While performing a Western blot, a graduate student spilled a small amount of the radiolabeled antibody on her left forearm. Although very little harm was done to the skin, the radiation did cause minor damage to the DNA of the exposed skin by severing covalent bonds between the nitrogenous bases and the deoxyribose sugar, leaving several apurinic/apyrimidinic sites. Damaged cells would most likely repair these sites by which of the following mechanisms?

Nonhomologous end joining repair

A 34-year-old woman comes to the physician for evaluation of a breast lump she noticed 2 days ago while showering. She has no history of major illness. Her mother died of ovarian cancer at age 38, and her sister was diagnosed with breast cancer at age 33. Examination shows a 1.5-cm, nontender, mobile mass in the upper outer quadrant of the left breast. Mammography shows pleomorphic calcifications. Biopsy of the mass shows invasive ductal carcinoma. The underlying cause of this patient's condition is most likely a mutation of a gene involved in which of the following cellular events?

Repair of double-stranded DNA breaks

Inhibition of programmed cell death

Regulation of intercellular adhesion

Activity of cytoplasmic tyrosine kinase

Arrest of cell cycle in G1 phase

A 54-year-old woman with breast cancer comes to the physician because of redness and pain in the right breast. She has been undergoing ionizing radiation therapy daily for the past 2 weeks as adjuvant treatment for her breast cancer. Physical examination shows erythema, edema, and superficial desquamation of the skin along the right breast at the site of radiation. Sensation to light touch is intact. Which of the following is the primary mechanism of DNA repair responsible for preventing radiation-induced damage to neighboring neurons?

Nonhomologous end joining repair

An 11-year-old boy is brought to the physician for the evaluation of frequent falling. His mother reports that the patient has had increased difficulty walking over the last few months and has refused to eat solid foods for the past 2 weeks. He has met all developmental milestones. The patient has had multiple ear infections since birth. His temperature is 37°C (98.6°F), pulse is 90/min, and blood pressure is 120/80 mm Hg. Examination shows foot inversion with hammertoes bilaterally. His gait is wide-based with irregular and uneven steps. Laboratory studies show a serum glucose concentration of 300 mg/dL. Further evaluation of this patient is most likely to show which of the following findings?

Expansion of GAA trinucleotide repeats

Mutation of type I collagen gene

An investigator isolates bacteria from a patient who presented with dysuria and urinary frequency. These bacteria grow rapidly in pink colonies on MacConkey agar. During replication of these bacteria, the DNA strands are unwound at the origin of replication, forming two Y-shaped replication forks that open in opposite directions. At each replication fork, daughter strands are synthesized from the template strands in a 5′ to 3′ direction. On one strand, the DNA is synthesized continuously; on the other strand, the DNA is synthesized in short segments. The investigator finds that three enzymes are directly involved in elongating the DNA of the lagging strand in these bacteria. One of these enzymes has an additional function that the others do not possess. Which of the following steps in DNA replication is unique to this enzyme?

Excision of nucleotides with 5'→3' exonuclease activity

Elongation of lagging strand in 5'→3' direction

Prevention of reannealing of the leading strand and the lagging strand

Creation of ribonucleotide primers

Proofreading for mismatched nucleotides

An investigator is studying the replication of bacterial DNA with modified nucleotides. After unwinding, the double-stranded DNA forms a Y-shaped replication fork that separates into two strands. At each of these strands, daughter strands are synthesized. One strand is continuously extended from the template strands in a 5′ to 3′ direction. Which of the following is exclusively associated with the strand being synthesized away from the replication fork?

Elongation in the 3'→5' direction

Synthesis of short RNA sequences

Homologous recombination for USMLE Step 1: High-Yield Biochemistry Lessons

HR Mechanism - The Sister-Act Rescue

⭐ Homologous recombination is a high-fidelity, error-free mechanism for repairing double-strand DNA breaks, primarily active in the S and G2 phases of the cell cycle when a sister chromatid is available as a template.

Repairs double-strand breaks (DSBs), primarily active in the S/G2 phase.
Uses the undamaged sister chromatid as a perfect template, ensuring a high-fidelity, error-free outcome.

Homologous Recombination vs. Non-Homologous End Joining

Key Proteins - The Repair Crew

Damage Sensors & Initiators:
- ATM (Ataxia-Telangiectasia Mutated): A master kinase that detects double-strand breaks (DSBs) and activates the repair cascade through phosphorylation.
- MRN Complex (MRE11-RAD50-NBS1): The first responder. Binds directly to broken DNA ends and helps recruit ATM.
Mediators & Loaders:
- BRCA1 & PALB2: Act as scaffolds, recruiting the core machinery. PALB2 links BRCA1 to BRCA2.
- BRCA2: Essential for loading the RAD51 recombinase onto the processed single-stranded DNA.
Core Recombinase:
- RAD51: The central enzyme that forms a filament on ssDNA, enabling the search for homology and subsequent strand invasion.

⭐ Exam Favorite: Mutations in BRCA1 and BRCA2 genes, key mediators of homologous recombination, are strongly associated with hereditary breast and ovarian cancers.

Clinical Correlations - When Good Repair Goes Bad

Synthetic lethality in BRCA-mutated cancers with PARPi

Hereditary Breast & Ovarian Cancer (HBOC):
- Mutations in BRCA1 & BRCA2 genes impair HR, conferring a high lifetime risk for breast, ovarian, prostate, and pancreatic cancers.
Fanconi Anemia:
- Rare genetic disorder with mutations in genes essential for repairing DNA crosslinks, involving the HR pathway.
- Presents with bone marrow failure, physical anomalies, and ↑ cancer risk (AML, squamous cell carcinoma).
Ataxia-Telangiectasia:
- Mutation in the ATM gene, a kinase that activates HR proteins (e.g., BRCA1).
- Features cerebellar ataxia, telangiectasias, immunodeficiency, and ↑ cancer predisposition (lymphoma, leukemia).

⭐ The concept of synthetic lethality is exploited by PARP inhibitors (e.g., Olaparib). These drugs are particularly effective against cancers with BRCA1/2 mutations because the cell has lost both its major DNA repair pathways, leading to targeted cell death.

HR vs. NHEJ - A Tale of Two Fixes

Homologous Recombination vs. Non-Homologous End Joining

Feature	Homologous Recombination (HR)	Non-Homologous End Joining (NHEJ)
Fidelity	High (Error-free)	Low (Error-prone)
Cell Cycle	S/G2 Phase	G1 Phase
Template	Sister Chromatid	None
Speed	Slow	Fast
Key Proteins	BRCA1/2, RAD51	Ku70/80, DNA-PKcs

High‑Yield Points - ⚡ Biggest Takeaways

Homologous Recombination (HR) is a high-fidelity, error-free DNA repair mechanism for double-strand breaks (DSBs).

It primarily occurs during the late S and G2 phases of the cell cycle, using an undamaged sister chromatid as a template.

Key proteins essential for HR include BRCA1 and BRCA2.

Mutations in BRCA1/2 are strongly linked to hereditary breast, ovarian, and prostate cancers.

PARP inhibitors are synthetically lethal to HR-deficient cells, a key cancer therapy principle.

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