R10 Recombination

Question 1

What are the two roles of meiotic recombination ?

Answer 1

• Creates novel gene combinations, allowing for genetic diversity
• Ensures accurate segregation of homologues pairs in Meiosis I. Shown in the crossovers.

Question 2

What is the recombination frequency ?

Answer 2

number of recombinant offspring (genotype different to parents)/ total offspring x 100

if less than 50% then the genes are linked, if more than they are not linked.

Question 3

What is linkage mapping ?

Answer 3

tells us how far apart genes are on chromosomes using the RF value, closer they are the lower the RF value

Question 4

Homologous recombination is important for … ?

Answer 4

• Genome stability:
  o Repair of broken replication forks and DSBs
  o Accurate chromosome segregation in
  meiosis
• Genetic diversity:
  o Meiotic and mitotic recombination of
  different alleles
  o Horizontal gene transfer in bacteria

Question 5

How does homologous recombination (crossing over) happen ?

Answer 5

Initiated by a DSB

DSB resection: helicase and 5’->3’ nucleases work together to degrade the 5’ end to make a 3’ overhang

strand invasion: one 3’ overhang invades the homologous chromosome

repair synthesis: DNA pol extends broken strand by copying the sequence along the broken region

double holiday junction: broken sequence is restored, the donor and recipient DNA is linked by two holiday junctions (HJ)

HJ resolution: HJ resolvases, leaves us with non-crossover or cross over DNA.

Question 6

What is the role of Rad51?

Answer 6

A HR recombinase, filaments on the ssDNA, catalyses homology search and strand invasion and then removed from the dsDNA where pol extends the invading strand

BRAC2 gene promotes the Rad51 filaments

Has less affinity for dsDNA than ssDNA.

Question 7

What affects the resolution of the holiday junction ?

Answer 7

The angle of the strands will determine if non-crossing over or crossing over forms

Question 8

What is spo11?

Answer 8

endonuclease, expressed in meiosis to make the DSB to initiate crossing over

Question 9

When is centromeric and non-centromeric cohesin degraded ?

Answer 9

Centromeric cohesin breaks at meisosis II and non-centromeric cohesin breaks before meiosis

Question 10

What is the role of seperase in meiosis ?

Answer 10

Cleaves the non-centromeric and centromeric cohesin

Question 11

What are the recombination types ?

Answer 11

Homologous - based on the amount of homology (having genes in the same loci)

site-specific - limited homology

illegitimate - no homology

transposition - no homology, moving of transposable elements

Question 12

How can an illegitimate recombinant form ?

Answer 12

The HIV virus enters the t-cell, recognised by CD4 receptors, the viralRNA is converted to dsDNA and integrated into the human genome

intergrase cleaves the host DNA and connects the strands

Question 13

What is a phage ?

Answer 13

Viruses that infect bacteria

Question 14

What are some examples of a replication barrier ?

Answer 14

R-loops (highly transcribed genes), G-quartet, t-RNA genes, inactive origin, heterochromatin, telomeres and centromeres

Question 15

What is a fork called if it cannot pass a replication barrier formed by a lesion (SSB and corsslinking) ?

Answer 15

a stalled replication fork

Question 16

What happens to a stalled replication fork ?

Answer 16

A stalled fork can regress into a 4 way junction (Holiday junction) which is less likely to break as there is no ssDNA. Then it is resolved using HJ resolvases, looks exactly like a broken replication fork.

• The broken end is recognised and processed as a DSB, Rad51 nucleoprotein filament is formed on the resected 3’ end.
• The broken end invades unbroken chromatid
• Replication fork (RF) is re-established but the two-sister chromatid are connected by a nicked holiday junction. The repair of broken RF operates via homologous recombination.
• Then resolved by HJ resolvases
• The nicks are ligated by the replicative ligase, replication continues.

Question 17

Why are genes in the centre of two other genes less likely to be recombined ?

Answer 17

This would require 2 crossovers to cut it from the surrounding genes

Question 18

Why are linear chromosomes compatible for crossing over whereas circular aren’t ?

Answer 18

Circular would require a dicentric (2 DSB) recombination

Question 19

How has recombination evolved ?

Answer 19

1) Broken replication fork (prokaryotes)
2) Homologous recombination (prokaryotes) – genome stability
3) Meiosis (eukaryotes) – genetic diversity
4) Linear chromosomes (eukaryotes) – chromosome segregation in meiosis
5) Telomeres (eukaryotes) – stability of linear chromosomes
6) Downregulation of telomerase expression (eukaryotes) – cancer suppression in large animals and humans