Question

1) Pick one of the phases of mitosis, and compare it to the similar phase in meiosis I and meiosis II. For example, compare and contrast prophase of mitosis with prophase I and prophase II in meiosis. What is similar, and what is different? Use correct terminology (sister chromotids, sister chromosomes, centrosome, ect.)

2) What was something new that you learned from chapter 9? How would you explain this new information to someone interested in learning about inheritance? Did it change your perspective on genetics? How and/or why?

Topic in chapter 9

Mendel’s law of segregation describes the inheritance of a single character.

Mendel developed four hypotheses, described below using modern terminology.

​There are alternative versions of genes (called alleles) that account for variations in inherited characters.

​For each character, an organism inherits two alleles of a gene, one from each parent.

An organism that has two identical alleles for a gene is said to be homozygous for that gene.

An organism that has two different alleles for a gene is said to be heterozygous for that gene.

Answer 1

Mitosis is the process by which cells divide and form two daughter cells each with the same number of chromosomes as in the parent cell. Mitosis is also known as equational division. It is the kind of division that we observe in somatic or non-reproductive cells that leads to the growth of the organism in size (in case of multicellular organisms) and in number (in case of unicellular organisms like bacteria). Mitosis is a strictly regulated cellular event which proceeds through a number of stages namely Prophase, Metaphase, Anaphase and Telophase. The events during each of these stages rhythmically prepare the cell to divide its chromosomes (already replicated during interphase) equally among the daughter cells.

Meiosis is the process by which haploid gametes are formed from diploid parent cells. It takes place in reproductive cells which undergo gametogenesis. Meiosis is also known as reduction division as the number of chromosomes in the daughter cells is half of that in the parent cell. Meiosis helps in maintaining ploidy in organisms generation after generation. Sexual reproduction is an outcome of meiosis. As in mitosis, meiosis also proceeds through various phases. The entire process is divided into two stages – Meiosis I and Meiosis II. Each of these stages proceeds through phases like Prophase, Metaphase, Anaphase and Telophase as in mitosis. However meiosis I differ from meiosis II in several ways. Let us compare the Anaphase of meiosis I&II and mitosis to better understand the key differences. (Prophase, Metaphase and Telophase are more or less similar in mitosis and meiosis. Comparing the Anaphase stage will bring out the differences between both type of divisions.)

A general summary of the events in cell division is as follows:

The DNA replicates and doubles during the interphase of both mitosis and meiosis. During prophase, the chromatin condenses and organizes as bivalent chromosomes and the nuclear membrane starts disintegrating. The centrosome divides and starts migrating to the poles. During metaphase the homologues pair up and arrange themselves in the metaphase plate. During anaphase, the migration of chromosomes to both poles occurs with the help of spindle fibers and in telophase the chromosomes reach the poles and nuclear membrane develops.

When mitosis begins, there will be double the number of chromosomes in the cell (a DNA replicated during interphase). The duplicated DNA strands condense to form sister chromatids that forms the bivalent (butterfly shaped) structures. The DNA sequence of the sister chromatids are exactly similar. Homologous chromosomes are the chromosomes carrying the same set of genes, received from the male and female parents, in a diploid cell. The homologous chromosomes may not be identical as the alleles may differ (in homozygotes the homologous chromosomes are identical).

Anaphase of Mitosis:

During this stage, the sister chromatids of the homologous chromosomes aligned at the metaphase plate separates and moves to the opposite poles. Each of the sister chromatids breaks of from the bivalent structure by the action of Separase and spindle fibers. This is the stage that ensures that each daughter cell formed will get exactly the same number of chromosomes as in the parent cell.

So during anaphase of mitosis, the sister chromatids separates.

Anaphase of Meiosis I:

During meiosis I, just like in Mitosis, the chromosome number will be double at the beginning. The result of meiosis I is two daughter cells, each with half the number of chromosomes as in the parent cell. During metaphase I, the homologous chromosomes align at the metaphase plate as in mitosis, but instead of the sister chromatids, the homologous chromosomes separate. Both copies of a bivalent (that is, the sister chromatids) thus move to the same daughter cell. The segregation of alleles takes place during this phase.

Anaphase of Meiosis II:

Meiosis II is similar to Mitosis in several ways. During Anaphase II, the sister chromatids separate, just as in mitosis. This ensures that each gamete receives only one set of chromosomes (n) and one of each homologous pair. Each of the two daughter cell formed after Meiosis I will give rise to 2 daughter cells after Meiosis II.

2.

The reason behind the inheritance of characters from parents to offspring was a subject of curiosity to mankind for long. It was Gregor Johannes Mendel who discovered the principles of heredity conclusively through his experiments on pea plants. His findings can address the 2 basic questions that anyone can have on the inheritance of characters.

1. Why does the progeny formed by sexual reproduction resemble parents?

2. What is the basis of inheritance?

Organisms that reproduce sexually produce offspring that resemble parents in many characters but are not identical to either of the parents. This is because of the formation and fusion of gametes. Each organism has a characteristic number of chromosomes. When two organisms of the same number of chromosomes mate, it would result in the production of progeny with double the chromosome number if there is no mechanism for the reduction of chromosome number. Therefore gametes are formed by a process called meiosis that reduces the number of chromosomes by half. Normally each and every cell carries two sets of chromosomes, each from both parents. Meiosis produce cells that carry only one set of chromosomes. When two such chromosomes fuse, it restores the number of chromosomes in the progeny.

Why do diploid organisms have two sets of chromosomes? It is because they are the product of gamete fusion and each parent contributes to the genetic content. So each diploid cell carries two chromosomes carrying the same set of genes. But these genes exist in two or more alternate forms known as alleles. So a pair of chromosomes means that it carries the same or different forms of the same set of genes. If the alleles are same the organism is said to be homozygous for that particular character and if different, heterozygous.