Chapter 10, Problem 15STP

Summary Introduction

To hypothesize:

The reason meiosis occurs in two stages- meiosis I and meiosis II.

Introduction:

Reproductive cells which pass on genetic traits from the parent to child are produced by the process of meiosis. In order to maintain the same number of chromosomes in each generation, an organism undergoes meiosis. It is necessary in organisms that reproduce sexually. There are two stages in meiosis; meiosis I and meiosis II.

Answer to Problem 15STP

Meiosis occurs in two stages mainly because:

• Meiosis I results in exchange of genes between homologous chromosomes during crossing over.This promotes genetic variation in a species.
• The two stages of meiosis ensure that the chromosome number in each species is conserved.
• Two divisions of meiosis are needed to produce gametes which are haploid.

Explanation of Solution

There are two stages in meiosis; meiosis I and meiosis II. This is because:

• During prophase I of meiosis, the homologous chromosomes align with each other and exchange some part of their chromosome so that the resultant chromosomes are recombinants- that is they have a mix of genes derived from both parental chromosomes.Crossing over results in genetic variation in a species.
• Meiosis I and II result in four haploid cells which undergo fertilization. This results in diploid zygote and the chromosome number is maintained in the species.
• Two divisions of meiosis are needed to produce gametes. Meiosis I occurs only in reproductive cells and meiosis II is similar to mitosis. Before the reproductive cells enter meiosis, they are diploid. After meiosis I two daughter cells are formed which are haploid. After meiosis II each haploid daughter cell forms two haploid cells. Therefore, four haploid gametes are formed at the end of meiosis II.