Homologous recombination can repair a break accurately with no partner sequence.

Why is a double-strand break the most dangerous DNA lesion?

• Only one backbone is nicked
• Both backbones are severed with no intact template in the same duplex
• The bases are merely mispaired

What does the resected end do in homologous recombination?

• It invades the partner and copies the missing sequence
• It trims both ends and glues them directly
• It degrades the whole chromosome arm

Why does homologous recombination dominate in S and G2?

• The enzymes only wake up in those phases
• Sisters are present then to serve as partners
• Breaks never happen outside S and G2

How do the two repair routes compare in accuracy?

• End joining is accurate and recombination is sloppy
• Both routes always leave large deletions
• Recombination is accurate while end joining often leaves small indels

A linkage map shows 1 percent recombinants between two markers. What does that mean?

• One crossover per hundred meioses in that interval
• One gene per hundred base pairs
• One hundred crossovers per single meiosis

A break happens in G1 with no sister chromatid nearby. Which route repairs it, and at what cost?

• Recombination, with perfect restoration guaranteed
• End joining, quick but possibly scarred
• Neither route; the cell must divide instead

Jo claims end joining is the accurate route because it is faster. What is wrong?

• She is right; faster enzymes make fewer mistakes
• Recombination is slower and therefore sloppier
• Speed is not accuracy; joining has no template, so indels remain