Question

Crossing over occurs in
A) Four strand stage
B) Three strand stage
C) Two strand stage
D) Single strand stage

Answer 1

Hint: Between prophase I and metaphase I, crossing over happens and is the mechanism in which two homologous non-sister chromatids hook up with each other and share various genetic material segments to generate two recombinant chromosome chromatids.

Complete Answer:
Crossing over is the sharing of genetic material between the non-sister chromatids of two homologous chromosomes during sexual reproduction, which ends in recombinant chromosomes. It is one of the final stages of genetic recombination, which happens during a period called synapsis in the pachytene stage in prophase I of meiosis. Before the synaptonemal system grows, synapsis starts and is not finished until close to the end of prophase I.

Now, let us find the solution from the options-
- At the four-strand stage, crossing over happens during a cell cycle. It happens in the pachytene flower. The genetic material interaction between the bivalent non-sister chromatids is named as crossing over.
- Segment interchange happens at the four-strand stage seen between chromosomes as well as between chromatids. Thus, option B is not the correct option.
- There is ample evidence to indicate that crossing occurs at the pachytene level after four strands, two strands contributing to each paired chromosome, have already been recreated by the chromosomes. Thus option C is not the correct option.
- Although two strand crossing will contribute to four recombinants, two parental variations and two recombinants would be generated by four strand crossing over all four gametes generated from a mother cell. Thus, option D is not the correct option.

Thus, the correct answer is option (A) Four strand stage.

Note: Crossing-over value is the related level of crossing over between two gene loci. For a given range of environmental and genetic factors, recombination appears to be stable in a certain area of a linking structure (chromosome), and the same applies to the cross-over factor used in the development of genetic maps.