Crossing over

In: Science

Submitted By nazmaz
Words 468
Pages 2
MECHANISM OF MEIOTIC CROSSING OVER:
The mechanism of crossing over includes the following stages i.e., Synapsis, Duplication of chromosomes, Crossing over and Terminalization.
i. Synapses: It is the intimate pairing between two homologous chromosomes (one maternal and another paternal) during zygotene stage of prophase I of meiosis. Synapsis often starts when the homologous ends of the two chromosomes come together on the nuclear envelope and it continue inward in a zipper-like manner from both ends aligning the two homologous chromosomes side by side. By synapsis each gene is thus brought into juxtaposition i.e. side by side. The resulting pairs of the homologous chromosomes are called bivalents.

ii. Duplication of chromosomes: The synapses are followed by the duplication of chromosomes (in pacytene stage). During this stage, each homologous chromosome of bivalents split longitudinally and form two identical sister chromatids which remain together by the centromere. This stage of four chromatids in the pair is called tetrad.

iii. Crossing over by breakage and union: Crossing over occurs in the homologous chromosomes only during the four stranded or tetrad stage. Homologous chromosome continue to stay in synapis for days during pacytene stage and chromosomal crossing over occurs due to exchange of chromosomal material between non-sister chromatids of each tetrad. During crossing over, two non-sister chromatids first break at the corresponding points due to activity of enzyme endonuclease. Then a segment on one side of each break connects with a segment on the opposite side of the break, so these two non-sister chromatids cross each other. The fusion of chromosomal segments with that of the opposite one takes place due to the action of enzyme ligase. The crossing over of two chromatids is known as chiasma formation. The crossing over, thus includes the…...

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