Lecture 9

Question 1

What does homology-directed DSB repair use as a template

Answer 1

Homologous DNA

Sister chromatid late S-phase, G2

Homologous chromosome in G1

Question 2

What is presynapsis

Answer 2

Generation of ssDNA at site of DSB

Question 3

What is synapsis

Answer 3

Pairing of 1 single-strand end invading intact homologous duplex

Forms heteroduplex

Question 4

What is postsynapsis

Answer 4

Repair of damaged duplex by DNA synthesis using strands from intact duplex as template

Separation of 2 duplexes

Question 5

Explain process of homology directed repair of DSB

Answer 5

1. ssDNA (~50nts) formed by helicase and exonuclease at break site
2. 3’ end invades intact duplex and base pairs - forms heteroduplex and D-loop
3. Specific pairing in D-loop - 90-100% accurate over20-30bp
4. Newly synthesized strand releasedcfrom template - D-loop moves along template
5. DNA synthesis initiated from 3’OH of invading strand in D-loop - uses undamaged strand as template
6. DNA synthesis continues till new DNA pairs 3’ overhang