13 | Recombination Flashcards
Homologous Recombination
Encompasses genetic exchange at sequences that are identical, or nearly so.
Recombinational DNA Repair…
Is, to a large extent, focused on repair of double-stranded breaks
DSB
(Double-Strand Break)
Usually the result of a break in the single-stranded template strand being encountered by the replication fork during duplication.
It is considered the most dangerous DNA lesion.
Recombinase
Catalyzing enzyme that enables the 3’ strand extension to invade the homologous chromosome “DNA strand invasion”
DNA Strand Invasion
The quintessential step of recombination where:
(1) Invading strand displaces one-strand on the homologous strand then;
(2) Base pairs with the other
SDSA
(Synthesis-dependent Strand Annealing)
Pathway of filling gaps with DNA Polymerase and sealing the strand with DNA ligase to complete repair
DSBR
(Double-strand Break Repair)
Pathway via use of Holliday junction to complete ligation repair while DNA strands still linked. Catalyzed by specialized nucleases and sealed by DNA ligase.
Crossover
Exchange of homologous DNA segments during cleavage and rejoining during DSBR
Branch migration
During DNA repair, after strand invasion is complete, enzymes move DNA branches along the strand with some base pairs interrupted and others left unaffected
Fork regression
When branch migration forces the replication fork backwards
Gap repair
A variant of DNA repair where the complimentary strand necessary for repair of the lesion is found in the undamaged arm of the replication fork
RecBCD Enzyme
Heterotrimeric enzyme of E.coli capable of both helicase (DNA unzipping) and nuclease (base-pairing) activity
Chi Site (or sequence)
A short stretch of DNA in the genome of a bacterium near which homologous recombination is more likely than expected to occur.
Chi sites serve as stimulators of DNA double-strand break repair in bacteria.
RecFOR
Collective of loading proteins for RecA
RecA
The prototype of a class of proteins, found in all organisms, whose recombinases promote the central step in the recombination reaction by aligning single-strand DNA with a homologous strand on a double-stranded DNA to allow strand invasion.
Homologs
Two copies of a chromosome
Meiotic Prophase I Recombination Stages (4)
(1) Leptotene
(2) Zygotene
(3) Pachytene
(4) Diffuse stage
Leptotene, what happens DURING?
Double-strand breaks
Leptotene to Zygotene, what happens during transition?
Strand invasion
Zygotene to Pachytene, what happens during transition?
Replicative extension
Pachytene, what happens during?
Double-crossover intermediates
Pachytene to Diffuse Stage, what happens during transition?
Crossover
Spo11
A protein closely related TO eukaryotic Topoisomerase II that catalyzes formation of Double-strand Breaks during initiation of eukaryotic recombination
Dmc1 & Rad51
The two RecA class recombinases that are the eukaryotic counterparts of RecA, sharing structural and sequential homology with RecA and they also form nucleoprotein extension filaments
