class 19 DNA recombination

examples of homologous sequences

very similar sequences to one another

• maternal and paternal copies of a chromosome

• sister chromatids after replication

what does homologous recomb. require

homologous sequences

what is homologous recombination

it is the part of meiosis 1 where homologous (maternal and paternal) chromosomes cross over

homologous recomb process/pathway

first DSB is induced during meiosis I (typically bad, so highly regulate) - ONLY ONE OF THE HOMOLOGS

5’ exocnuclease creates 3’ overhang.

then the longer strand can base pair with the homologous chromosome that’s still intact (rad 51 mediated strand invasion)

the displaced strand is captured and also used to fill in the second cut strand.

DNA polymerase extends overhangs, ligase joins strands.

this results in two crossing over points where the homologous chromosomes are covalently bound

Branch migration can be used to adjust the crossing over point, and the length of the heteroduplex region.

mismatch repair is used to deal with mismatches. selection of the “correct strand“ is random.

transposition

cut and paste type of recombination that can occur without a long region of homology

these sequences have repeats at the ends that allow fro recombination.

uses transposase enzymes - bind to the end repeat sequences and bends the DNA to connect the two ends together. this leaves behind the original DNA with a DSB where the transposon sequence was cut out.

• DSB reapired by NHEJ

then transposon can reintegrate somewhere else.