Recombination

Question 1

Briefly provide a description of crossing over.

Answer 1

During meiosis homologous chromosome will align, and at specific sites along the sequence, recombination will take place. These sites are called chiasmata.

Question 2

What is Holliday’s model for recombination?

Answer 2

Homologous chromosomes align, and one strand from each chromosome is broken or cut. The nick in each strand of DNA is repaired, but through binding to the nick of the other chromosome, creates a Holliday junction. The crossing over region, moves down the strand, via branch migration, forming a heteroduplex region. These region is then resolved, resulting in two complete and separate recombined chromosomes.

Question 3

What is the current model for recombination?

Answer 3

Is based on the Holliday model. A double-strand break occurs in both chromosomes, and some of the DNA surrounding these regions were degraded. Forming single-stranded DNA which can then base pair with sequences on the homologous chromosome. DNA synthesis and ligation from this point lead to a Holliday junction

Question 4

What are the main enzymes involved in recombination?

Answer 4

RecBCD - a protein complex which nicks, unwinds and degrades the DNA to generate single-stranded DNA.
RecA - catalyses base-pairing between the single-stranded DNA and sequences in the homolog.
RuvA and RuvB - Drive branch migration
RuvC - Resolves Holliday junctions by DNA cleavage.