DNA is under constant attack — from UV radiation, reactive oxygen species, replication errors, and chemical mutagens. A single human cell sustains tens of thousands of lesions per day, so genome fidelity depends on a layered network of repair systems. These connect directly to cancer biology and hereditary syndromes, which is how the MCAT tests them.
This guide covers two categories: mechanisms that catch errors during replication, and pathways that repair lesions after the fact. The emphasis is recognizing the major pathways and the logic of each, not memorizing enzyme rosters.
Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.
Overview: The Logic of DNA Damage
Know the logicThe repair pathway is determined by the nature of the lesion. Use this table as a conceptual anchor for which pathway handles what.
| Type of damage | Common cause | Primary repair pathway |
|---|---|---|
| Misincorporated nucleotide | Replication error | Proofreading; mismatch repair (MMR) |
| Insertion/deletion loop | Replication slippage | Mismatch repair (MMR) |
| Pyrimidine (thymine) dimer | UV radiation | Nucleotide excision repair (NER); direct repair (photolyase) |
| Oxidized/deaminated base | Reactive oxygen species; spontaneous | Base excision repair (BER) |
| Abasic site (depurination) | Spontaneous hydrolysis | Base excision repair (BER) |
Repair During Replication
Proofreading by DNA Polymerase
Know the logicThe first line of defense is the 3′→5′ exonuclease (proofreading) activity built into the replicative polymerase (DNA Pol III in E. coli).
Polymerase synthesizes 5′→3′. When a wrong nucleotide is added, the mismatch distorts the active site, stalls the polymerase, and shifts the 3′ terminus into the exonuclease site. The bad nucleotide is excised (3′→5′, "backwards") and synthesis resumes. This co-replicational correction sharply lowers the error rate. (Note: primase, which lays down RNA primers, has no proofreading — another reason RNA primers are later replaced with DNA made by a proofreading polymerase.)
Quick check: DNA polymerase's proofreading exonuclease works in the 3′→5′ direction. Why must it be 3′→5′ rather than 5′→3′?
Answer: Because synthesis proceeds 5′→3′, the most recently added nucleotide — the one most likely to be a mismatch — sits at the 3′ end. A 3′→5′ exonuclease removes this terminal nucleotide; a 5′→3′ exonuclease would chew the wrong end.
Mismatch Repair (MMR)
Know the logicSome mismatches escape proofreading. Mismatch repair (MMR) is a post-replicative system that catches these. Together, proofreading and MMR keep the mutation rate extremely low.
The core challenge is strand discrimination — both strands look like legitimate DNA, so how does the cell know which one carries the error? It must identify the newly synthesized strand (which holds the error) versus the parental template (which holds the correct base). In bacteria, transient methylation acts as a timestamp: the parental strand is already methylated, the new strand briefly is not. MMR excises a stretch of the new strand around the mismatch; polymerase and ligase fill and seal using the parental strand as template.
Quick check: A wrong nucleotide escapes proofreading, and MMR is also defective. What is the downstream consequence?
Answer: With both layers gone, the mismatch is copied in the next replication round, becoming a permanent point mutation in a daughter cell. Accumulating mutations can drive tumorigenesis — which is why defective repair raises cancer risk.
Repair of Mutations
Must knowLesions arise from spontaneous events (deamination, depurination, tautomeric shifts that cause mispairing) and induced damage (UV, chemical mutagens, ionizing radiation). Once a lesion exists in genomic DNA, a different toolkit takes over. These pathways differ in how much DNA they remove and replace.
Direct Repair
Passage-levelThe simplest repair: chemically reverse the damage without removing any nucleotides. Only a few lesions can be fixed this way.
Photolyase (in prokaryotes, plants, and lower eukaryotes — but not humans) directly reverses pyrimidine (thymine) dimers, using visible-light energy (photoreactivation) to break the covalent linkage and restore the bases. (Direct removal of an alkyl group from a damaged base is a second example.) The unifying idea: the original base is restored in place, not excised.
Quick check: What distinguishes direct repair (like photolyase) from excision repair pathways?
Answer: Direct repair chemically reverses the lesion in place without removing nucleotides. Excision pathways (BER, NER) cut out the damaged base or a stretch of nucleotides and resynthesize using the complementary strand as template.
Base Excision Repair (BER)
Must knowWhen a single base is chemically damaged — oxidized, deaminated, or lost — but the backbone is intact, base excision repair removes and replaces that one base.
The steps:
- A DNA glycosylase recognizes the damaged base and cleaves the N-glycosidic bond, removing it (e.g., uracil-DNA glycosylase removes uracil from deaminated cytosine).
- This leaves an abasic (AP) site — sugar present, no base.
- An AP endonuclease nicks the backbone.
- DNA polymerase inserts the correct nucleotide using the complementary strand as template.
- DNA ligase seals the nick.
Why it matters: The most common spontaneous lesion is depurination (loss of A/G via N-glycosidic hydrolysis), creating AP sites that BER handles. Also, cytosine deamination produces uracil; if unrepaired, uracil pairs with adenine, converting a G:C pair to A:T after replication.
Quick check: Cytosine spontaneously deaminates to uracil. Why must uracil-DNA glycosylase remove this uracil before replication?
Answer: If uracil (which behaves like thymine) is present at replication, polymerase incorporates adenine opposite it, permanently converting the G:C pair to A:T (a transition mutation). Removing uracil via BER first lets the original cytosine be restored from the intact guanine on the complementary strand.
Nucleotide Excision Repair (NER)
Must knowWhen damage is bulky — distorting the whole double helix rather than one base — nucleotide excision repair cuts out an oligonucleotide patch (~25–30 nt) around the lesion and resynthesizes.
Key substrates: UV pyrimidine (cyclobutane) dimers, and large adducts from chemical carcinogens (e.g., tobacco smoke).
Know the logicRecognize the helix distortion → cut the strand on both sides of the lesion, releasing the short oligonucleotide → polymerase fills the gap from the undamaged strand → ligase seals. This "recognize → excise → fill in → seal" scheme is shared by BER and MMR; NER differs in removing a patch around a bulky lesion rather than a single base.
Clinical connection — Xeroderma Pigmentosum (XP): XP results from defective NER. Patients are extremely UV-sensitive with greatly elevated skin-cancer risk, because UV pyrimidine dimers go unrepaired and accumulate as mutations. This is the high-yield example of NER failure.
Quick check: A young patient has extreme sun sensitivity and multiple skin cancers from defective NER. Why does this specifically cause skin cancer?
Answer: Without NER, UV pyrimidine dimers in sun-exposed cells are not removed. They stall polymerase and become fixed mutations during replication — eventually hitting tumor suppressors and proto-oncogenes, driving skin cancer. This is the xeroderma pigmentosum phenotype.
A Note on Double-Strand Breaks
Passage-levelA double-strand break (DSB) — both strands severed at one point (e.g., ionizing radiation) — is the most dangerous lesion, because no intact complementary strand remains to template repair. Dedicated DSB pathways rejoin the ends; their mechanisms are out of scope here. Just recognize a DSB as a distinct, especially serious lesion.
Common Confusions & Tricks
1. Proofreading vs. MMR. Proofreading (3′→5′ exonuclease of the polymerase) is co-replicational — acts immediately as the wrong nucleotide is added. MMR is post-replicational — a separate layer acting after the fork passes.
2. BER vs. NER. BER targets small, non-distorting lesions (a single modified base). NER targets bulky, helix-distorting lesions (dimers, large adducts). The MCAT giveaway: UV light → thymine dimers → NER (or photolyase in non-humans).
3. Photolyase is NOT in humans. For a patient with UV damage, the relevant human pathway is NER. Photolyase is bacteria/plants/invertebrates only — frequently tested.
4. Uracil in DNA is wrong (almost always). Cytosine deamination → uracil in DNA → repaired by BER (uracil-DNA glycosylase). Uracil in RNA is normal; don't confuse the two.
5. Defective repair → elevated cancer risk. When a pathway fails, lesions become fixed mutations; if they hit tumor suppressors or proto-oncogenes, cancer risk rises. Xeroderma pigmentosum (defective NER → skin cancer) is the canonical example.
Key Takeaways
- Proofreading (3′→5′ exonuclease of the replicative polymerase) corrects mismatches during replication.
- Mismatch repair (MMR) catches remaining mismatches after the fork passes; relies on strand discrimination (recognizing the new strand) so the correct template base is kept. Proofreading + MMR keep the mutation rate extremely low.
- Direct repair reverses damage without excision: photolyase reverses UV dimers (bacteria/plants, NOT humans).
- Base excision repair (BER): glycosylase removes a single damaged base → AP site → AP endonuclease nicks → polymerase fills → ligase seals. Handles small, non-distorting lesions.
- Nucleotide excision repair (NER): removes a short oligonucleotide containing a bulky, helix-distorting lesion (UV dimers, adducts), then resynthesizes and ligates. Defect → xeroderma pigmentosum (XP): extreme UV sensitivity and skin cancer.
- Damage → pathway matching: UV/bulky adducts → NER; oxidized/deaminated/missing bases → BER; replication mismatches → proofreading then MMR.
- Failed repair → fixed mutations; mutations in tumor suppressors/proto-oncogenes raise cancer risk.