Textbook Problem

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Crossing over happens during which phase of meiosis?

1. a. prophase I
2. b. prophase II
3. c. anaphase I
4. d. anaphase II

Summary Introduction

Introduction:

The crossing over involves the exchange of the DNA segments between the homologous chromosomes that will finally end in the formation of recombinant chromosomes as a result of sexual reproduction. Usually crossing over takes place at the pachytene stage of prophase I (meiosis I) that is one of the final phases of genetic recombination. Crossing over helps to produce altered alleles in the homologous chromosome due to the exchange of the DNA between them. Thus, new order of allele possessing gametes can be produced that ultimately leads to genetic diversity of an organism.

Explanation of Solution

Reason for correct answer:

Prophase I is the first and longest phase of the first meiotic division (meiosis I) that includes five more stages. Chromosome crossing over starts at the pachytene stage of the prophase I of meiosis I. Crossing over is facilitated by the formation of a synaptonemal complex. At present, the chromosomes that appeared as long threads start to condense. During the condensation, the homologous chromosomes tend to cross each other. The crossing over occurs among the non-sister chromatids of the two nearby homologous chromosome. Hence, the correct answer is option a.

Option a. is given as “prophase I”.

Chromosome crossing over takes place in the prophase I stage of meiosis I where the non- sister chromatids cross over to form new ordered allele.

Hence, the correct answer is option a.

Reason for incorrect answer:

Option b. is given as, “anaphase I”. No, the crossing over do not take place in the anaphase I of the meiosis I...