Flashcards in Replication, Recombination and Repair Deck (98)
What is recombination?
The shuffling of alleles between homologous chromosomes
What does recombination allow?
The selection of genes
A means of escape for favourable alleles and elimination for unfavourable alleles
Name 3 types of recombination
What is homolgous recombination?
Responsible for meiotic crossing over and integration of transferred DNA into bacterial genomes
When does homologous replication occur in eukaryotes?
At the 4 strand stage of meiosis
What is the Holliday model?
A method of homologous recombination between double stranded DNA molecules with identical or nearly identical sequences.
What is the Holliday model applicable to?
Molecules with limited regions of homology or a single molecule with two separate regions of homology recombining with itself.
Describe the process of eukaryotic recombination.
Each sister chromatid contains duplex DNA
Homologous chromosomes pair and form bivalents
Pairing extends along chromosomes in the process of synapsis
Synaptonemal complex formed with homologous chromosomes side by side
Chromosomes separate but held together at chiasmata
Chromosomes condense, chiasmata remain, all 4 chromatids visible
Describe the Holliday model
Exchange of polynucleotides between 2 duplexes forms heteroduplex
Stabilised by base pairing between transferred strands
Gaps sealed by DNA ligase
What is resolution?
Spliced or patched
Cleaving across branchpoint reverts structure back into two individual double stranded DNA molecules
What is patched resolution?
A non-recombinant resolution due to cleavage in the same direction
What is spliced resolution?
A recombinant resolution due to cleavage in opposite directions
How does a hetroduplex region occur?
Two DNA molecules line up
Single stranded nicks appear at equivalent position in helix
What is the Meselson-Radding modification
Single-stranded nick in one helix which invades unbroken strand in the homologous position
Invasion produces D-loop
Cleavage of displaced strand generates heteroduplex
Where does recombination occur?
At double stranded breaks and single stranded gaps
Where do DSBs and SSGs come from?
DNA-damaging agents and normal DNA replication
What is the purpose of recombination?
Necessary to re initiate DNA replication
Repairs DSBs and rectifies aborted replication process
What does an exonuclease do in recombination?
Forms a double stranded break in the recipient strand
Digests 5' side of nick forming 3' overhang
What does RecA do?
Coats the 3'-ssDNA overhang
What are recombination specific helicases?
RecBCD and RecQ
What does RecBCD do?
Loads RecA protein onto overhang
What does RecQ do?
Works in conjunction with RecJ exonuclease to function at single stranded gaps
What happens when RecA fails to assemble?
RecF, RecO and RecR assemble
What is the presynaptic filament?
The nucleoprotein complex formed by the assembly of contigous RecA protein on ssDNA which undergoes homology search and strand invasion
What does the RuvAB complex do?
Branch migrates the crossover point
What does the RuvC protein do?
Holliday junction specific endonuclease which resolves the Holliday junction
What does the RuvABC complex do?
Recognises and cleaves the Holliday junction
What is oriC?
DNA replication in E.coli is origin dependent and must start from oriC
What is PriA?
A DNA replication protein that is required to reinitiate the replisome so Pol III can extend the nucleoprotein complex