Chapter 11, Problem 1TYK

The chromosome constitution number of this individual is $2\pi =6.$

This drawing represents:

a. mitotic metaphase.

b. meiotic metaphase I.

c. meiotic metaphase II.

d. a gamete.

e. sixnonhomologous chromosomes.

Summary Introduction

Introduction:

The cell cycle consists of two main divisions known as mitosis and meiosis. Mitosis is the first division during the cell cycle in which the number of chromosomes remains same as the number of chromosomes in the parent cell; on the contrary, in meiosis, the number of chromosomes gets reduced to half.

Answer to Problem 1TYK

Correct answer:

Metaphase 1 of meiosis.

Explanation of Solution

Justification for the correct answer:

Option (b) is meiotic metaphase 1. There are two meiotic divisions after mitosis, namely, meiosis 1 and meiosis 2. During the meiosis 1, the paired chromosomes, also known as bivalent, arrange themselves on the metaphase plate. These paired chromosomes align at the middle of the plate and get attached to the meiotic spindle. The spindle is formed from the structure of the protein, which is responsible for dividing the genetic material in the cell. Hence, option (b) is correct.

Explanation for the incorrect answer:

Option (a) is metaphase in mitosis. Mitosis is the first part of the cell cycle that takes place before the meiosis. Metaphase stage in mitosis involves the arrangement of most condensed chromosomes and these chromosomes then get coiled at this stage. During the metaphase stage of meiosis, the chromosomes are not paired. So, it is an incorrect option.

Option (c) is metaphase 2 phase of meiosis 2. The chromosomes in the meiosis 2 are similar to the chromosomes found in the metaphase stage of mitosis. During the meiosis 2, the chromosomes get condensed and attach themselves to the spindles arising from the centromere present at the opposite poles. These condensed chromosomes are aligned on a metaphase plate but they are in a condensed form. So, it is an incorrect option.

Option (d) is a gamete. During fertilization fusion of one haploid gamete with another haploid gamete results in the formation of a diploid zygote. Number of chromosomes in a haploid cell is 23, two haploid cells fuse together to form a diploid cell. During meiosis number of chromosomes are reduced to 23 from 46 chromosomes. So, it is an incorrect option.

Option (e) states that number of non-homologous chromosomes is 6. Paired chromosomes are the one in which one chromosome is a homolog to another chromosome. These paired chromosomes have the same gene content and possess the same morphology as well. Non-homologous chromosomes are paired chromosomes in which both the chromosomes possess a different gene content. So, it is an incorrect option.

Hence, options (a), (c), (d), and (e) are incorrect.

Conclusion

Therefore, it may be concluded that in the metaphase division of meiosis 1 alignment of paired chromosomes on the metaphase plate takes place. These paired chromosomes get attached to the spindle fibers during this (metaphase 1) stage.