Sunday, July 10, 2011

Class IX, BIOLOGY, "Meiosis"


It is that type of cell division in which cytoplasm and nucleus divides twice and as a result of this, four daughter cells are formed and chromosome number is reduced to half. It means that one diploid (2n) parent cell divides to form four haploid (n) daughter cells.
Meiosis consists of two sub divisions:
1. Meiosis I
2. Meiosis II

1. Meiosis I

It has following stages:

Prophase I

1. It is lengthy than prophase of mitosis. It is very important phase. It is divided into five stages during which there is continuous condensation of chromosomes.
2. The important process of this phase is synapsis in which homologous chromosomes pair with each other length wise.
3. Each pair consists of four chromatids or two chromosomes.
4. After synapsis, the process of crossing over takes place. In this, homologous chromosomes exchange their chromatids parts at certain places.
5. At end of this phase, nuclear membrane breaks up. Nucleolus disappears and chromosomes scatter over the spindle.
6. Like mitosis, mitotic apparatus is also formed here.

Metaphase I
1. The chromosomes arrange on scatter of the spindle.
2. Here, homologous bivalents arrange at equatorial plate of spindle.
3. Only one spindle fiber is attached to each chromosome.

Anaphase I
1. Homologous pairs of chromosomes are separated.
2. Spindle fibers contract.
3. Chromosomes begin to move towards the opposite poles.
4. This phase is different from metaphase of mitosis because half the number of chromosomes moves towards each pole and each chromosome still has two chromatids.

Telephase I
1. Half the number of chromosomes reach at opposite poles.
2. Chromosomes again increase their length.
3. Nucleolus reappears. Nuclear membrane is reformed and in this way two daughter nuclei are formed.
4. Now cytoplasm divides and two daughter cells are formed. Each cell is haploid (n).

2. Meiosis II

It is similar to mitosis. The haploid cells formed in meiosis I pass through phases of meiosis II and ultimately four haploid (n) daughter cells are formed. These cells afterwards change into spores (in plants or gametes (animals)

Significance of Meiosis

1. Meiosis takes place only in germ mother cells which form gametes or spores.
2. It maintains the chromosome number of a species constant generation after generation.
3. If gametes had the same number of chromosomes as in somatic cells, the number of chromosomes would have doubled after each generation in a species.
4. The number of chromosomes is constant for each species. During meiosis; gametes (both and) formed are haploid.
5. Gametes unite to form a diploid zygote.
6. During meiosis, pairing of chromosomes takes place which is called synapsis.
7. Exchange of genetic material occurs during meiosis. In this way variations are produced which are raw material for evolution.

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