The precise mechanism, which brings about breakage and reunion at specific points on the chromosome, is not fully understood.
Several theories have been put forward to account for the mechanism of crossing over. The following are some of them:
1. Classical theory:
This view proposed by Karl Sax in 1932 states that, chiasma is formed due to alternate opening out between sister and non sister chromatids. At the points of overlap breakage and reunion occur resulting in cross over.
2. Duplication theory:
When the chromosomes duplicate, longitudinal splitting occurs and the two non sister chromatids, which are close to each other may over lap, leading to break, exchange and reunion.
3. Breakage and exchange theory:
Stern and Hotta (1969) have described the involvement of two enzymes viz. endonuclease and ligase in the break and reunion respectively of the chromatids.
4. Breakage theory:
This theory proposed by Muller argues that there is a mechanical break in non sister chromatids first, leading to the exchange and then reunion. No cross over would have taken place before breakage.
5. Copy choice theory:
This theory was first proposed by belling (1932) – Belling opines that the mechanism of crossing over involves two steps- First step is the duplication of chromatids resulting in the formation of new genes.
Second step relates to the new connections between genes and chromosomes. When DNA synthesis takes place during pachytene, the portion of the new helix synthesized may copy a non sister helix rather than a sister helix thus bringing about recombination.
This is called the copy choice. The main objection to this theory is that it assumes DNA duplication after synapses, while it is known that, duplication would have taken place much earlier to synapses.
According to the copy choice theory, the recombination of the chromatids is entirely due to the DNA synthesized anew.
It believes that when the chromosomes duplicate, the new chromatid will use the chromatid of one parent as a template and then switches on to the chromatid of another parent; as a result the newly formed chromatid is a recombined product.