In an organism with a diploid number of 2n=10, how many individual chromosomal structures will align on the metaphase plate during (a) mitosis, (b) meiosis I and (c) meiosis II.

The correct answer is (a) Mitosis = 10, (b) Meiosis I = 10, (c) Meiosis II = 5.

Explanation:

Mitosis is the process by which the genetic material from the parent cell is divided equally among the daughter cells. This process of mitosis is preceded the a replication phase where the genetic material of the parent cell is doubled and then divided among the daughter cells. The ploidy or the chromosome number of the parent is maintained in the daughter cell. Meiosis is the process by which the genetic material from the parent cell is divided among daughter cells, where the daughter cells are the gametes. Here, the number of chromosomes in the daughter cells is half of that present in the parent cell. A diploid individual having 10 chromosomes, have 5 of the chromosomes obtained from its mother and 5 from its father. During mitosis, each of the 10 chromosomes will be divided between the two daughter cells. This is done by replication or doubling of the 10 chromosomes. This replication process will result in the generation of 2 chromatids per chromosome. During the metaphase stage of mitosis when these chromosomes align along the metaphase plate, there will be 10 chromosomes aligning and each chromosome will comprise of 2 chromatids, so total there will be 20 chromatids. These chromatids separate during the anaphase and generate 20 chromosomes in the dividing parent cell. In the metaphase stage of meiosis I, the homologous (each obtained from mother and father) pair of chromosomes arrange themselves in tetrad or bivalents (two chromosomes together). Therefore, if the total number of chromosome is 10, there are 5 bivalents that can be formed from them. However, the total number of chromosomes that align along the metaphase plate is 10. Each chromosome has 2 chromatids, so total number of chromatids is 20. During anaphase I phase, the bivalents or homologous chromosomes separate from each other, and the number of chromosomes and chromatids in each daughter cell after meiosis I is 5 and 10 respectively. The metaphase stage of meiosis II is exactly same as that of mitosis. Here the 5 chromosomes comprising of 10 chromatids, obtained after meiosis I, are aligned along the metaphase plate. During anaphase II, the 10 chromatids are further divided into the two gametes such that each gamete gets 5 chromosomes (after separation chromatid forms the chromosome).