DNA REPAIR MECHANISMS

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DNA REPAIR MECHANISMS
(CSIR-NET | GATE BT | DBT-BET | ICMR | PhD Entrance)

Welcome back to Biotech Notes Hub! DNA replication is highly accurate, but it is not perfect. Furthermore, human cells experience approximately 10,000 to 100,000 DNA lesions per day due to endogenous metabolism and environmental radiation. If left uncorrected, these lesions lead to mutations, cellular senescence, or cancer.

This is arguably the highest-weightage topic in molecular biology for competitive exams. Examiners consistently test the specific enzymes, the difference between Global Genome vs. Transcription-Coupled repair, the proteins of homologous recombination, and the diseases caused by repair defects (like Xeroderma Pigmentosum and Lynch Syndrome).

This strictly optimized, error-free masterclass breaks down every repair pathway with crystal-clear mechanisms, comparison tables, mnemonics, and a high-yield quiz.

Quick Navigation Index

• 1. The DNA Repair Tree
• 2. Direct DNA Repair (Photoreactivation & MGMT)
• 3. Mismatch Repair (MMR) & Lynch Syndrome
• 4. Excision Repair (BER & NER)
• 5. Translesion Synthesis (TLS / SOS Repair)
• 6. Double-Strand Break Repair (HR vs NHEJ)
• 7. Master Comparison & Drug Tables
• 8. Research Update: Synthetic Lethality
• 9. Frequently Asked Questions (FAQs)
• 10. Master Level Quiz

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1. The DNA Repair Tree

To master this topic, you must first mentally organize the pathways. Some repair pathways fix single bases, some fix bulky adducts, and others fix shattered chromosomes.

DNA Repair
│
├── Direct Repair (No DNA synthesis required)
│
├── Excision Repair
│     ├── Mismatch Repair (MMR)
│     ├── Base Excision Repair (BER)
│     └── Nucleotide Excision Repair (NER)
│           ├── Global Genome Repair (GG-NER)
│           └── Transcription Coupled Repair (TC-NER)
│
├── Translesion DNA Synthesis (TLS / SOS Error-Prone Repair)
│
└── Double-Strand Break Repair
     ├── Homologous Recombination (HR)
     └── Non-Homologous End Joining (NHEJ)

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2. Direct DNA Repair

Direct repair is the fastest and most elegant mechanism. The damaged base is chemically reversed to its normal state without breaking the DNA backbone or removing any nucleotides.

1. Photoreactivation

• Repairs: UV-induced Thymine dimers.
• Enzyme: Photolyase.
• Mechanism: Photolyase binds to the dimer. It absorbs Blue/Visible light to generate the energy needed to break the covalent bonds of the dimer, restoring normal thymines.
• Exam Fact: This mechanism is present in bacteria, plants, and many animals, but is absent in humans and placental mammals (we rely entirely on NER).

2. Alkylation Repair (The Suicide Enzyme)

• Damage: O⁶-Methylguanine (a highly mutagenic lesion that pairs with Thymine instead of Cytosine).
• Enzyme: MGMT (O⁶-Methylguanine DNA Methyltransferase).
• Mechanism: MGMT physically transfers the rogue methyl group from the DNA guanine directly onto a cysteine residue within its own active site.
• Exam Fact: Once MGMT accepts the methyl group, it becomes permanently inactivated and is degraded. Because it is consumed in the reaction, it is classically termed a Suicide Enzyme.

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3. Mismatch Repair (MMR) & Lynch Syndrome

MMR acts immediately behind the replication fork to catch and correct the rare errors (like G-T pairings or small insertions/deletions) that escaped the proofreading exonuclease of DNA Polymerase.

CSIR Mnemonic: Bacterial MMR

Remember S → L → H

• MutS: Senses the mismatch bulge.
• MutL: Links MutS to MutH.
• MutH: Hacks (cuts) the unmethylated new strand.

Bacterial MMR Mechanism

1. Discrimination: Dam methylase marks the old parental strand by methylating Adenines at GATC sites. The new strand is temporarily unmethylated. 2. Detection: MutS recognizes the mismatch. MutL brings over MutH. 3. Cleavage: MutH specifically cuts the unmethylated (new) strand. 4. Repair: UvrD helicase unwinds the DNA, exonucleases chew away the error, DNA Pol III fills the gap, and Ligase seals it.

Eukaryotic MMR

Eukaryotes have MutS homologs (MSH2, MSH6, MSH3) and MutL homologs (MLH1, PMS2). However, eukaryotes lack MutH and do not use methylation for strand discrimination. Instead, they use preexisting nicks (like the gaps between Okazaki fragments) to identify the newly synthesized strand.

Clinical Importance: Mutations in human MSH2 or MLH1 cause Lynch Syndrome (Hereditary Non-Polyposis Colorectal Cancer, HNPCC).

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4. Excision Repair (BER & NER)

Base Excision Repair (BER)

BER specifically repairs small, non-helix-distorting base lesions (like Deamination, Depurination, and Oxidation).

CSIR Mnemonic: BER Pathway

Remember GAPL

• Glycosylase: Removes the damaged base, creating an AP (Apurinic/Apyrimidinic) site.
• AP Endonuclease: Cuts the DNA backbone at the empty AP site.
• Polymerase: Fills the gap (Pol Beta for short patch; Pol Delta/Epsilon for long patch).
• Ligase: Seals the nick.

Example: Uracil DNA Glycosylase perfectly removes Uracil that forms when Cytosine is spontaneously deaminated.

Nucleotide Excision Repair (NER)

NER targets massive, bulky, helix-distorting lesions, primarily UV-induced Thymine Dimers and bulky chemical adducts.

Bacterial NER Mechanism (The Uvr System)

UvrA & UvrB: Scan the DNA and detect the massive bulge caused by the dimer. UvrC (Excinuclease): Makes two cuts—one on the 5-prime side and one on the 3-prime side of the lesion. UvrD (Helicase): Strips away the entire 12-13 nucleotide damaged fragment. Completion: DNA Pol I fills the massive gap; DNA Ligase seals it.

Eukaryotic NER Sub-pathways

• Global Genome Repair (GG-NER): Scans the entire silent genome. Uses XPC for damage recognition.
• Transcription Coupled Repair (TC-NER): Prioritizes actively transcribing genes. It is triggered when RNA Polymerase physically crashes into a DNA lesion and stalls. Uses CSA and CSB proteins.

Diseases of NER: Defective NER proteins (XPA-XPG) cause Xeroderma Pigmentosum (extreme UV sensitivity, skin cancer). Defective TC-NER (CSA/CSB) causes Cockayne Syndrome (premature aging, neurological defects, but interestingly, no increased cancer risk).

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5. Translesion Synthesis (TLS / SOS Repair)

When normal, highly-accurate replicative polymerases (like Pol III or Epsilon) hit a massive DNA lesion (like a thymine dimer), they stall. If the fork remains stalled, the cell dies. To survive, the cell uses a "last resort" mechanism.

The Bacterial SOS Response

• Extensive DNA damage creates large amounts of single-stranded DNA (ssDNA).
• RecA binds to the ssDNA and becomes activated.
• Active RecA acts as a co-protease, causing the LexA repressor protein to cleave itself.
• The destruction of LexA turns on dozens of SOS survival genes, including specialized TLS polymerases (Pol IV and Pol V).

The Catch (Error-Prone): TLS polymerases have massive, open active sites that can physically fit over bulky thymine dimers. However, they lack 3-prime to 5-prime proofreading activity. They blindly insert random nucleotides to bypass the damage. This saves the cell from dying but introduces heavy mutations.

Eukaryotic Disease: A defect in the eukaryotic TLS polymerase Pol Eta (η) causes Variant Xeroderma Pigmentosum (XP-V).

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6. Double-Strand Break Repair (HR vs NHEJ)

A double-strand break (DSB), often caused by X-rays or gamma rays, is the most lethal form of DNA damage. If a chromosome breaks in half, it must be stitched back together immediately.

Figure 1: DSB Repair Pathways. NHEJ simply grabs the broken ends and smashes them together, often resulting in small deletions. HR uses the intact sister chromatid as a pristine template, performing strand invasion to accurately synthesize the missing DNA sequence.

1. Homologous Recombination (HR)

• Accuracy: Extremely high (Error-free).
• Template: Requires the identical Sister Chromatid. Therefore, it primarily occurs during the S and G2 phases of the cell cycle.
• Mechanism: The MRN complex detects the break. Exonucleases resect the ends. RAD51 (assisted by BRCA1 and BRCA2) coats the single-strand and forces it to invade the sister chromatid to use it as a template.
• Clinical Note: Mutations in BRCA1 or BRCA2 severely disable HR, leading to hereditary breast and ovarian cancers.

2. Non-Homologous End Joining (NHEJ)

• Accuracy: Low (Error-prone).
• Template: None required. It operates throughout the entire cell cycle, including G1.
• Mechanism: Ku70 and Ku80 proteins immediately cap the broken ends. They recruit DNA-PKcs. The Artemis nuclease trims any jagged edges. Finally, DNA Ligase IV and XRCC4 smash and seal the ends together.
• Result: Because the ends are trimmed blindly, NHEJ almost always causes small insertions or deletions (indels) at the repair site.

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7. Master Comparison & Drug Tables

Feature	Homologous Recombination (HR)	Non-Homologous End Joining (NHEJ)
Accuracy	High (Error-free)	Low (Error-prone, causes indels)
Template Required?	Yes (Sister Chromatid)	No
Cell Cycle Phase	S and G2 phases	All phases (Dominant in G1)
Key Proteins	MRN, RAD51, BRCA1/2, ATM	Ku70/80, DNA-PKcs, Artemis, Ligase IV

Disease	Defective Repair Pathway	Primary Symptoms
Xeroderma Pigmentosum	NER (Global Genome)	Extreme UV sensitivity, highly elevated skin cancer risk.
Cockayne Syndrome	TC-NER (Transcription Coupled)	Premature aging, neurodegeneration (no cancer increase).
Lynch Syndrome (HNPCC)	MMR (MSH2, MLH1)	Hereditary colorectal cancer.
Breast Cancer (Familial)	HR (BRCA1, BRCA2)	Loss of high-fidelity double-strand break repair.

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8. Research Update: Synthetic Lethality

The Genius of PARP Inhibitors in BRCA-Mutated Cancers

How do you kill a cancer cell without harming normal cells? Exploit its broken DNA repair system using a concept called Synthetic Lethality.

• The Background: Healthy cells repair single-strand breaks using the BER pathway, heavily reliant on the enzyme PARP. If a double-strand break occurs, healthy cells use HR (via BRCA proteins) to fix it.
• The Cancer Flaw: BRCA-mutated breast cancer cells have completely lost their HR pathway. However, they survive by relying heavily on PARP to fix daily single-strand damage.
• The Drug: Olaparib (a PARP inhibitor).
• The Result: When a patient takes Olaparib, PARP is disabled. Single-strand breaks accumulate and eventually collapse the replication fork, creating lethal double-strand breaks. Healthy cells survive this because they still have functional BRCA to perform HR. The cancer cells, lacking BRCA, cannot repair the double-strand breaks and undergo catastrophic apoptosis!

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9. Frequently Asked Questions (FAQs)

Why is Global Genome NER different from Transcription Coupled NER?

GG-NER constantly patrols the entire genome (including silent DNA) using the XPC protein to find damage. It is slow but thorough. TC-NER is a rapid-response unit. If an RNA polymerase is actively transcribing a vital gene and crashes into a DNA lesion, the polymerase stalls and immediately calls in CSA/CSB proteins to fix the damage so transcription can resume.

If SOS Repair (TLS) causes so many mutations, why did bacteria evolve to use it?

It is a matter of survival. Standard polymerases cannot physically read past bulky lesions; they stall, and the bacteria will die. The TLS polymerases (Pol IV, Pol V) are sloppy, but they allow the cell to bypass the damage and finish replication. For a bacterium, surviving with a few random mutations is always better than dying immediately.

How does Base Excision Repair (BER) differ from Nucleotide Excision Repair (NER)?

BER handles small, specific chemical alterations (like deamination or oxidation) by snipping out just the single damaged nitrogenous base first using a Glycosylase. NER handles massive, bulky damage (like UV dimers) that physically distort the double helix by chopping out a large 12-30 nucleotide chunk of the entire DNA backbone.

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10. Master Level Quiz

CSIR NET & GATE Level Master Quiz

Test your rapid recall. These 10 questions match the exact logic and phrasing of high-level life science examinations.

1. The enzyme O⁶-methylguanine-DNA methyltransferase (MGMT) is unique in the realm of DNA repair mechanisms. Which of the following best describes its classification and function?

A) It is a glycosylase that initiates BER by removing the methylated base. B) It is a suicide enzyme that performs Direct Repair by permanently accepting the methyl group onto its own active site. C) It acts as an endonuclease during Nucleotide Excision Repair. D) It is the primary ligase used in Non-Homologous End Joining.

[Correct Answer: B] MGMT repairs alkylation damage directly without breaking the DNA backbone. Because the methyl group is transferred covalently to the enzyme itself, the enzyme is irreversibly inactivated (consumed in the reaction), earning the title "suicide enzyme".

2. A patient presents with severe neurological defects, growth retardation, and premature aging, but lacks any increased susceptibility to skin cancer. Genetic testing reveals a defect in the CSA and CSB proteins. What specific DNA repair pathway is compromised?

A) Global Genome Nucleotide Excision Repair (GG-NER) B) Homologous Recombination (HR) C) Mismatch Repair (MMR) D) Transcription-Coupled Nucleotide Excision Repair (TC-NER)

[Correct Answer: D] These are the classic symptoms of Cockayne Syndrome. A defect in TC-NER means cells cannot repair damage in actively transcribing genes, leading to cell death and neurodegeneration. In contrast, XP (defective GG-NER) leads to massive skin cancer risk.

3. During the bacterial Mismatch Repair (MMR) pathway, the cell must differentiate between the original template strand and the newly synthesized strand containing the error. Which enzymatic activity provides the specific molecular tag used for this discrimination?

A) Photolyase activation. B) Dam methylase adding methyl groups to Adenines at GATC sequences. C) Polyadenylation of the 3-prime end. D) Ubiquitination of the old strand.

[Correct Answer: B] Immediately after replication, the DNA is hemimethylated. The old parental strand is fully methylated by Dam methylase, while the new strand is temporarily bare. The MutH endonuclease specifically recognizes this and cuts only the unmethylated (new) strand.

4. Which of the following correctly orders the recruitment of proteins during the Non-Homologous End Joining (NHEJ) repair of a double-strand break?

A) RAD51 → BRCA1 → Ligase I B) XPC → TFIIH → UvrD C) Ku70/Ku80 → DNA-PKcs → Artemis → Ligase IV D) MutS → MutL → MutH

[Correct Answer: C] NHEJ is initiated when the Ku70/Ku80 heterodimer binds the broken DNA ends. They recruit the kinase DNA-PKcs. Artemis trims the overhanging edges, and the complex of XRCC4/Ligase IV seals the break.

5. The spontaneous deamination of Cytosine yields Uracil in the DNA double helix. Which specific class of enzyme catalyzes the very first step of the repair process to correct this lesion?

A) DNA Ligase B) AP Endonuclease C) DNA Glycosylase D) Translesion Polymerase

[Correct Answer: C] This is the Base Excision Repair (BER) pathway. The first step is catalyzed by Uracil DNA Glycosylase, which breaks the N-glycosidic bond to remove the Uracil base, generating an empty AP site. AP Endonuclease then acts in the second step.

6. The clinical success of PARP inhibitors (e.g., Olaparib) in treating BRCA-mutated breast cancers relies heavily on the biological principle of Synthetic Lethality. How does this therapy selectively kill tumor cells?

A) It directly introduces double-strand breaks into the cancer genome. B) It prevents single-strand breaks from being repaired by BER, converting them into double-strand breaks during replication, which the tumor cannot repair due to its missing HR pathway. C) It activates the SOS response, causing error catastrophe. D) It permanently inactivates Topoisomerase I.

[Correct Answer: B] BRCA-mutated tumors lack Homologous Recombination (HR). By inhibiting PARP, you disable their backup repair system (BER). Normal cells survive because their HR pathway is intact, but the tumor cells suffer a "synthetic lethal" blow and undergo apoptosis.

7. Hereditary Non-Polyposis Colorectal Cancer (Lynch Syndrome) is primarily caused by germline mutations in genes encoding which of the following proteins?

A) BRCA1 and BRCA2 B) XPA and XPC C) MSH2 and MLH1 D) Ku70 and DNA-PKcs

[Correct Answer: C] Lynch syndrome is the classic disease associated with defective eukaryotic Mismatch Repair (MMR). MSH2 (MutS homolog) and MLH1 (MutL homolog) are the most frequently mutated genes in this syndrome.

8. What is the fundamental biophysical reason that Translesion Synthesis (TLS) polymerases (such as Pol Eta or Pol IV) are considered "error-prone"?

A) They lack 5-prime to 3-prime polymerase activity. B) They are easily degraded by proteases. C) They possess massive active sites to accommodate bulky lesions but completely lack 3-prime to 5-prime proofreading exonuclease activity. D) They utilize RNA instead of DNA nucleotides.

[Correct Answer: C] Normal polymerases have tight active sites and robust proofreading. TLS polymerases have wide open, "sloppy" active sites to physically bypass roadblocks like thymine dimers. Because they lack proofreading, any wrong base inserted during the bypass becomes a permanent mutation.

9. During Homologous Recombination (HR), which specific protein is responsible for coating the single-stranded DNA and mediating the critical "strand invasion" step into the intact sister chromatid?

A) Ku80 B) RAD51 C) MutS D) Photolyase

[Correct Answer: B] RAD51 is the central recombinase in eukaryotes (analogous to RecA in bacteria). It forms a nucleoprotein filament on the resected single-stranded DNA and searches the sister chromatid for homology, executing the strand invasion necessary for error-free repair.

10. In the bacterial SOS response, an accumulation of single-stranded DNA activates the RecA protein. What is the immediate downstream action of activated RecA that turns on the SOS genes?

A) It phosphorylates the origin of replication. B) It induces the auto-cleavage of the LexA repressor protein. C) It acts as an RNA polymerase to transcribe the genes. D) It directly seals double-strand breaks.

[Correct Answer: B] Under normal conditions, LexA represses the transcription of SOS repair genes. When massive DNA damage occurs, active RecA forces LexA to cleave itself. With the repressor destroyed, the SOS genes (including the error-prone TLS polymerases) are rapidly transcribed.