R10 Recombination Flashcards
What are the two roles of meiotic recombination ?
- Creates novel gene combinations, allowing for genetic diversity
- Ensures accurate segregation of homologues pairs in Meiosis I. Shown in the crossovers.
What is the recombination frequency ?
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.
What is linkage mapping ?
tells us how far apart genes are on chromosomes using the RF value, closer they are the lower the RF value
Homologous recombination is important for … ?
- 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
How does homologous recombination (crossing over) happen ?
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.
What is the role of Rad51?
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.
What affects the resolution of the holiday junction ?
The angle of the strands will determine if non-crossing over or crossing over forms
What is spo11?
endonuclease, expressed in meiosis to make the DSB to initiate crossing over
When is centromeric and non-centromeric cohesin degraded ?
Centromeric cohesin breaks at meisosis II and non-centromeric cohesin breaks before meiosis
What is the role of seperase in meiosis ?
Cleaves the non-centromeric and centromeric cohesin
What are the recombination types ?
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
How can an illegitimate recombinant form ?
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
What is a phage ?
Viruses that infect bacteria
What are some examples of a replication barrier ?
R-loops (highly transcribed genes), G-quartet, t-RNA genes, inactive origin, heterochromatin, telomeres and centromeres
What is a fork called if it cannot pass a replication barrier formed by a lesion (SSB and corsslinking) ?
a stalled replication fork
What happens to a stalled replication fork ?
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.
Why are genes in the centre of two other genes less likely to be recombined ?
This would require 2 crossovers to cut it from the surrounding genes
Why are linear chromosomes compatible for crossing over whereas circular aren’t ?
Circular would require a dicentric (2 DSB) recombination
How has recombination evolved ?
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
Which type of recombination is the most wide-spread among living organisms?
Illegitimate recombination
Site-specific recombination
Homologous recombination
None of the above
-Illegitimate recombination - specific only to some cells and viruses
-Site-specific recombination- specific only to some cells and viruses
-Homologous recombination - the correct answer as present in all known cells
