MRE11, to initiate DNA repair, within 13 seconds.^[47]

γH2AX, the phosphorylated form of H2AX is also involved in the early steps leading to chromatin decondensation after DNA double-strand breaks. The histone variant H2AX constitutes about 10% of the H2A histones in human chromatin.^[48] γH2AX (H2AX phosphorylated on serine 139) can be detected as soon as 20 seconds after irradiation of cells (with DNA double-strand break formation), and half maximum accumulation of γH2AX occurs in one minute.^[48] The extent of chromatin with phosphorylated γH2AX is about two million base pairs at the site of a DNA double-strand break.^[48] γH2AX does not, itself, cause chromatin decondensation, but within 30 seconds of irradiation, RNF8 protein can be detected in association with γH2AX.^[49] RNF8 mediates extensive chromatin decondensation, through its subsequent interaction with CHD4,^[50] a component of the nucleosome remodeling and deacetylase complex NuRD.

DDB2 occurs in a heterodimeric complex with DDB1. This complex further complexes with the ubiquitin ligase protein CUL4A^[51] and with PARP1.^[52] This larger complex rapidly associates with UV-induced damage within chromatin, with half-maximum association completed in 40 seconds.^[51] The PARP1 protein, attached to both DDB1 and DDB2, then PARylates (creates a poly-ADP ribose chain) on DDB2 that attracts the DNA remodeling protein ALC1.^[52] Action of ALC1 relaxes the chromatin at the site of UV damage to DNA. This relaxation allows other proteins in the nucleotide excision repair pathway to enter the chromatin and repair UV-induced cyclobutane pyrimidine dimer damages.

After rapid chromatin remodeling, cell cycle checkpoints are activated to allow DNA repair to occur before the cell cycle progresses. First, two kinases, ATM and ATR are activated within 5 or 6 minutes after DNA is damaged. This is followed by phosphorylation of the cell cycle checkpoint protein Chk1, initiating its function, about 10 minutes after DNA is damaged.^[53]

DNA damage checkpoints[edit]

After DNA damage, cell cycle checkpoints are activated. Checkpoint activation pauses the cell cycle and gives the cell time to repair the damage before continuing to divide. DNA damage checkpoints occur at the G1/S and G2/M boundaries. An intra-S checkpoint also exists. Checkpoint activation is controlled by two master kinases, ATM and ATR. ATM responds to DNA double-strand breaks and disruptions in chromatin structure,^[54] whereas ATR primarily responds to stalled replication forks. These kinases phosphorylate downstream targets in a signal transduction cascade, eventually leading to cell cycle arrest. A class of checkpoint mediator proteins including BRCA1, MDC1, and 53BP1 has also been identified.^[55] These proteins seem to be required for transmitting the checkpoint activation signal to downstream proteins.

DNA damage checkpoint is a signal transduction pathway that blocks cell cycle progression in G1, G2 and metaphase and slows down the rate of S phase progression when DNA is damaged. It leads to a pause in cell cycle allowing the cell time to repair the damage before continuing to divide.

Checkpoint Proteins can be separated into four groups: phosphatidylinositol 3-kinase (PI3K)-like protein kinase, proliferating cell nuclear antigen (PCNA)-like group, two serine/threonine(S/T) kinases and their adaptors. Central to all DNA damage induced checkpoints responses is a pair of large protein kinases belonging to the first group of PI3K-like protein kinases-the ATM (Ataxia telangiectasia mutated) and ATR (Ataxia- and Rad-related) kinases, whose sequence and functions have been well conserved in evolution. All DNA damage response requires either ATM or ATR because they have the ability to bind to the chromosomes at the site of DNA damage, together with accessory proteins that are platforms on which DNA damage response components and DNA repair complexes can be assembled.

An important downstream target of ATM and ATR is p53, as it is required for inducing apoptosis following DNA damage.^[56] The cyclin-dependent kinase inhibitor p21 is induced by both p53-dependent and p53-independent mechanisms and can arrest the cell cycle at the G1/S and G2/M checkpoints by deactivating cyclin/cyclin-dependent kinase complexes.^[57]

The prokaryotic SOS response[edit]

The SOS response is the changes in gene expression in Escherichia coli and other bacteria in response to extensive DNA damage. The prokaryotic SOS system is regulated by two key proteins: LexA and RecA. The LexA homodimer is a transcriptional repressor that binds to operator sequences commonly referred to as SOS boxes. In Escherichia coli it is known that LexA regulates transcription of approximately 48 genes including the lexA and recA genes.^[58] The SOS response is known to be widespread in the Bacteria domain, but it is mostly absent in some bacterial phyla, like the Spirochetes.^[59]
The most common cellular signals activating the SOS response are regions of single-stranded DNA (ssDNA), arising from stalled replication forks or double-strand breaks, which are processed by DNA helicase to separate the two DNA strands.^[42] In the initiation step, RecA protein binds to ssDNA in an ATP hydrolysis driven reaction creating RecA–ssDNA filaments. RecA–ssDNA filaments activate LexA autoprotease activity, which ultimately leads to cleavage of LexA dimer and subsequent LexA degradation. The loss of LexA repressor induces transcription of the SOS genes and allows for further signal induction, inhibition of cell division and an increase in levels of proteins responsible for damage processing.

In Escherichia coli, SOS boxes are 20-nucleotide long sequences near promoters with palindromic structure and a high degree of sequence conservation. In other classes and phyla, the sequence of SOS boxes varies considerably, with different length and composition, but it is always highly conserved and one of the strongest short signals in the genome.^[59] The high information content of SOS boxes permits differential binding of LexA to different promoters and allows for timing of the SOS response. The lesion repair genes are induced at the beginning of SOS response. The error-prone translesion polymerases, for example, UmuCD’2 (also called DNA polymerase V), are induced later on as a last resort.^[60] Once the DNA damage is repaired or bypassed using polymerases or through recombination, the amount of single-stranded DNA in cells is decreased, lowering the amounts of RecA filaments decreases cleavage activity of LexA homodimer, which then binds to the SOS boxes near promoters and restores normal gene expression.

Eukaryotic transcriptional responses to DNA damage[edit]

Eukaryotic cells exposed to DNA damaging agents also activate important defensive pathways by inducing multiple proteins involved in DNA repair, cell cycle checkpoint control, protein trafficking and degradation. Such genome wide transcriptional response is very complex and tightly regulated, thus allowing coordinated global response to damage. Exposure of yeast Saccharomyces cerevisiae to DNA damaging agents results in overlapping but distinct transcriptional profiles. Similarities to environmental shock response indicates that a general global stress response pathway exist at the level of transcriptional activation. In contrast, different human cell types respond to damage differently indicating an absence of a common global response. The probable explanation for this difference between yeast and human cells may be in the heterogeneity of mammalian cells. In an animal different types of cells are distributed among different organs that have evolved different sensitivities to DNA damage.^[61]

In general global response to DNA damage involves expression of multiple genes responsible for postreplication repair, homologous recombination, nucleotide excision repair, DNA damage checkpoint, global transcriptional activation, genes controlling mRNA decay, and many others. A large amount of damage to a cell leaves it with an important decision: undergo apoptosis and die, or survive at the cost of living with a modified genome. An increase in tolerance to damage can lead to an increased rate of survival that will allow a greater accumulation of mutations. Yeast Rev1 and human polymerase η are members of Y family translesion DNA polymerases present during global response to DNA damage and are responsible for enhanced mutagenesis during a global response to DNA damage in eukaryotes.^[42]

Aging[edit]

Pathological effects of poor DNA repair[edit]

Experimental animals with genetic deficiencies in DNA repair often show decreased life span and increased cancer incidence.^[15] For example, mice deficient in the dominant NHEJ pathway and in telomere maintenance mechanisms get lymphoma and infections more often, and, as a consequence, have shorter lifespans than wild-type mice.^[62] In similar manner, mice deficient in a key repair and transcription protein that unwinds DNA helices have premature onset of aging-related diseases and consequent shortening of lifespan.^[63] However, not every DNA repair deficiency creates exactly the predicted effects; mice deficient in the NER pathway exhibited shortened life span without correspondingly higher rates of mutation.^[64]

If the rate of DNA damage exceeds the capacity of the cell to repair it, the accumulation of errors can overwhelm the cell and result in early senescence, apoptosis, or cancer. Inherited diseases associated with faulty DNA repair functioning result in premature aging,^[15] increased sensitivity to carcinogens and correspondingly increased cancer risk (see below). On the other hand, organisms with enhanced DNA repair systems, such as Deinococcus radiodurans, the most radiation-resistant known organism, exhibit remarkable resistance to the double-strand break-inducing effects of radioactivity, likely due to enhanced efficiency of DNA repair and especially NHEJ.^[65]

Longevity and caloric restriction[edit]

A number of individual genes have been identified as influencing variations in life span within a population of organisms. The effects of these genes is strongly dependent on the environment, in particular, on the organism’s diet. Caloric restriction reproducibly results in extended lifespan in a variety of organisms, likely via nutrient sensing pathways and decreased metabolic rate. The molecular mechanisms by which such restriction results in lengthened lifespan are as yet unclear (see^[66] for some discussion); however, the behavior of many genes known to be involved in DNA repair is altered under conditions of caloric restriction. Several agents reported to have anti-aging properties have been shown to attenuate constitutive level of mTOR signaling, an evidence of reduction of metabolic activity, and concurrently to reduce constitutive level of DNA damage induced by endogenously generated reactive oxygen species.^[67]

For example, increasing the gene dosage of the gene SIR-2, which regulates DNA packaging in the nematode worm Caenorhabditis elegans, can significantly extend lifespan.^[68] The mammalian homolog of SIR-2 is known to induce downstream DNA repair factors involved in NHEJ, an activity that is especially promoted under conditions of caloric restriction.^[69] Caloric restriction has been closely linked to the rate of base excision repair in the nuclear DNA of rodents,^[70] although similar effects have not been observed in mitochondrial DNA.^[71]

The C. elegans gene AGE-1, an upstream effector of DNA repair pathways, confers dramatically extended life span under free-feeding conditions but leads to a decrease in reproductive fitness under conditions of caloric restriction.^[72] This observation supports the pleiotropy theory of the biological origins of aging, which suggests that genes conferring a large survival advantage early in life will be selected for even if they carry a corresponding disadvantage late in life.

Medicine and DNA repair modulation[edit]

Hereditary DNA repair disorders[edit]

Defects in the NER mechanism are responsible for several genetic disorders, including:

• Xeroderma pigmentosum: hypersensitivity to sunlight/UV, resulting in increased skin cancer incidence and premature aging
• Cockayne syndrome: hypersensitivity to UV and chemical agents
• Trichothiodystrophy: sensitive skin, brittle hair and nails

Mental retardation often accompanies the latter two disorders, suggesting increased vulnerability of developmental neurons.

Other DNA repair disorders include:

• Werner’s syndrome: premature aging and retarded growth
• Bloom’s syndrome: sunlight hypersensitivity, high incidence of malignancies (especially leukemias).
• Ataxia telangiectasia: sensitivity to ionizing radiation and some chemical agents

All of the above diseases are often called «segmental progerias» («accelerated aging diseases») because those affected appear elderly and experience aging-related diseases at an abnormally young age, while not manifesting all the symptoms of old age.

Other diseases associated with reduced DNA repair function include Fanconi anemia, hereditary breast cancer and hereditary colon cancer.

Cancer[edit]

Because of inherent limitations in the DNA repair mechanisms, if humans lived long enough, they would all eventually develop cancer.^[73][74] There are at least 34 Inherited human DNA repair gene mutations that increase cancer risk. Many of these mutations cause DNA repair to be less effective than normal. In particular, Hereditary nonpolyposis colorectal cancer (HNPCC) is strongly associated with specific mutations in the DNA mismatch repair pathway. BRCA1 and BRCA2, two important genes whose mutations confer a hugely increased risk of breast cancer on carriers,^[75] are both associated with a large number of DNA repair pathways, especially NHEJ and homologous recombination.

Cancer therapy procedures such as chemotherapy and radiotherapy work by overwhelming the capacity of the cell to repair DNA damage, resulting in cell death. Cells that are most rapidly dividing – most typically cancer cells – are preferentially affected. The side-effect is that other non-cancerous but rapidly dividing cells such as progenitor cells in the gut, skin, and hematopoietic system are also affected. Modern cancer treatments attempt to localize the DNA damage to cells and tissues only associated with cancer, either by physical means (concentrating the therapeutic agent in the region of the tumor) or by biochemical means (exploiting a feature unique to cancer cells in the body). In the context of therapies targeting DNA damage response genes, the latter approach has been termed ‘synthetic lethality’.^[76]

Perhaps the most well-known of these ‘synthetic lethality’ drugs is the poly(ADP-ribose) polymerase 1 (PARP1) inhibitor olaparib, which was approved by the Food and Drug Administration in 2015 for the treatment in women of BRCA-defective ovarian cancer. Tumor cells with partial loss of DNA damage response (specifically, homologous recombination repair) are dependent on another mechanism – single-strand break repair – which is a mechanism consisting, in part, of the PARP1 gene product.^[77] Olaparib is combined with chemotherapeutics to inhibit single-strand break repair induced by DNA damage caused by the co-administered chemotherapy. Tumor cells relying on this residual DNA repair mechanism are unable to repair the damage and hence are not able to survive and proliferate, whereas normal cells can repair the damage with the functioning homologous recombination mechanism.

Many other drugs for use against other residual DNA repair mechanisms commonly found in cancer are currently under investigation. However, synthetic lethality therapeutic approaches have been questioned due to emerging evidence of acquired resistance, achieved through rewiring of DNA damage response pathways and reversion of previously inhibited defects.^[78]

DNA repair defects in cancer[edit]

It has become apparent over the past several years that the DNA damage response acts as a barrier to the malignant transformation of preneoplastic cells.^[79] Previous studies have shown an elevated DNA damage response in cell-culture models with oncogene activation^[80] and preneoplastic colon adenomas.^[81] DNA damage response mechanisms trigger cell-cycle arrest, and attempt to repair DNA lesions or promote cell death/senescence if repair is not possible. Replication stress is observed in preneoplastic cells due to increased proliferation signals from oncogenic mutations. Replication stress is characterized by: increased replication initiation/origin firing; increased transcription and collisions of transcription-replication complexes; nucleotide deficiency; increase in reactive oxygen species (ROS).^[82]

Replication stress, along with the selection for inactivating mutations in DNA damage response genes in the evolution of the tumor,^[83] leads to downregulation and/or loss of some DNA damage response mechanisms, and hence loss of DNA repair and/or senescence/programmed cell death. In experimental mouse models, loss of DNA damage response-mediated cell senescence was observed after using a short hairpin RNA (shRNA) to inhibit the double-strand break response kinase ataxia telangiectasia (ATM), leading to increased tumor size and invasiveness.^[81] Humans born with inherited defects in DNA repair mechanisms (for example, Li-Fraumeni syndrome) have a higher cancer risk.^[84]

The prevalence of DNA damage response mutations differs across cancer types; for example, 30% of breast invasive carcinomas have mutations in genes involved in homologous recombination.^[79] In cancer, downregulation is observed across all DNA damage response mechanisms (base excision repair (BER), nucleotide excision repair (NER), DNA mismatch repair (MMR), homologous recombination repair (HR), non-homologous end joining (NHEJ) and translesion DNA synthesis (TLS).^[85] As well as mutations to DNA damage repair genes, mutations also arise in the genes responsible for arresting the cell cycle to allow sufficient time for DNA repair to occur, and some genes are involved in both DNA damage repair and cell cycle checkpoint control, for example ATM and checkpoint kinase 2 (CHEK2) – a tumor suppressor that is often absent or downregulated in non-small cell lung cancer.^[86]

Genes involved in DNA damage response pathways and frequently mutated in cancer (HR = homologous recombination; NHEJ = non-homologous end joining; SSA = single-strand annealing; FA = fanconi anemia pathway; BER = base excision repair; NER = nucleotide excision repair; MMR = mismatch repair)

	HR	NHEJ	SSA	FA	BER	NER	MMR
ATM
ATR
PAXIP
RPA
BRCA1
BRCA2
RAD51
RFC
XRCC1
PCNA
PARP1
ERCC1
MSH3

Epigenetic DNA repair defects in cancer[edit]

Classically, cancer has been viewed as a set of diseases that are driven by progressive genetic abnormalities that include mutations in tumour-suppressor genes and oncogenes, and chromosomal aberrations. However, it has become apparent that cancer is also driven by
epigenetic alterations.^[87]

Epigenetic alterations refer to functionally relevant modifications to the genome that do not involve a change in the nucleotide sequence. Examples of such modifications are changes in DNA methylation (hypermethylation and hypomethylation) and histone modification,^[88] changes in chromosomal architecture (caused by inappropriate expression of proteins such as HMGA2 or HMGA1)^[89] and changes caused by microRNAs. Each of these epigenetic alterations serves to regulate gene expression without altering the underlying DNA sequence. These changes usually remain through cell divisions, last for multiple cell generations, and can be considered to be epimutations (equivalent to mutations).

While large numbers of epigenetic alterations are found in cancers, the epigenetic alterations in DNA repair genes, causing reduced expression of DNA repair proteins, appear to be particularly important. Such alterations are thought to occur early in progression to cancer and to be a likely cause of the genetic instability characteristic of cancers.^[90][91][92]

Reduced expression of DNA repair genes causes deficient DNA repair. When DNA repair is deficient DNA damages remain in cells at a higher than usual level and these excess damages cause increased frequencies of mutation or epimutation. Mutation rates increase substantially in cells defective in DNA mismatch repair^[93][94] or in homologous recombinational repair (HRR).^[95] Chromosomal rearrangements and aneuploidy also increase in HRR defective cells.^[96]

Higher levels of DNA damage not only cause increased mutation, but also cause increased epimutation. During repair of DNA double strand breaks, or repair of other DNA damages, incompletely cleared sites of repair can cause epigenetic gene silencing.^[97][98]

Deficient expression of DNA repair proteins due to an inherited mutation can cause increased risk of cancer. Individuals with an inherited impairment in any of 34 DNA repair genes (see article DNA repair-deficiency disorder) have an increased risk of cancer, with some defects causing up to a 100% lifetime chance of cancer (e.g. p53 mutations).^[99] However, such germline mutations (which cause highly penetrant cancer syndromes) are the cause of only about 1 percent of cancers.^[100]

Frequencies of epimutations in DNA repair genes[edit]

Deficiencies in DNA repair enzymes are occasionally caused by a newly arising somatic mutation in a DNA repair gene, but are much more frequently caused by epigenetic alterations that reduce or silence expression of DNA repair genes. For example, when 113 colorectal cancers were examined in sequence, only four had a missense mutation in the DNA repair gene MGMT, while the majority had reduced MGMT expression due to methylation of the MGMT promoter region (an epigenetic alteration).^[101] Five different studies found that between 40% and 90% of colorectal cancers have reduced MGMT expression due to methylation of the MGMT promoter region.^{[102][103][104][105][106]}

Similarly, out of 119 cases of mismatch repair-deficient colorectal cancers that lacked DNA repair gene PMS2 expression, PMS2 was deficient in 6 due to mutations in the PMS2 gene, while in 103 cases PMS2 expression was deficient because its pairing partner MLH1 was repressed due to promoter methylation (PMS2 protein is unstable in the absence of MLH1).^[107] In the other 10 cases, loss of PMS2 expression was likely due to epigenetic overexpression of the microRNA, miR-155, which down-regulates MLH1.^[108]

In a further example, epigenetic defects were found in various cancers (e.g. breast, ovarian, colorectal and head and neck). Two or three deficiencies in the expression of ERCC1, XPF or PMS2 occur simultaneously in the majority of 49 colon cancers evaluated by Facista et al.^[109]

The chart in this section shows some frequent DNA damaging agents, examples of DNA lesions they cause, and the pathways that deal with these DNA damages. At least 169 enzymes are either directly employed in DNA repair or influence DNA repair processes.^[110] Of these, 83 are directly employed in repairing the 5 types of DNA damages illustrated in the chart.

Some of the more well studied genes central to these repair processes are shown in the chart. The gene designations shown in red, gray or cyan indicate genes frequently epigenetically altered in various types of cancers. Wikipedia articles on each of the genes highlighted by red, gray or cyan describe the epigenetic alteration(s) and the cancer(s) in which these epimutations are found. Review articles,^[111] and broad experimental survey articles^[112][113] also document most of these epigenetic DNA repair deficiencies in cancers.

Red-highlighted genes are frequently reduced or silenced by epigenetic mechanisms in various cancers. When these genes have low or absent expression, DNA damages can accumulate. Replication errors past these damages (see translesion synthesis) can lead to increased mutations and, ultimately, cancer. Epigenetic repression of DNA repair genes in accurate DNA repair pathways appear to be central to carcinogenesis.

The two gray-highlighted genes RAD51 and BRCA2, are required for homologous recombinational repair. They are sometimes epigenetically over-expressed and sometimes under-expressed in certain cancers. As indicated in the Wikipedia articles on RAD51 and BRCA2, such cancers ordinarily have epigenetic deficiencies in other DNA repair genes. These repair deficiencies would likely cause increased unrepaired DNA damages. The over-expression of RAD51 and BRCA2 seen in these cancers may reflect selective pressures for compensatory RAD51 or BRCA2 over-expression and increased homologous recombinational repair to at least partially deal with such excess DNA damages. In those cases where RAD51 or BRCA2 are under-expressed, this would itself lead to increased unrepaired DNA damages. Replication errors past these damages (see translesion synthesis) could cause increased mutations and cancer, so that under-expression of RAD51 or BRCA2 would be carcinogenic in itself.

Cyan-highlighted genes are in the microhomology-mediated end joining (MMEJ) pathway and are up-regulated in cancer. MMEJ is an additional error-prone inaccurate repair pathway for double-strand breaks. In MMEJ repair of a double-strand break, an homology of 5–25 complementary base pairs between both paired strands is sufficient to align the strands, but mismatched ends (flaps) are usually present. MMEJ removes the extra nucleotides (flaps) where strands are joined, and then ligates the strands to create an intact DNA double helix. MMEJ almost always involves at least a small deletion, so that it is a mutagenic pathway.^[24] FEN1, the flap endonuclease in MMEJ, is epigenetically increased by promoter hypomethylation and is over-expressed in the majority of cancers of the breast,^[114] prostate,^[115] stomach,^[116][117] neuroblastomas,^[118] pancreas,^[119] and lung.^[120] PARP1 is also over-expressed when its promoter region ETS site is epigenetically hypomethylated, and this contributes to progression to endometrial cancer^[121] and BRCA-mutated serous ovarian cancer.^[122] Other genes in the MMEJ pathway are also over-expressed in a number of cancers (see MMEJ for summary), and are also shown in cyan.

Genome-wide distribution of DNA repair in human somatic cells[edit]

Differential activity of DNA repair pathways across various regions of the human genome causes mutations to be very unevenly distributed within tumor genomes.^[123][124] In particular, the gene-rich, early-replicating regions of the human genome exhibit lower mutation frequencies than the gene-poor, late-replicating heterochromatin. One mechanism underlying this involves the histone modification H3K36me3, which can recruit mismatch repair proteins,^[125] thereby lowering mutation rates in H3K36me3-marked regions.^[126] Another important mechanism concerns nucleotide excision repair, which can be recruited by the transcription machinery, lowering somatic mutation rates in active genes^[124] and other open chromatin regions.^[127]

Epigenetic alterations due to DNA repair[edit]

Damage to DNA is very common and is constantly being repaired. Epigenetic alterations can accompany DNA repair of oxidative damage or double-strand breaks. In human cells, oxidative DNA damage occurs about 10,000 times a day and DNA double-strand breaks occur about 10 to 50 times a cell cycle in somatic replicating cells (see DNA damage (naturally occurring)). The selective advantage of DNA repair is to allow the cell to survive in the face of DNA damage. The selective advantage of epigenetic alterations that occur with DNA repair is not clear.

Repair of oxidative DNA damage can alter epigenetic markers[edit]

In the steady state (with endogenous damages occurring and being repaired), there are about 2,400 oxidatively damaged guanines that form 8-oxo-2′-deoxyguanosine (8-OHdG) in the average mamalian cell DNA.^[128] 8-OHdG constitutes about 5% of the oxidative damages commonly present in DNA.^[129] The oxidized guanines do not occur randomly among all guanines in DNA. There is a sequence preference for the guanine at a methylated CpG site (a cytosine followed by guanine along its 5′ → 3′ direction and where the cytosine is methylated (5-mCpG)).^[130] A 5-mCpG site has the lowest ionization potential for guanine oxidation.

Oxidized guanine has mispairing potential and is mutagenic.^[132] Oxoguanine glycosylase (OGG1) is the primary enzyme responsible for the excision of the oxidized guanine during DNA repair. OGG1 finds and binds to an 8-OHdG within a few seconds.^[133] However, OGG1 does not immediately excise 8-OHdG. In HeLa cells half maximum removal of 8-OHdG occurs in 30 minutes,^[134] and in irradiated mice, the 8-OHdGs induced in the mouse liver are removed with a half-life of 11 minutes.^[129]

When OGG1 is present at an oxidized guanine within a methylated CpG site it recruits TET1 to the 8-OHdG lesion (see Figure). This allows TET1 to demethylate an adjacent methylated cytosine. Demethylation of cytosine is an epigenetic alteration.

As an example, when human mammary epithelial cells were treated with H₂O₂ for six hours, 8-OHdG increased about 3.5-fold in DNA and this caused about 80% demethylation of the 5-methylcytosines in the genome.^[131] Demethylation of CpGs in a gene promoter by TET enzyme activity increases transcription of the gene into messenger RNA.^[135] In cells treated with H₂O₂, one particular gene was examined, BACE1.^[131] The methylation level of the BACE1 CpG island was reduced (an epigenetic alteration) and this allowed about 6.5 fold increase of expression of BACE1 messenger RNA.

While six-hour incubation with H₂O₂ causes considerable demethylation of 5-mCpG sites, shorter times of H₂O₂ incubation appear to promote other epigenetic alterations. Treatment of cells with H₂O₂ for 30 minutes causes the mismatch repair protein heterodimer MSH2-MSH6 to recruit DNA methyltransferase 1 (DNMT1) to sites of some kinds of oxidative DNA damage.^[136] This could cause increased methylation of cytosines (epigenetic alterations) at these locations.

Jiang et al.^[137] treated HEK 293 cells with agents causing oxidative DNA damage, (potassium bromate (KBrO3) or potassium chromate (K2CrO4)). Base excision repair (BER) of oxidative damage occurred with the DNA repair enzyme polymerase beta localizing to oxidized guanines. Polymerase beta is the main human polymerase in short-patch BER of oxidative DNA damage. Jiang et al.^[137] also found that polymerase beta recruited the DNA methyltransferase protein DNMT3b to BER repair sites. They then evaluated the methylation pattern at the single nucleotide level in a small region of DNA including the promoter region and the early transcription region of the BRCA1 gene. Oxidative DNA damage from bromate modulated the DNA methylation pattern (caused epigenetic alterations) at CpG sites within the region of DNA studied. In untreated cells, CpGs located at −189, −134, −29, −19, +16, and +19 of the BRCA1 gene had methylated cytosines (where numbering is from the messenger RNA transcription start site, and negative numbers indicate nucleotides in the upstream promoter region). Bromate treatment-induced oxidation resulted in the loss of cytosine methylation at −189, −134, +16 and +19 while also leading to the formation of new methylation at the CpGs located at −80, −55, −21 and +8 after DNA repair was allowed.

Homologous recombinational repair alters epigenetic markers[edit]

At least four articles report the recruitment of DNA methyltransferase 1 (DNMT1) to sites of DNA double-strand breaks.^{[138][139][140][141]} During homologous recombinational repair (HR) of the double-strand break, the involvement of DNMT1 causes the two repaired strands of DNA to have different levels of methylated cytosines. One strand becomes frequently methylated at about 21 CpG sites downstream of the repaired double-strand break. The other DNA strand loses methylation at about six CpG sites that were previously methylated downstream of the double-strand break, as well as losing methylation at about five CpG sites that were previously methylated upstream of the double-strand break. When the chromosome is replicated, this gives rise to one daughter chromosome that is heavily methylated downstream of the previous break site and one that is unmethylated in the region both upstream and downstream of the previous break site. With respect to the gene that was broken by the double-strand break, half of the progeny cells express that gene at a high level and in the other half of the progeny cells expression of that gene is repressed. When clones of these cells were maintained for three years, the new methylation patterns were maintained over that time period.^[142]

In mice with a CRISPR-mediated homology-directed recombination insertion in their genome there were a large number of increased methylations of CpG sites within the double-strand break-associated insertion.^[143]

Non-homologous end joining can cause some epigenetic marker alterations[edit]

Non-homologous end joining (NHEJ) repair of a double-strand break can cause a small number of demethylations of pre-existing cytosine DNA methylations downstream of the repaired double-strand break.^[139] Further work by Allen et al.^[144] showed that NHEJ of a DNA double-strand break in a cell could give rise to some progeny cells having repressed expression of the gene harboring the initial double-strand break and some progeny having high expression of that gene due to epigenetic alterations associated with NHEJ repair. The frequency of epigenetic alterations causing repression of a gene after an NHEJ repair of a DNA double-strand break in that gene may be about 0.9%.^[140]

Evolution[edit]

The basic processes of DNA repair are highly conserved among both prokaryotes and eukaryotes and even among bacteriophages (viruses which infect bacteria); however, more complex organisms with more complex genomes have correspondingly more complex repair mechanisms.^[145] The ability of a large number of protein structural motifs to catalyze relevant chemical reactions has played a significant role in the elaboration of repair mechanisms during evolution. For an extremely detailed review of hypotheses relating to the evolution of DNA repair, see.^[146]

The fossil record indicates that single-cell life began to proliferate on the planet at some point during the Precambrian period, although exactly when recognizably modern life first emerged is unclear. Nucleic acids became the sole and universal means of encoding genetic information, requiring DNA repair mechanisms that in their basic form have been inherited by all extant life forms from their common ancestor. The emergence of Earth’s oxygen-rich atmosphere (known as the «oxygen catastrophe») due to photosynthetic organisms, as well as the presence of potentially damaging free radicals in the cell due to oxidative phosphorylation, necessitated the evolution of DNA repair mechanisms that act specifically to counter the types of damage induced by oxidative stress.

Rate of evolutionary change[edit]

On some occasions, DNA damage is not repaired or is repaired by an error-prone mechanism that results in a change from the original sequence. When this occurs, mutations may propagate into the genomes of the cell’s progeny. Should such an event occur in a germ line cell that will eventually produce a gamete, the mutation has the potential to be passed on to the organism’s offspring. The rate of evolution in a particular species (or, in a particular gene) is a function of the rate of mutation. As a consequence, the rate and accuracy of DNA repair mechanisms have an influence over the process of evolutionary change.^[147] DNA damage protection and repair does not influence the rate of adaptation by gene regulation and by recombination and selection of alleles. On the other hand, DNA damage repair and protection does influence the rate of accumulation of irreparable, advantageous, code expanding, inheritable mutations, and slows down the evolutionary mechanism for expansion of the genome of organisms with new functionalities. The tension between evolvability and mutation repair and protection needs further investigation.

Technology[edit]

A technology named clustered regularly interspaced short palindromic repeat (shortened to CRISPR-Cas9) was discovered in 2012. The new technology allows anyone with molecular biology training to alter the genes of any species with precision, by inducing DNA damage at a specific point and then altering DNA repair mechanisms to insert new genes.^[148] It is cheaper, more efficient, and more precise than other technologies. With the help of CRISPR–Cas9, parts of a genome can be edited by scientists by removing, adding, or altering parts in a DNA sequence.

See also[edit]

References[edit]

External links[edit]

• Media related to DNA repair at Wikimedia Commons
• Roswell Park Cancer Institute DNA Repair Lectures
• A comprehensive list of Human DNA Repair Genes
• 3D structures of some DNA repair enzymes
• Human DNA repair diseases
• DNA repair special interest group
• DNA Repair Archived 12 February 2018 at the Wayback Machine
• DNA Damage and DNA Repair
• Segmental Progeria
• DNA-damage repair; the good, the bad, and the ugly