Recombination Flashcards
What is recombination?
homologous chromosomes bind and may or may not exchange large portions of DNA
Only __ out of 4 chromatids can be recombinant
2
Recombination occurs only between…
homologous regions and is highly accurate
Recombination happens during?
Prophase 1 of meiosis
What are the 3 stages of recombination?
Synapse, Chiasmata (holiday junctions), and resolution (recombination possible outcomes)
synapse
Homologous chromosomes interact using proteins
Chiasmata (holiday junctions)
point of exchange of genetic information between homologous chromosomes
Resolution has what two possible outcomes?
Gene conversion and crossing-over
Gene conversion
small amount of DNA exchanged corresponding to holiday junction
(mismatches can be repaired to resemble 1 allele or other homologous to match current chromosome or other
Crossing-over
large amounts of DNA exchanged (everything following chiasmata site)
Know that Holiday Junction forms what shape
Hexagon
Step 1: Double-strand break formation
Dmc1 breaks phosphodiester bonds of both strands of one chromatid
Dmc1
engages in strand exchange activity
Step 2: resection
5’ ends of each broken strand are degraded to create 3’ single-stranded traits
Step 3: first strand invasion
One single-strand tail invades a homologous (non-sister chromatid and forms stable heteroduplex displacement loop (D-loop) from invaded chromatin is stabilized by single-strand binding protein
Step 4: formation of double holiday junctions
D-loop enlarged by new DNA synthesis at 3’ end of invading strand
New DNA synthesis fills in gap in bottom strand using displaced strand as template
Step 5: branch migration
Heteroduplex region of both DNA molecules is lengthened
Step 6: holiday intermediate
we have rotation, such that holiday junction is formed
Step 7: alternative resolutions
cutting of holiday junctions by endonucleases is equally likely in either vertical or horizontal plane
cutting of holiday junctions by endonucleases in either vertical or horizontal plane is equally likely
Step 8: probability of crossover occurring
Non-crossover occurs when both junctions are resolved in the same plane
crossover occurs with the two junctions are resolved in different planes
Non-crossover events occur when cuts are
two vertical or two horizontal
crossover events occur when cuts are
one vertical, one horizontal
Heteroduplex
region of DNA between breakpoints
one strand is maternal and the other is paternal
strands can have mismatches
how to eliminate mismatches?
DNA repair enzymes eliminate mismatches
Gene conversion v Crossing Over
Gene Conversion
Just area of heteroduplex formation exchanged
Mismatch repair can make it resemble gene original gene or other homologous gene
Crossing Over
large portions of DNA exchanged
everything after chismata site
Purpose of recombination
Genetic diversity of gametes
keep homologous chromosomes together and sorted properly during meiosis 1
prevents non disjunctions during meiosis 1
