DNA Recombination and Repair

Question 1

Define DNA recombination

Answer 1

breaking and rejoining DNA in new combinations

Question 2

What are the two kinds of recombination?

Answer 2

• homologous recombination
• site-specific recombination

Question 3

Site-specific recombination

Answer 3

• occurs specifically to the nucleotide within a particular sequence
• specific enzymes
• important in making antibodies

Question 4

Homologous recombination

Answer 4

• occur when you have two DNA molecules with almost the same sequence
• can occur anywhere within that length

Question 5

Where does homologous recombination occur?

Answer 5

• during meiosis -> chromosomes are homologous but not identical
• in bacteria (e.g. transducing phage

Question 6

What is the Holliday model for homologous genetic recombination

Answer 6

1. initial cleavage event
2. strand invasion
3. holliday junction
4. branch migration
5. resolution

Question 7

What is the Holliday junction

Answer 7

a four-way DNA junction

Question 8

Describe the ion-induced folding of the Holliday junction

Answer 8

adding metal ions (e.g. Mg) neutralises the charge of the backbone and causes the structure to fold

Question 9

Branch migration

Answer 9

• branch points migrate along the DNA in either direction
• if left to own devices it would be a random walk process

Question 10

Recombination mechanisms

Answer 10

• ds gap repair
• fork regression

Question 11

Double-strand break repair

Answer 11

• break in one chromatid is resected on one strand by CtlP/MRN complex and Exo1 nucleases
• generation of 3’ ss DNA
• bound and protected by RPA
• RPA is removed and replaced by RAD51 recombinase by BRCA2
• strand invasion (D-loop)
• repair synthesis

Question 12

Fork regression

Answer 12

fork effectively backs up

Question 13

Role of RD51

Answer 13

promotes homology searching, strand invasion, and pairing with sister chromatid allowing for DNA synthesis to be primed