Holliday structure, the most important intermediate in
recombination is also known as Holliday junction or half chiasma. The first
molecular model of the genetic exchange that occurs between homologous
chromosomes during recombination was proposed by Robert Holliday. This Holliday model involves several steps.
First the homologous chromosomes are both nicked at
identical location. Then the strands from one side of the nicks invade the
homologous chromosomes, base pairing with no complementary strands. The
invading strands are next covalently linked to the original strands at the nick
site, forming what is called a Holliday
junction. The Holliday junction migrates away from the original nick site,
a process called branch migration. As it does so, the DNA strands are swapped
between the chromosomes. This creates heteroduplex regions on both chromosomes,
where man made or bases sequence differences between homologous chromosomes
result in a region of DNA with a low percentage of mismatched base pairs.
The length of branch migration may vary, but some point breaks are
made in the DNA that end migration and resolves the entangled DNA strands into
two separate chromosomes. There are two ways that breaks can be made; one
results in recombinant chromosomes.
If the crossed strands are cleaved by endonucleases, the after ligation
within the chromosomes there will be two the chromosomes there will be two non
recombinant chromosome with short heteroduplex regions. Alternatively if one
rotates on DNA helix 180 0,
a process called isomerisation, we can visualise how un crossed strands can be
broken. After the uncrossed strands are cleaved by an endonucleases, ligation
can produce recombinant chromosomes with short heteroduplex regions
