Hom Recomb Flashcards
HR steps
- Resection; 2. Scanning/Searching; 3. Strand invasion & Extension; 4a. SDSA; 4b. double holliday junction formation; 5. resolving the HJ
describe step 1 resection
5’–>3’ exonuclease resects DNA to mke it ssDNA (needed for searching); extended 3’ end bound by homolgy-scanning protein (RecA in proks, Rad51 in euks)
describe step 2, scanning/searching
ssDNA is in a “search” conformation–> stretched out to be 1.5x longer than regular B DNA; scanning brings complex (nucleoprotein filament) randomly to local dsDNA to search for homology; scanning does not require ATP or helicase–works through “breathing” dsDNA–>bubbles naturally creates in AT rich seqs
successful hom search seq length depends on the organism– 25 bp in bac, 100 bp in yeast, 500-1000bp in humans–more cautious
ya
describe step 3, strand invasion and extension
if hom is found the ssDNA invades the dsDNA creating a d-loop; as pol extends the 3’ end of the break into the hom region the left side of the break with new synth’d DNA now contains a region complementary to the other 3’ end
describe step 4, synth dependent strand annealing
once there is complementarity on on either end of the break the hom is no longer needed; newly synth’d 3’ end pairs with resected 3’ end and extion from each fills in the gaps–>can use new template; ligation fills in the gaps
describe step 4b, double HJ formation
nah
how will HJs be fixed?
by resolvases; RuvA in bacteria
how are radiodurans radiation resistant?
undergo prolonged dessication; do not divide (but damage builds up)–>exposed to viable conditions, start repair–>
how does d. radiodurans fix their chromosomes
after high levels rad–>uses SDSA to extend overlapping fragments and then hom recomb to rebuild complete circular chromosomes
loss of heterozyg. from hom recomb (DSB DNA getting a different allele) can lead to cancers
ya
knock RecA out from D. radiodurans–> can’t repair genome (so sequence homology scanner)
ya
