Step 1 topics → Biochemistry → DNA Repair Pathways

DNA Repair Pathways

USMLE Step 1 trap: Confuses NER and BER by swapping the lesion types each pathway handles. Nucleotide excision repair (NER) removes bulky helix-distorting lesions; BER removes small, non-helix-distorting base modifications.

DNA repair pathways are one of the highest-yield molecular biology topics on USMLE Step 1, and they're tested in three distinct ways: pure mechanism recall (which pathway fixes which lesion), disease correlation (which clinical syndrome maps to which broken pathway), and clinical vignette interpretation (a patient presents with a pattern of cancers or UV sensitivity — you have to work backward to the defect). Students consistently confuse NER with BER — both involve excision, but NER handles bulky helix-distorting lesions like UV-induced thymine dimers, while BER handles small chemical modifications — and confuse which DSB repair pathway is active in G1 versus S/G2. The five pathways you need cold are NER, BER, MMR, HR, and NHEJ. Each has a signature lesion type, a signature disease when broken, and a specific cell-cycle context for the DSB pathways.

The exam loves to exploit the fact that students blur NER and BER together because they sound similar and both involve excision. They are not interchangeable. The lesion type is the key discriminator: bulky, helix-distorting lesions (thymine dimers from UV, chemical adducts) go to NER; small, chemically modified bases (oxidized, alkylated, deaminated) go to BER. USMLE Step 1 will describe a UV-sensitive patient with skin cancers and expect you to know this is NER, not BER. Similarly, a patient with early-onset colorectal cancer clustering in a family should trigger MMR/Lynch syndrome, not any of the other pathways.

The cell-cycle context for double-strand break repair is another classic trap. HR requires a sister chromatid as a template, so it only works in S and G2. NHEJ doesn't need a template — it just ligates broken ends — making it the default in G1. This distinction is frequently tested on USMLE Step 1 in both direct mechanism questions and in passages about radiation sensitivity or cancer predisposition syndromes.

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Common misconceptions

Common mistake
Wrong: Base excision repair (BER) removes bulky helix-distorting lesions like thymine dimers.
Right: Nucleotide excision repair (NER) removes bulky helix-distorting lesions; BER removes small, non-helix-distorting base modifications.
BER handles small, subtle base modifications — oxidation, alkylation, deamination — where the overall helix structure is preserved. NER exists specifically because some lesions (like UV-induced thymine dimers and bulky chemical adducts) physically distort the DNA helix in a way that BER enzymes cannot handle. Think of it this way: BER is a scalpel for tiny chemical mistakes; NER is a bigger excision system triggered by structural distortion of the helix. Xeroderma pigmentosum is the proof: UV damage means thymine dimers, thymine dimers are helix-distorting, so the broken pathway is NER.
Common mistake
Wrong: Xeroderma pigmentosum results from a defect in mismatch repair.
Right: Xeroderma pigmentosum results from a defect in nucleotide excision repair, leaving UV-induced thymine dimers unrepaired.
Mismatch repair catches replication errors — bases that are simply mismatched but chemically normal. Xeroderma pigmentosum patients can't repair thymine dimers, which are covalent bonds between adjacent pyrimidines induced by UV radiation. That structural lesion requires NER, not MMR. The clinical giveaway is extreme UV sensitivity and skin cancers from sun exposure — this phenotype has nothing to do with replication fidelity, which is the domain of MMR.
Common mistake
Wrong: Homologous recombination is the primary repair pathway for double-strand breaks in G1 phase.
Right: NHEJ is the primary DSB repair pathway in G1 (no sister chromatid available); HR is used in S/G2 when a sister chromatid template is present.
HR is high-fidelity because it copies off an identical sister chromatid template — but a sister chromatid only exists after DNA replication, meaning HR is only available in S and G2 phases. In G1, the cell has no sister chromatid to use as a template. NHEJ fills that gap by directly ligating the two broken ends, no template required, though at the cost of occasional small insertions or deletions. So NHEJ dominates in G1 and early S; HR dominates in late S and G2. BRCA1/2 mutations impair HR, which is why BRCA-mutant cells are especially vulnerable to agents that cause DSBs.
Common mistake
Gap: Missing the link between Lynch syndrome, mismatch repair deficiency, and microsatellite instability
Lynch syndrome (hereditary nonpolyposis colorectal cancer) is caused by defective mismatch repair genes (MLH1, MSH2, etc.), leading to microsatellite instability.
Lynch syndrome (HNPCC) is caused by germline mutations in mismatch repair genes — most commonly MLH1 and MSH2. MMR normally corrects single-base mismatches and small insertion/deletion loops that occur during replication, particularly in repetitive sequences called microsatellites. When MMR is defective, errors in microsatellite sequences accumulate, producing microsatellite instability (MSI) — a molecular fingerprint of Lynch syndrome that's testable in isolation. The clinical result is early-onset colorectal cancer (and other cancers: endometrial, ovarian, gastric) in an autosomal dominant family pedigree. Know the pathway → gene → molecular marker → cancer spectrum chain.
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What the exam tests

  1. Given a specific type of DNA damage (e.g., thymine dimer, oxidized base, replication mismatch, double-strand break), identify which repair pathway — NER, BER, MMR, HR, or NHEJ — is responsible for correcting it.
  2. Given a disease or cancer predisposition syndrome (xeroderma pigmentosum, Lynch syndrome/HNPCC, BRCA-associated breast/ovarian cancer), identify the defective DNA repair pathway and its downstream consequence (e.g., microsatellite instability).
  3. Given a clinical vignette describing a patient's phenotype (UV sensitivity, specific cancer spectrum, family history pattern, age of onset), deduce which DNA repair pathway is defective and name the relevant genes or mechanism.

Can you avoid these mistakes?

A 25-year-old develops numerous squamous cell carcinomas on sun-exposed skin. His cells show an inability to remove pyrimidine dimers after UV exposure. Which DNA repair pathway is defective, and what is the diagnosis?
A 45-year-old woman is diagnosed with colorectal cancer. Her tumor cells show instability at multiple short tandem repeat sequences throughout the genome. Mutations in which genes and which repair pathway most likely explain this finding?
A researcher treats cells with a drug that causes double-strand breaks, then examines which repair pathway is used in cells arrested in G1 versus cells in G2. What result should the researcher expect, and why?
A patient with BRCA2 mutation develops breast cancer. BRCA2 normally participates in which DNA repair pathway, what type of lesion does that pathway fix, and in which cell cycle phase(s) is that pathway active?

Related topics

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