A Process of Cell Division That Occurs in the Germ Cells of Sexual Reproducing Animals

Meiosis is generally defined as a process that occurs in the germ cells of sexual reproducing animals. These germ cells are localized in the gonad. The cells in which meiosis takes place are called as meiocytes. The meiosis of gonads is called as ganocytes. The ganocytes are spermatogenesis in males and oogenesis in females.

Meiosis is a type of cell division, where daughter cells receive only half the complement of the originally diploid chromosomes. Meiosis is divided into two stages.

· MEIOSIS- I

Meiosis-I is divided into four stages.

1. PROPHASE- I: It is the longest stage of meiotic division. It is divided into five stages.

- LEPTOTENE: It is the first stage of meiosis. In this stage the chromosomes appear as beaded structure along the length. These chromosomes move towards one point of nuclear membrane to form the bouquet stage and nucleus becomes prominent. In this stage the nucleus is seen in the center of the cell. Due to the synthesis of RNA for the condensation of chromosomes, the volume of the nucleus increases.

- ZYGOTENE: Homologous chromosomes starts pairing by a process called synopsis. During sub phase the paternal and maternal chromosomes, with the help of homologous sequences come close together and pair with each other along their length. This type of selective synopsis helps in the formation of bivalent synopsis.

- PACHYTENE: In this stage, homologous chromosome of synaptic pair undergoes condensation and the crossing over of non sister chromatids takes place. The dense nucleus at which the recombination occurs appears as a cross (X) when the chromatids begins to separate. This breakage and reunion of non sister chromatids results in the formation of crossed shaped structure (X) called as chaisma. At this module, the non sister chromatids of paternal and maternal chromosomes exchange their segment. This exchange of chromosomal segment is known as crossing over.

- DIPLOTENE: The chromosome of homologous pair starts to repel each other. It starts from centromeric region and moves towards the end. The chaisma are still obtained and the separation of chromosomes further condensed. The number of chaisma may vary which depends on the numbers of homologous sequence in the chromosomes. The shortening of chromosomes continues and this is the best stage to observe chaisma.

- DIAKINESIS: Termination of chaisma is completed in this stage and the chromosome appears either as X or Y or O shaped. The variations of shapes are due to the number of chaisma. After their separation, chromosomes move towards the nuclear membrane. The degradation of nucleus and nuclear membrane takes place. This is the end of the prophase-I stage.

2. METAPHASE-I

The tetrad chromosomes become condensed and regular. They occupy equatorial plate and centromeres in each chromosome. They are directed towards the opposite poles of the cell. The spindle formation takes place.

3. ANAPHASE-I

The homologous chromosome pair separate apart due to the anaphasic movement. This is caused by the elongation of polar spindles and shortening of chromosomal spindles. The X-chromosomes moves to top of any of the poles. This stage ends with the complete separation of homologous chromosome, thus results in the formation of daughter nuclear content (haploid set)

4. TELOPHASE-I

In this phase the chromosome at each pole organizes in to daughter nuclei. The daughter nuclei are haploid since they have only one set of chromosome, where as the parent cell has two set of chromosomes. In this stage the spindle disappears and nucleus and nuclear membrane reappear. As the result in two daughter cell, one cell has X chromosomes and other without any sex chromosomes.

· MEIOSIS-II

It is the second nuclear division of meiosis and is similar to mitosis, hence called as equatorial division. It results in the formation of four haploid nuclei. Meiosis-I is divided into four stages

1. PROPHASE-II

In this stage two daughter cells show changes simultaneously. The centrioles again migrate and divide to form asters. The chromatin threads condense to form a network and chromosomal condensation continues further. The nuclear envelope and nucleolus shows disintegration. The formation of spindle starts during this stage.

2. METAPHASE -II

It is the shortest stage of meiosis. The chromosome's centromere lies in the equator exactly in right angle to the plane of metaphase I. The spindle fibers formed are of two types, namely chromosomal and polar spindle. The centromere lies in the center and arms of the chromatids radiate towards the opposite poles.

3. ANAPHASE -II

The condensed chromosome at the equator starts moving to their opposite poles. This movement is brought by the elongation of polar spindles and concentration of chromosomal spindles. The chromosomes of cells which are duplicated get separated. In this stage the chromatids are long and thick unlike the chromatids of the anaphase-I

4. TELOPHASE- II

In this stage, nucleolus and nuclear membrane reappears, spindle fibers and asters gradually disappears. The arms of the chromatids get uncoiled and results in the formation of four daughter cells, two with X chromosomes and other without any sex chromosomes.