The Meselson-Radding model of
meiotic recombination explains how the regions of
heteroduplex DNA on both sides of the initial
recombination event are not necessarily the same length, providing an advantage over the
Holiday model of recombination.
-
Firstly, a nick is made in one strand of one of the chromosomes (in the following, each line represents a single strand of a double helix)
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
-
Secondly, a new strand is produced, displacing the original one on one side of the nick (the new strand is represented by c, but is identical to a)
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaaccccccccccc aaaaaaaaaaaaaaa
/
a (displaced a strand)
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
-
Thirdly, the displaced a strand invades the other chromosome (this requires RecA)
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaaccccccccccc aaaaaaaaaaaaaaa
/
/
bbbbbbbbbbbbaaaaaaaaaa bbbbbbbbbbbbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
(the strand that has been replaced in chromosome 2 is degraded)
-
Strand migration occurs
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaacccccccccccbbbbbbbbbbb aa
\ /
X
/ \
bbbbbbbbbbbbaaaaaaaaaaaaaaaaaaaaaa bb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
-
The junction may be resolved in one of two ways. One produces non-recombinant chromosomes with a heteroduplex region. The other produces recombinant chromosomes.
(Type 1 - non-recombinant)
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 1
aaaaaaaaaaaacccccccccccbbbbbbbbbbbaaaaa
bbbbbbbbbbbbaaaaaaaaaaaaaaaaaaaaaabbbbb
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
(Type 2 - recombinant)
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbbbb
Chromosome 1
aaaaaaaaaaaacccccccccccbbbbbbbbbbbbbbbb
bbbbbbbbbbbbaaaaaaaaaaaaaaaaaaaaaaaaaaa
Chromosome 2
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbaaaaa