Theories on the mechanism of crossing over

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                                     Theories on the mechanism of crossing over

 

There are many theories to explain the mechanism of crossing over

 1. Chiasma Type Theory:

This theory was proposed by Janssens. This theory suggested that prior to crossing over, the chromosomes of each bivalent geld plicated to form a tetrad. Crossing over occurs only between the non-sister chromatids of a tetrad. In the diplotene stage, the nor sister chromatids overlap with one another and form chiasma or point of contact. In the chiasma, the chromatids break and they rejoin with the mutual exchange of segments. The other two chromatids remain intact. This theory is known as chiasma type theory.



 2. The Breakage First Theory:

 This theory was proposed by Muller. According to this theory the non-sister chromatids of homologous chromosomes first break off without crossing over. The broken segments rejoin to form new combinations.


 3. The Contact First Theory:

 This theory was proposed by Serebrovsky. According to this theory, the non-sister chromatids first touch and cross each other. The breakage occurs at the points of contact of the chromatids. The broken segments rejoin to form new combinations.

  4. Strain or Torsion Theory:

 This theory was proposed by Darlington. According to this theory, the homologous chromosomes (bivalent) are relationally coiled around each other in a specific fashion. The chromosomes duplicate and the two sister chromatids in each chromosome are also coiled one around the other. When the chromosomes start separating their relational coiling unravel in on direction and the sister chromatids unravel in the opposite direction The repulsion causes torsion, which exerts pressure or strain on the weak non-sister chromatids. The non-sister chromatids break at the point of contact and broken ends rejoin and form chiasma.


 5. Differential Contraction Theory:

 Huskin suggested that tension set up by differential contraction after pairing may cause breaks at overlap. Separation starts from the centromere towards the ends like a zip. This separation is called terminalization. This leads to the separation of homologous chromosomes.

 Based upon the number of chiasmata formed, crossing be classified into three types. They are as follows:

1. Single Crossing Over:

 In this type, only one chiasma is formed. Only one chromatid of each chromosome is involved in single crossing over.

 2. Double Crossing Over:

 In this type, two chiasmata are formed. The chiasmata may formed between the same chromatids or between different chromatids. Thus, two or three or all the four chromatids of the homologous chromosomes may be involved in the process of double crossing over

 a. Two strand double crossing over: 

In this type, the two non-sister chromatids are involved at both the crossing over points Here two chromatids are non-cross overs and maintain the parent combination of genes.

 b. Three strand double crossing overs: 

In this type, the chromatids are involved in the formation of chiasmata. Here three chromatids exchange their parts and one chromatid is non-cross 

c. Four strand double crossing overs: 

In this type, the four chromatids are involved in the formation of chiasmata.

 3. Multiple Crossing Over:

 In this type, more than two chiasmata are formed. Multiple crossing over is very rare because of interference in crossing over.


 Reference :

Genetics - R.PMeyan - Saras publication , Nagarcovil.

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