10 - recombination Flashcards
What are the 4 different types of recombination?
- homologous
- site-specific
- illegitimate
- transposition
What is transposition recombination?
when one transposon jumps from one location to the other or makes a copy and the copy jumps
what can recombination lead to?
- deletions
- insertions
- sequence duplication
- small changes - single base pair substitution
when does illegitimate recombination happen?
no homology
how does HIV use recombination?
- made from 2 RNA molecules
- recognised receptor on outside of cell and enters the cell and releases RNA
- RNA is reverse transcribed into double stranded DNA
- DNA enters nucleus via illegitimate recombination
-DNA is transcribed, more RNA is made - new viruses made
what is the process of HIV DNA entering the genome?
- viral ends resected to make 3’ overhang 2bp long
- host DNA strand cleaved 5pb apart
- viral strands ligates to cleaved ends by intergrase
- gaps are repaired by host enzymes
- can happen anywhere in the genome
what is an example onsite specific recombination?
phage lambda integrating into E. coli chromosome via site-specific recombination
What is the lytic cycle?
when a phage hijacks the cell to make more phages, cell the dies
What is the lysogenic cycle?
- when DNA is integrated into the host genome
What is a prophage?
bacterial cell that has a viral DNA in its genome
What is the process of the lytic cycle?
- phage attaches to host cell and injects DNA
- phage DNA circularises
- new phage DNA and proteins are made and viruses assembled
- cell lyses, releasing viruses
What is the process of the lysogenic cycle?
- phage attaches to host cell and injects DNA
- phage DNA circularises
- phage DNA integrates within bacterial chromosome
- cell is now a prophage
- bacteria reproduce
- sometimes bacterial will excise from chromosome and enter the lytic cycle
what is the details of site specific recombination?
- recombining molecules share a little homology
- ## either side of this is inverted repeats - recombination binding sites
Why is homologous recombination important?
genome stability
- repair of broken forks and DSBs
- accurate chromosome segregation in meiosis
genetic diversity
- meiotic and mitotic recombination of different alleles
- HGT in bacteria
Explain the process of homologous recombination
- DSB
- DSB resection- helicase and 5’-3’ exonuclease make 3’ overhangs
- strand invasion - 3’ overhang finds homologous chromosome and invades it via base pairing
- repair synthesis - DNA polymerase extends broken strand
- double holiday junction - repair is finished but connected by holiday junctions
- HJ resolution - can either have no cross over were 2 strands still contain original DNA or 2 stranded cross over
What is the role of RAD51 in DSB repair?
- Rad51 polymerises into a filament on single stranded DNA by BRCA2
- it finds the homology then invades the strand
- it is then removed so extension can happen
What is the role of BRCA2 on DSB repair?
ads the Rad51 onto ssDNA filaments
Explain single holiday junction resolution
- one cross over happens
What is Spo11?
- endonuclease expressed in meiosis only
- will break both strands in sister sister chromatids both never both sister chromatids
what does it mean if the recombination frequency (RF) is high?
the genes are far away on the chromosome
what does it mean if the recombination frequency (RF) is low?
the genes are close on the chromosome
What does it mean if the Rf is < 50%?
genes are linked
What does it mean if Rf = 50%?
genes are not linked
How are linkage maps built?
- highest Rf = furthest away
- smaller Rf = closer together
