Cell division:MEIOSIS

MEIOSIS:


The cells undergoes division and results in the formation of haploid daughter cells and reduces the number of chromosomal number by half.

Meiosis is also called as Reduction division or Heterotypic division.

Key  features of meiosis:
  1. It involves nuclear and cell division called meiosis-1 and meiosis-2 respectively.
  2. Only one cycle of DNA replication takes place.
  3. It involves paoring of homologous chromosomes and recombination between them.
At last four haploid cells are formed .

Meiosis-1:

Meiosis-1 initiates when the parental chromosomes replicate to produce identical sister chromatids.
    It has four phases. They are:

1.prophase-1

2.metaphase-1

3.anaphase-1

4.telophase-1

Prophase-1:

It is sub divided into five(5) phases.
1.Leptotene: chromosomes comes closer together and visible under microscope.
2.Zygotene: chromosomes start pairing together forming a synapse. Paired chromosomes are called homologous chromosomes. This chromosomal synapse is accompained by formation of complex structure called Synaptonemal complex. This complex formed is called as Bivalent or Tetrad of chromatids.
3.Pachytene: Tetrad is clearly visible in microscope. In this stage recombination nodules appear, site at which crossing over takes place between non-sister chromatids of homologous chromosomes. Crossing over is the exchange of genetic material. The enzyme involved in crossing over is the Recombinase. Recombination of homologous chromosomes is completed in this stage and  the chromosomes are linked at the sites of crossing over.
4.Diplotene: Homologous chromosomes get separated except at the sites of crossing over. Synaptonemal complex get disappear. X-shaped structure called chiasmata appears.
5.Diakinesis: Chiasmata moves towards the ends. By the end nucleolus and nuclear envelope disappears.

Metaphase-1

The bivalent chromosomes comes to equatorial plane. The microtubules from opposite pole attach to the chromosomes.

Anaphase-1

The chromosomes get separated but the chromatids remain attach to the centromeres.

Telophase-1

Nuclear membrane and nucleolus reappears.
Cytokinesis follows it and forms diad of cells.
Interkinesis takes place between two meiotic divisions.

Meiosis-2

Prophase-2

The chromosomes get compact and nuclear membrane disappears.

Metaphase-2

Chromosomes comes to equatorial plane and opposite poles of spindle attach to the kinetochores.

Anaphase-2

The chromosomes get separated but hold at the centromeres.

Telophase-2

Nuclear membrane reappears, followed by cytokinesis resulting in the formation of four haploid daughter cells.

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