Meiosis: Phases of Meiosis:

The preparatory steps of meiosis are identical to the interphase of the mitotic cell cycle. Interphase is divided into the same three phases i.e. G1, S phase, and G2. Interphase is followed by meiosis I and meiosis II.

Meiosis I:

In meiosis, I, the homologous chromosomes in a diploid cell separate and so haploid daughter cells are produced. It is the step in meiosis that gene sets genetic diversity.

Steps involved in Meiosis I:

Meiosis I occur in two main steps i.e. karyokinesis and cytokinesis

Subdivision of Karyokinesis of Meiosis

1: The karyokinesis of Meiosis l is subdivided into:

1. prophase I, ii. metaphase I, iii. anaphase I. iv. telophase l.
2. Prophase I:

Prophase I is the longest phase in meiosis. During this stage, individual chromosomes begin to condense within the nucleus. Then the homologous chromosomes line up with each other and form pairs.

Bivalent:

The combined homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids.

Chiasmata:

The two non-sister chromatids of homologous chromosomes become "zipped" together, forming complexes known as chiasmata, in a process known as synapsis.

Crossing over:

In the next stage, the non-sister chromatids of homologous chromosomes randomly exchange their segments and the phenomenon is known as crossing over. The exchange of segments results in a recombination of genetic information. After crossing over the homologous chromosomes separate from one another. However, they remain tightly bound at chiasmata, the regions where crossing-over occurred.

Spindle fibers:

Chromosomes condense further, the nucleoli disappear, and the nuclear envelope disintegrates. Centrioles, which were duplicated during interphase, migrate to the two poles of the cell. They give rise to spindle fibers. The kinetochore spindle fibers attach to the kinetochores of chromosomes. While the non-kinetochore spindle fibers from both sides interact with each other. There are two kinetochores on each tetrad, one for each kinetochore spindle fiber.

1. Metaphase I:

As kinetochore spindle fibers from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane forming the metaphase plate.

1. Anaphase 1:

Kinetochore spindle fibers shorten, breaking the chiasmata and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, one chromosome is pulled toward one pole, forming two diploid sets. Each chromosome still contains a pair of sister chromatids.

1. Telophase 1:

The first meiotic division ends when the chromosomes arrive at the poles. Each pole now has half the number of chromosomes but each chromosome still consists of a pair of chromatids. The spindle network disappears, and a new nuclear envelope surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. After meiosis, both haploid daughter cells enter a period of rest known as interkinesis or interphase II. The interphase il is different from the interphase of mitosis and meiosis 1 There is no S-phase and so no DNA replication occurs during this stage

Meiosis II:

It is the second part of the meiotic process. Much of this part is similar to mitosis.it is subdivided into

1. prophase II, ii. metaphase II, iii. anaphase II, iv. telophase II
2. Prophase ll:

Prophase II takes much less time compared to prophase I. In this prophase, the nucleoli and the nuclear envelope disappear and the chromatin condenses Centrioles move to the polar regions and make spindle fibers.

1. In metaphase II:

In metaphase II the chromosomes attach with the kinetochore spindle fibers 1 align at the equator of the cell. This is followed by anaphase II.

1. In anaphase ll:

In anaphase II the centromeres are cleaved and sister chromatids are pulled ….

The sister chromatids are now called sister chromosomes, and they are ….

toward opposing poles.

1. The telophase II:

The telophase Il is marked with uncoiling and disappearance of the chrom…. Nuclear envelopes reform; cleavage or cell wall formation eventually pre…. total of 4 daughter cells, each with a haploid set of chromosomes.