Recombination

Question 1

How does recombination work in meiosis?

Answer 1

Homologous recombination occurs 1-3 times per chromosome in 50% of the gametes. There are hot- and cold spots.

Question 2

What does it mean that a species is heterochiasmic or achiasmic?

Answer 2

In heterochiasmic organisms, the different gametes’ of the two sexes undergo recombination at different rates.

In achiasmic organisms, only one of the sexes’ gametes undergo recombination.

Question 3

What’s a Holliday junction and why is that model in use rather than Synthesis dependent strand annealing (SDSA)?

Answer 3

A Holliday junction is the structure that is created in the strand invasion during homologous recombination. The holliday junction can be resolved in two ways, resulting in either a patch or a switch of genetic material.

SDSA is a possible theory, but it only explains SDSA, not the outcome where genetic material is completely exchanged between the strands.

Question 4

Explain the process of homologous recombination with emphasis on the enzymes used.

Answer 4

1. Recombination sites are determined by PRDM9 binding.
2. SPO11 catalyzes the dsDNA breaks required for homologous recombination.
3. PRDM9 adds methylations to its binding sites.
4. The dsDNA breaks are resected (chewed back) by 5’ exonucleases.
5. 3’ overhangs get bound by DMC1.
6. MRX-complex facilitates strand invasion.

Question 5

Describe Synthesis dependent strand annealing (SDSA)

Answer 5

The SDSA model came to be due to that dsDNA breaks in drosophila couldn’t be explained with the Holliday junction.

Instead of a double Holliday junction, an SDSA junction consist of a single cross-over, allowing for patch-reparation of dsDNA breaks.