Chromosomes are not passed on intact, some part of maternaland paternal chromosome is exchanged through Homologous Recombination. Three models have been put forward to explain this phenomenon but only Holliday Model has been the most accepted.
Holliday Model of Homologous Recombination:
It was proposed by Robin Holliday in 1964. This model describes recombination of two homologous double stranded DNA molecules (with identical or nearly identical sequence) Its central feature is formation of a heteroduplex resulting from the exchange of polynucleotide segment between two homologous chromosome.
A) Two homologous chromosomes come closer and get properly aligned and form a single stranded break. This results in two aligned DNA duplex.
B) Now strands of each duplex, partly unwind and invade in opposite direction to form a cross between two DNA strand.
C) The hereroduplex is initially stabilized by base pairing between each transferred and intact polynucleotide of recipient molecule, this base pairing being possible because of sequence similarity between two molecules.
D) Subsequently the gaps are sealed by DNA ligase forming a Holliday structure/ Holliday Junction.
E) There occurs simultaneous unwinding and rewinding of duplex in such a way that there is no net change in the amount of base pairing but the position of cross over moves. This phenomenon is referred to as branch migration
F) The strands are able to rotate n same direction as the structure or junction is dynamic and forms a CHI structure.
G) Now this four stranded Holliday Junction is separated by symmetrical cuts in either of two directions:
- Horizontal Cut: If cut is made across the CHI horizontally a short segment of DNA Corresponding to the distance migrated by the branch is transferred. Flanking genes on parental strands however remains same i.e., only two strands exchange DNA segments.
- Vertical Cut: If cut is made across the CHI vertically, it results in reciprocal strand exchange such that end of one molecule is exchanged for the end of other molecule. This kind of DNA transfer is seen in crossing over. Flanking genes get exchanged in this cut. Recombination occurs in all 4 strands.