Question

Crossing over takes place on
A) Two stranded stage
B) Three stranded stage
C) One stranded stage
D) Four stranded stage

Answer 1

Hint**:** Crossing occurs during the pachytene stage. Within this point non-sister chromatids of homologous chromosomes will swap segments over homology regions.

Complete answer :

To solve this question, first we should know about crossing over. Crossing over is the sharing of genetic material from non-sister chromatids of homologous chromosomes during pachytene stage of meiosis I, resulting in the daughter cells possessing different variations and alleles.

Let us find the solution from the given options:

The non-sister chromatids of homologous chromosomes swap segments over regions of homology. Nevertheless, sex chromosomes are not entirely alike, and exchange information only over a small area of homology. Chiasmata form at the sites where exchange occurs. Chromosomes are paired and occur in clusters with synaptonemal. The paired chromosomes are actually composed of four chromatids. Each bivalent is made up of four chromatids. So, options A, B, and C are incorrect options.

The evidence that crossing over occurs in four stranded stage comes from Neurospora crassa. They produce ascospores which are arranged in the ratio 2:2:2:2 instead of 2:2. Among these four types of ascospores two are of parental type and two are of recombinant type. Thus, four stranded stages is the correct answer.

Thus, the right option is D i.e. Four stranded stages.

Note: Each of an adult organism’s cells is produced from the initial zygote and, thus, are genetically similar. As this adult prepares for reproduction, several adapted cells undergo meiosis, which produces four daughter haploid cells.