crossing over, mechanism and its significance
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*Note:-about crossing over #important question for NEB 12 (8marks)
Crossing over , its mechanism and significance
# crossing over
crossing over is the process of exchanging genetic material between two sister chromatids of homologous chromosome as a result s gene is formed. it occurs duirng the pachytene stage of prophase I of meiosis I.
# Mechanism of crossing over
The mechanism of crossing over takes place in following process:
Duirng zygotene stage of prophase I the paternal and maternal homologous chromosome come close together and paired with each other , this process is called synapsis. and the paired chromosome are called bivalents.
During pachytenwe stage of prophase I the homologous chromosome splits to form two identical sister chromatids.thus, the bivalents consist of 4 chromatids called Tetrad.
the nonsister chromatids of homologous pair twist each other at one or more point. the chromatids segment break at a corresponding point due to endonucleus enzymes and the segment of one side fusees with the opposite side due to ligase enzymes.
it is the point of attachemntt where he crossing over occurs.
After complete of crossing over the movementt of chiasma is called Terminalization. due to it homologous chromosome completely separate.
i)synapsis:-
Duirng zygotene stage of prophase I the paternal and maternal homologous chromosome come close together and paired with each other , this process is called synapsis. and the paired chromosome are called bivalents.
ii) duplication of chromosome:-
During pachytenwe stage of prophase I the homologous chromosome splits to form two identical sister chromatids.thus, the bivalents consist of 4 chromatids called Tetrad.
iii) crossing over:-
the nonsister chromatids of homologous pair twist each other at one or more point. the chromatids segment break at a corresponding point due to endonucleus enzymes and the segment of one side fusees with the opposite side due to ligase enzymes.
iv) Chiasmata formation :-
it is the point of attachemntt where he crossing over occurs.
v) Terminalization:-
After complete of crossing over the movementt of chiasma is called Terminalization. due to it homologous chromosome completely separate.
# significance of crossing over
* it produce new combination of gene
* it increase the chance of variation.
*various organism was created through the recombination of gene
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