Question 1

What are double-strand breaks and why do they pose a threat to cells?

Accepted Answer

Double-strand breaks are breaks in both strands of the DNA double helix, representing one of the most severe forms of DNA damage. These breaks threaten genome stability and can lead to cell death, mutations, or cancer if not repaired correctly. Cells have evolved specialized repair mechanisms to detect and fix these lesions before they compromise genetic integrity.

Question 2

How does homologous recombination repair double-strand breaks?

Accepted Answer

Homologous recombination uses a sister chromatid or homologous chromosome as a template to accurately repair double-strand breaks. The process involves strand invasion, where damaged DNA invades the template DNA, allowing precise copying of the missing sequence. This mechanism ensures high-fidelity repair and maintains genetic information accurately.

Question 3

What is non-homologous end joining and when is it used?

Accepted Answer

Non-homologous end joining (NHEJ) is a rapid repair pathway that directly ligates broken DNA ends without requiring a template. This mechanism is active throughout the cell cycle and is particularly important in G1 phase when sister chromatids are unavailable. While faster than homologous recombination, NHEJ can introduce small insertions or deletions at repair sites.

Question 4

What role do repair proteins play in detecting and processing double-strand breaks?

Accepted Answer

Repair proteins recognize double-strand breaks and initiate signaling cascades that halt cell cycle progression. Key proteins assess damage severity and recruit additional factors to process broken ends. These proteins coordinate the choice between repair pathways and ensure appropriate cellular responses to maintain genome stability.

Question 5

How does the cell cycle respond when double-strand breaks are detected?

Accepted Answer

When double-strand breaks occur, checkpoint proteins halt cell cycle progression, allowing time for repair before DNA replication or cell division proceeds. This pause prevents propagation of mutations and gives repair machinery opportunity to fix damage. If repair fails, cells may undergo apoptosis to prevent transmission of genetic errors.

Question 6

What happens when double-strand break repair goes wrong?

Accepted Answer

Failed or incorrect double-strand break repair can result in chromosomal rearrangements, deletions, or translocations that compromise genetic integrity. These errors may activate oncogenes or inactivate tumor suppressors, increasing cancer risk. Persistent unrepaired breaks can trigger cell death pathways, protecting organisms from propagating damaged cells.

Question 7

Are there different repair pathways available depending on the cell cycle phase?

Accepted Answer

Yes, repair pathway choice depends on cell cycle phase and DNA availability. During S and G2 phases, homologous recombination is preferred because sister chromatids provide accurate templates. In G1 phase, non-homologous end joining dominates since sister chromatids are absent. This temporal regulation ensures optimal repair accuracy based on available cellular resources.

Fixing Double-strand Breaks

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