Qiao 3

Question 0

What are the steps, in general terms, of homologous recombination?

Answer 0

1. A double-stranded break (DSB) is made in one of the two homologous chromosomes.
2. Exonuclease trims the 5’ ends at the DSB, leaving 3’ ssDNA overhangs.
3. Strand invasion between the two chromosomes.
4. DNA synthesis (5’ to 3’) until strands are joined, forming two Holliday junctions.
5. Strands are cut, creating chromosomes with or without crossover depending on cut location.

Question 1

What are the two types of recombination?

Answer 1

Homologous: during meiosis, between nearly identical regions of DNA, gene order is maintained
Site-specific: mobile genetic elements transposed between non-homologous sites, gene order can be altered and information can be added or deleted

Question 2

What are two classic examples of site-specific recombination?

Answer 2

• Mating-type switching in yeast

- Immunoglobulin diversity or V(D)J recombination

Question 3

Two (of several) DNA damage recognition proteins (“sensors”) are ___ and ____.

Answer 3

ATR (ataxia telangiectasia and Rad3-related protein) is a PK activated by persistent ssDNA. Activated ATR phosphorylates Chk1, initiating signal cascade that arrests cell cycle.
ATM (ataxia telangiectasia mutated) is a PK that is recruited by DSBs and activates the DNA damage checkpoint.

Question 4

What is the primary ‘effector’ of checkpoint activation (also known as the genome guardian)?

Answer 4

p53

Question 5

Germline mutations of the p53 tumor suppressor gene result in what syndrome?

Answer 5

Li–Fraumeni syndrome, extremely rare autosomal dominant hereditary disorder that predisposes patients to cancer (25x more likely to have cancer by age 50)

Question 6

p53 regulates cell growth through which three pathways? What are the four major triggers of p53-mediated growth inhibition?

Answer 6

Hyperproliferative signals, DNA damage, shortening telomeres, hypoxia; cell cycle arrest, senescence, apoptosis