Important Theories That Can Explain the Mechanism of Crossing Over are listed below:
1. Janssen’s partial chiasma type theory.
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2. Belling’s copy choice theory.
3. Darlington’s breakage and reunion theory.
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1. Breakage and reunion theory:
This theory is based on the assumptions that:
1. Prior to crossing over each chromosome of each bivalent get duplicated to form tetrad.
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2. Crossing over occurs only between non-sister chromatids.
3. Crossing over involves the mechanical breaks in non-sister chromatids due to twisting around each other and reunion or recombination of chromatids take place.
According to this theory first of all, chromatids break and then form chiasmata. Crossing over does not produce chiasmata but it is caused by chiasmata.
2. Copy choice theory:
This theory was proposed by Belling in 1933. According to this theory, the paired chromosomes duplicate their genes before the fibres join them to form strands.
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When the chromosomes are twisted around each other, reciprocal exchange of the chromatids take place during pachytene or just before.
There may be some recombination during the period of DNA synthesis affecting short, unpaired segments of the chromosomes.
A small part of new DNA helix being synthesized may copy a non-sister helix rather than a sister helix to produce recombination in a very short period.
There are two main objections:
1. Only two chromatids out of four involved in crossing over thus newly formed chromatids would be altered by recombination.
2. Duplication should occur during late meiotic prophase but now it is clear that DNA duplication occurs even before synapsis.
Hence, these theories assume that crossing over occurs in the interphase rather than pachytene.
3. Partial chiasma type theory:
According to this theory breaks occur only in two chromatids out of the four at the pachytene stage. These four again rejoined and the chiasmata are formed i.e. chiasmata is the result of crossing over.
