Homologous Recombinations

Genetics BIOL 2004 Kamran Virginia Tech

homologous recombination

exchange of genetic info between two homologous DNA moleculues

characteristics of homologous DNA molecules

• nearly identical DNA sequences

• similar gene order

Homologous recombination is the basis for which processes?

• crossing over

• gene conversion

• DNA repair

enzymes involved in recombination

• SSB proteins

• Recombination proteins

• DNA polymerase

• DNA ligase

• DNA gyrase/topoisomerase

What do SSB proteins do?

stabilize the ssDNA

What do recombination proteins do?

align homologous sequences

What does DNA polymerase do?

fills in missing DNA

What does DNA ligase do?

seals breaks in DNA backbone

What does DNA gyrase/topoisomerase do?

relieves torsional stress

Holliday Junction Model

• explains how recombination occurs through strand exchange between homologs

• central structure = Holliday junction

First step in the Holliday Junction Model

• homologous DNA moleculues align

• recombination enzymes cut DNA strands in the same location

Second step in the Holliday Junction Model

• free end of each broken strand crosses over and pairs with the complementary strand of the other DNA moleculue = heteroduplex DNA

• base pairing occurs between molecules

heteroduplex DNA

DNA double helix containing a strand from each homologous chromosome

Holliday Junction

• forms in step 3

• crossed DNA strands create four-stranded X shaped structure

Third step in the Holliday Junction Model

• Holliday Junction formed

• genetic info can be shared

Fourth step in the Holliday Junction Model

• branch migration = Holliday junction movels along the DNA molecule

branch migration

• movement of Holliday Junction

• crossover point slides along DNA, strands exchange pairing partners, heteroduplex DNA gets bigger

Fifth step of the Holliday Junction Model

• resolvases cut DNA to separate Holliday junction

What do resolvases do?

cut the DNA to separate the Holliday junction

What happens if the Holliday junction is cut on the horizontal plane?

product is NONcrossover recombinants

What happens if the Holliday Junction is cut on the vertical plane?

product is crossover recombinants

Double Stranded Break Model

• explains how recombination occurs

• modern model

Double Stranded Break

• two dsDNA homologs align

• one of the homologs gets cut through both strands

Resection of DNA ends

• enzymes remove nucleotides from 5’ end

• 3’ strand now overhangs

Strand Invasion

• overhanging 3’ end invades other homologous chromosome

• pairs with complementary sequence

  • forms heteroduplex DNA

DNA Synthesis

• DNA polymerase extends invading strand

Second Strand Capture

• second broken strand pairs with the displaced strand from the homologous chromosome

= two Holliday junctions

Branch Migration and Resolution

• two Holliday junctions move through branch migration

• cleaved and resolved like regular Holliday junctions

When does gene conversion happen?

when heteroduplex DNA contains mismatched base pairs

Why does gene conversion take place?

homologous chromosomes can carry different alleles

How does mismatched heteroduplex DNA form?

e.g. Aa

How are mismatched bases repaired?

• one allele converted to another

• changes allele ration (NONMENDELIAN segregation)