Master Molecular Biology
for USMLE Step 1

What the USMLE Step 1 Tests in Molecular Biology

USMLE Step 1 Molecular Biology tests the ability to apply DNA replication, transcription, translation, and repair mechanisms to clinical scenarios. Candidates must diagnose genetic disorders from pedigree patterns and mutation types (e.g., nonsense vs. frameshift), interpret Southern blot and PCR results, and link defective enzymes to diseases like xeroderma pigmentosum (NER defect) or ataxia telangiectasia (ATM kinase). Questions often present a patient with cancer predisposition (e.g., BRCA1/2 in breast/ovarian cancer), a child with developmental delay and a trinucleotide repeat disorder (e.g., fragile X syndrome with >200 CGG repeats), or drug resistance (e.g., imatinib failure in CML due to BCR-ABL kinase domain mutation). Candidates must select confirmatory tests (e.g., karyotype for Philadelphia chromosome, FISH for HER2/neu amplification) and predict response to targeted therapies like trastuzumab or vemurafenib. Emphasis is on integrating molecular mechanisms with histopathology and pharmacology.

High-Yield Concepts

• DNA Repair Defects and Cancer Syndromes: Xeroderma pigmentosum: defect in nucleotide excision repair (NER) of UV-induced thymine dimers → >1000-fold increased risk of skin cancers (BCC, SCC, melanoma). First-line: lifelong sun avoidance and regular dermatology surveillance. Hereditary non-polyposis colorectal cancer (HNPCC/Lynch syndrome): defect in mismatch repair (MLH1, MSH2) → microsatellite instability; Amsterdam II criteria (≥3 relatives with colorectal/endometrial cancer across 2 generations, 1 <50 years).
• Trinucleotide Repeat Disorders: Anticipation and Testing: Fragile X syndrome: CGG repeat >200 in FMR1 gene (full mutation) → intellectual disability, long face, macroorchidism. Premutation (55-200 repeats) → risk of fragile X-associated tremor/ataxia syndrome (FXTAS) in males. Huntington disease: CAG repeat >40 in HTT gene → autosomal dominant, chorea, dementia; age of onset inversely correlates with repeat length. Confirm via PCR and Southern blot.
• Oncogenes and Targeted Therapy Resistance: BCR-ABL fusion (Philadelphia chromosome) in CML: imatinib (tyrosine kinase inhibitor) is first-line; resistance often due to T315I mutation in ABL kinase domain → switch to ponatinib or allogeneic stem cell transplant. BRAF V600E mutation in melanoma: vemurafenib (BRAF inhibitor) → response rate ~50%; resistance via MAPK pathway reactivation (e.g., MEK mutations).
• Tumor Suppressor Genes: Two-Hit Hypothesis: Retinoblastoma: RB1 gene on chromosome 13q14; hereditary form (first hit germline, second hit somatic) → bilateral, early onset; sporadic form (both hits somatic) → unilateral. Li-Fraumeni syndrome: germline TP53 mutation → increased risk of sarcomas, breast cancer, brain tumours, adrenal carcinoma. Screening: annual whole-body MRI and breast MRI from age 20.
• Epigenetics: Imprinting and Uniparental Disomy: Prader-Willi syndrome: loss of paternal 15q11-13 (deletion or maternal uniparental disomy) → hypotonia, obesity, hyperphagia, intellectual disability. Angelman syndrome: loss of maternal 15q11-13 (deletion or paternal uniparental disomy) → severe intellectual disability, ataxia, inappropriate laughter, seizures. Diagnosis: methylation-specific PCR or FISH.
• Pharmacogenomics: Thiopurine Methyltransferase (TPMT): TPMT deficiency (autosomal recessive; ~0.3% of population homozygous) → severe myelosuppression with standard doses of azathioprine or 6-mercaptopurine (used in IBD, ALL). Pre-treatment genotyping or enzyme activity measurement: normal (high activity) → standard dose; intermediate → 50% dose reduction; deficient → avoid or use 10% dose. Alternative: allopurinol co-therapy only with dose adjustment.
• DNA Replication Inhibitors and Cell Cycle Checkpoints: Hydroxyurea: inhibits ribonucleotide reductase → blocks DNA synthesis; used in sickle cell disease (increases HbF) and myeloproliferative neoplasms. Cisplatin: forms intrastrand DNA crosslinks → triggers G2/M arrest; resistance via increased nucleotide excision repair or loss of p53 function. Checkpoint kinase inhibitors (e.g., prexasertib) in clinical trials for BRCA-mutant cancers.
• CRISPR-Cas9 and Gene Therapy Applications: CRISPR-Cas9: uses guide RNA to target Cas9 nuclease to specific DNA sequence, creating double-strand breaks repaired by NHEJ (gene knockout) or HDR (gene correction). Approved therapy: exagamglogene autotemcel (Casgevy) for sickle cell disease and β-thalassaemia (reactivates fetal haemoglobin via BCL11A disruption). Ethical considerations: off-target effects, germline editing moratorium.

Common Traps in Molecular Biology Questions

• Confusing xeroderma pigmentosum (NER defect) with ataxia telangiectasia (ATM kinase defect, radiosensitivity, cerebellar ataxia, elevated AFP).
• Thinking that all trinucleotide repeat disorders show anticipation—Huntington and fragile X do, but Friedreich ataxia (GAA repeat) does not.
• Assuming that a positive family history of breast cancer always means BRCA1/2—consider TP53 (Li-Fraumeni), PTEN (Cowden), or CHEK2.
• Mistaking uniparental disomy for imprinting: Prader-Willi and Angelman are due to imprinting defects, but uniparental disomy is one mechanism causing loss of expression from one parental allele.
• Forgetting that imatinib resistance in CML is often due to BCR-ABL kinase domain mutations, not BCR-ABL amplification—so FISH for BCR-ABL may remain positive, but sequencing is needed.
• Overlooking that TPMT testing is recommended before starting azathioprine in inflammatory bowel disease and rheumatology, not just in oncology.

How to Revise Molecular Biology for the USMLE Step 1

Prioritise understanding of mutation types (nonsense, missense, frameshift, splice site) and their functional consequences (loss-of-function vs. gain-of-function). Focus on DNA repair pathways (NER, BER, MMR, HR, NHEJ) and their associated clinical syndromes—you will see vignettes of patients with sun sensitivity, cancer predisposition, or neurological deficits. Practise interpreting pedigree patterns (autosomal dominant, recessive, X-linked, mitochondrial) and calculating recurrence risks. For pharmacogenomics, know the key enzymes (CYP2D6, TPMT, UGT1A1, G6PD) and their clinical impacts. Questions often integrate molecular biology with histology (e.g., microsatellite instability in colorectal cancer) or pharmacology (e.g., mechanism of action of PARP inhibitors in BRCA-mutant tumours). Use practice items from NBME subject exams and UWorld; focus on questions that require linking a molecular defect to a specific drug or management strategy.

Practise it: MedLumen has 30 Molecular Biology questions for the USMLE Step 1, each with a full explanation and references.