The Holiday model was the first model to describe
meiotic recombination. It is quite simple, but does not describe all
observed results and so has been mostly
discarded in favour of newer models. However, it does serve as a good introduction to
recombination.
- The two chromosomes are next to each other
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
(Note that each chromosome is a double helix)
-
One strand from each chromosome is nicked and transfers to the other chromosome
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaa
\ /
X
/ \
bbbbbbbbbbbbbbbb bbbbbbbbbbbbbbbbbbbbbbbbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
-
The branch may migrate, creating a region of heteroduplex DNA
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaaaaaabbbbbbbbbb aaaaaaaaaaaaaaaaa
\ /
X
/ \
bbbbbbbbbbbbbbbbaaaaaaaaaa bbbbbbbbbbbbbbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
(that is, there is now a section where part of each double helix is originally from the adjacent double helix)
-
The Holiday junction is resolved in one of two ways. Both leave heteroduplex DNA, but only one is recombinant.
(Type 1 - heteroduplex but non-recombinant)
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaaaaaabbbbbbbbbbaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbaaaaaaaaaabbbbbbbbbbbbbbbbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
(Type 2 - heteroduplex and recombinant)
aaaaaaaaaaaaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbb
Chromosome 1
aaaaaaaaaaaaaaaabbbbbbbbbbbbbbbbbbbbbbbbbbbbb
bbbbbbbbbbbbbbbbaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbaaaaaaaaaaaaaaaaaa