Crossing over, Mechanism of crossing over, Types of crossing over- on the basis no. of chiasma formation

Crossing over-
Complete linkage is very rarely found in nature. In most of the organism, it has been seen that the offspring formed from the parents are also produced some offsprings having characters different from their parents.

Crossing over is a natural genetic recombination process which occurs during the pachytene stage of the prophase I of meiosis.

The process of separation of linked genes in this way was called crossing by MORGAN.

According to MORGAN- The exchange of segment between non - sister chromatids of homologous chromosomes is called crossing over.

Mechanism of crossing over-
Mechanism of crossing over completed in following steps:-

1) Synapsis-  The process of crossing over occurs before the formation of gametes during meiotic division.
During leptotene of prophase I of meiotic division, the homologous chromosome begins to become distinct and during its zygotene stage of prophase I, the synapsis or paring between the two homologous chromosome is initiated.

This pair of homologous chromosome are called bivalents.

2) Duplication of chromosomes-
The synapsis is followed by duplication of chromosomes.
During duplication each homologous chromosome of bivalent splits longitudinally and form two identical sister chromatids which remain attached to each other at centromere.

At this stage, each bivalent contains four chromatids so it is called tetrad.

3) Crossing over or chiasma formation- During pachytene of prophase-I non - sister chromatids of bivalents break at one or more corresponding points due to endonuclease enzyme and after exchange of chromosomal segments, they again unite together.

In this way, the breakage and reunion of chromatids is called crossing over.

During diplotene, a cross (X) like structure is formed at the point of crossing over in bivalent which is called chiasma or chiasmata.

The no. Of crossing over or chiasmata formation depends on the length of the chromosomes because the longer the chromosomes, the Greater the no. Of chiasmata.

4) Terminalisation - After chiasma formation the non - sister chromatids begins to separate slowly from centromere towards chiasma and chiasma itself begins to show movement towards the terminal ends of tetrad in zipper fashion. This movement of chiasma is called Terminalisation.

The homologous chromosomes get Completely separated from each other by this process.

Types of crossing over:-
On the basis of no. Of chiasma formation-
1) Single cross over
2) Double cross over
3) Multiple cross over

1) single cross over- In this, only one chiasma is found between two non - sister chromatids of bivalents as a result of which two non cross over type and two single cross over type chromatids are formed.

2) Double cross over type- In this, two chiasma are formed between two non - sister chromatids of bivalents as a result of which two double cross over and two non - cross over type of chromatids are formed.

3) Multiple cross over- In this, more than two chiasma are formed between two non - sister chromatids and thus crossing over takes place at more than two points on the same chromosome.