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In a cell cycle crossing over occurs at
(a) Single strand stage
(b) Two strand stage
(c) Four strand stage
(d) Eight strand stage

Last updated date: 20th Jun 2024
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Hint: Crossing over is the mutual exchange and reciprocal recombination of chromatid segments, DNA fragments, or block of genes between the non-sister chromatids of homologous chromosomes. Crossing over occurs during prophase I of meiosis before tetrads are aligned along the equator in metaphase I. Only two of the four chromatids are involved in Crossing over.

Complete answer:
In a cell cycle, crossing over occurs at the four-strand stage. Crossing over involves the symmetrical breakage of chromatids, and the reciprocal exchange and crosswise reunion of segments between non-sister chromatids, often breaking linkage. This results in the recombination of genes. Crossing over occurs at the pachytene stage of prophase I of Meiosis I.
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During the pachytene, stage one chromatid of each homologue overlaps or crosses over a corresponding non-sister chromatid of the other homologue at one or more points. These crossing points are called chiasma. At this point, the non-sister chromatids divide transversely with the help of endonuclease. The broken segments are then mutually exchanged by them. Each exchanged segment fuses with a corresponding portion of non-sister chromatids with the help of ligases. The whole process which involves the breakage, mutual exchange, and reciprocal reunion of chromosome segments is called crossing over.

Additional Information:
- Crossing over and the independent assortment of alleles are the major mechanisms for producing new, non-parental gene combinations and genetic variations.
- Natural selection acts upon these recombinant genotypes and genetic variations and thereby enhances the fitness, survival value, and evolutionary potentiality of organisms.
- In eukaryotes, crossing over may be mitotic or meiotic. Mitotic crossing over occurs in somatic cells and finial cells.
So, the correct answer is the '(c) four-strand stage’.

- Mitotic crossing over involves five major events; synapsis of homologous chromosomes and tetrad formation, crossing over of non-sister chromatids and chiasma formation, symmetrical breakage of non-sister chromatids, reciprocal recombination through the mutual exchange and crosswise reunion of broken segments, dissociation of the homologues by the decriminalization of chiasma.
- Synapsis is the close pairing of homologous chromosomes during the zygotene stage of the first meiotic division.
- Double-crossing over is the crossing over which involves two chiasma, two, three, or four chromatids.