- Related Topics:
- cell
- Nucleic acid
- DNA
- p53
- mismatch repair
DNA repair
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What is DNA?
Deoxyribonucleic acid (DNA) is a molecule composed of two chains of nucleotides that wind around each other to resemble a twisted ladder. DNA carries genetic information and ensures its accurate transmission from one generation to the next. However, DNA can be damaged by spontaneous mutations, replication errors, and chemical modifications.
DNA repair refers to the mechanisms a cell uses to maintain the integrity of its genetic code. There are three main types of repair mechanisms: direct reversal, excision repair, and postreplication repair. Failure of these repair mechanisms can result in conditions such as xeroderma pigmentosum and hereditary nonpolyposis colorectal cancer.
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Why does DNA sometimes get damaged?
DNA can be damaged by various factors, both internal and external. These include:
* Environmental factors Exposure to ultraviolet (UV) light, ionizing radiation, and certain chemicals can modify DNA.
* Oxidative damage Free radicals, produced during normal metabolism or from external sources, can oxidize DNA bases.
* Replication errors Although DNA replication has proofreading mechanisms, sometimes mismatched base pairs escape this process.
* Spontaneous mutations DNA bases can react with their environment, such as water, leading to structural changes and incorrect base pairing.If DNA damage is not repaired, it can lead to mutations, cancer, and other genetic diseases.
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What is DNA repair?
DNA repair refers to the mechanisms a cell uses to maintain the integrity of its genetic code, ensuring species survival by enabling faithful inheritance of parental DNA. It also preserves individual health, as mutations can lead to cancer and other genetic diseases.
DNA repair mechanisms include direct reversal, excision repair, and postreplication repair. Direct reversal is specific to the damage, like using light-driven enzymes to separate fused pyrimidine bases. Excision repair can be specific or nonspecific, excising mismatched bases or distorted regions. Postreplication repair fills gaps at damaged sites through recombination or error-prone repair. Failures in DNA repair can lead to conditions such as hereditary nonpolyposis colorectal cancer and xeroderma pigmentosum.
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How do cells fix damaged DNA?
Cells employ various DNA repair mechanisms to maintain the integrity of their genetic code. These mechanisms include:
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Direct Reversal: Specific to the damage, such as DNA photolyase separating pyrimidine bases fused by UV light.
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Excision Repair: Can be base-specific, where DNA glycosylases identify and remove mismatched bases, or nonspecific, where the repair machinery recognizes distortions in the double helix and excises the distorted region.
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Mismatch repair: Mismatch repair systems identify and repair defective DNA segments that arise in the normal course of a cell’s life.
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Postreplication Repair: This occurs downstream of the lesion, utilizing recombination repair to copy the sequence from an undamaged sister chromosome or error-prone repair using the damaged strand as a template.
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Recombination: Involves the fragmenting, shuffling, and rejoining of DNA to assist in the repair of damaged DNA.
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What are some common types of DNA repair mechanisms?
DNA repair mechanisms are essential for maintaining the integrity of the genetic code, counteracting errors from replication or damage. Several key mechanisms ensure accurate cell function and genetic inheritance.
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Mismatch repair corrects errors post-replication, guided by template strand markers. Loss of this system can lead to cancer due to accumulated mutations.
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Base excision repair involves DNA glycosylases that identify and remove damaged or cytotoxic DNA bases, preventing mutations during replication.
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Excision repair pathway recognizes and nicks damaged DNA, excising a small single-stranded segment, which is then repaired by DNA polymerase and ligase.
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Homologous recombination exchanges genetic material between similar DNA strands, critical for repairing double-stranded breaks and increasing genetic diversity.
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Why is DNA repair important for health and preventing diseases?
DNA repair mechanisms are crucial for preserving health and preventing disease because they maintain the integrity of the genetic code. These mechanisms fix errors that occur during DNA replication or from damage after replication. DNA damage can result from various sources, including spontaneous mutations, replication errors, chemical modifications, oxidizing agents, and radiation.
If DNA is not repaired, mutations can accumulate, potentially leading to cancer and other genetic diseases. For example, issues in mismatch repair systems can cause a buildup of mutations, which may lead to uncontrolled cell division and cancer. DNA repair is so important that the mutation rate for a species is more dependent on the effectiveness of its repair mechanisms than on the rate at which breaks occur.
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DNA repair, any of several mechanisms by which a cell maintains the integrity of its genetic code. DNA repair ensures the survival of a species by enabling parental DNA to be inherited as faithfully as possible by offspring. It also preserves the health of an individual. Mutations in the genetic code can lead to cancer and other genetic diseases.
Successful DNA replication requires that the two purine bases, adenine (A) and guanine (G), pair with their pyrimidine counterparts, thymine (T) and cytosine (C). Different types of damage, however, can prevent correct base pairing, among them spontaneous mutations, replication errors, and chemical modification. Spontaneous mutations occur when DNA bases react with their environment, such as when water hydrolyzes a base and changes its structure, causing it to pair with an incorrect base. Replication errors are minimized when the DNA replication machinery “proofreads” its own synthesis, but sometimes mismatched base pairs escape proofreading. Chemical agents modify bases and interfere with DNA replication. Nitrosamines, which are found in products such as beer and pickled foods, can cause DNA alkylation (the addition of an alkyl group). Oxidizing agents and ionizing radiation create free radicals in the cell that oxidize bases, especially guanine. Ultraviolet (UV) rays can result in the production of damaging free radicals and can fuse adjacent pyrimidines, creating pyrimidine dimers that prevent DNA replication. Ionizing radiation and certain drugs, such as the chemotherapeutic agent bleomycin, can also block replication, by creating double-strand breaks in the DNA. (These agents can also create single-strand breaks, though this form of damage often is easier for cells to overcome.) Base analogs and intercalating agents can cause abnormal insertions and deletions in the sequence.
There are three types of repair mechanisms: direct reversal of the damage, excision repair, and postreplication repair. Direct reversal repair is specific to the damage. For example, in a process called photoreactivation, pyrimidine bases fused by UV light are separated by DNA photolyase (a light-driven enzyme). For direct reversal of alkylation events, a DNA methyltransferase or DNA glycosylase detects and removes the alkyl group. Excision repair can be specific or nonspecific. In base excision repair, DNA glycosylases specifically identify and remove the mismatched base. In nucleotide excision repair, the repair machinery recognizes a wide array of distortions in the double helix caused by mismatched bases; in this form of repair, the entire distorted region is excised. Postreplication repair occurs downstream of the lesion, because replication is blocked at the actual site of damage. In order for replication to occur, short segments of DNA called Okazaki fragments are synthesized. The gap left at the damaged site is filled in through recombination repair, which uses the sequence from an undamaged sister chromosome to repair the damaged one, or through error-prone repair, which uses the damaged strand as a sequence template. Error-prone repair tends to be inaccurate and subject to mutation.
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Often when DNA is damaged, the cell chooses to replicate over the lesion instead of waiting for repair (translesion synthesis). Although this may lead to mutations, it is preferable to a complete halt in DNA replication, which leads to cell death. On the other hand, the importance of proper DNA repair is highlighted when repair fails. The oxidation of guanine by free radicals leads to G-T transversion, one of the most common mutations in human cancer.
Hereditary nonpolyposis colorectal cancer results from a mutation in the MSH2 and MLH1 proteins, which repair mismatches during replication. Xeroderma pigmentosum (XP) is another condition that results from failed DNA repair. Patients with XP are highly sensitive to light, exhibit premature skin aging, and are prone to malignant skin tumours because the XP proteins, many of which mediate nucleotide excision repair, can no longer function.