Meiosis, Meiosis | Cell division | Biology (article), What Is Meiosis?

 Meiosis

Meiosis is the process by which one diploid (2n) cell divides to generate four haploid (1n) daughter cells.

Diploid means the cell in which the chromosomes is in pair and haploid means the  cell with half the number of  chromosomes i.e. not in form of pair.

Phases of Meiosis

Meiosis was discovered and described first time in 1876,

 by German biologist Oscar Hertwig.

The preparatory steps of meiosis are identical to the

 interphase of mitosis.

Interphase is followed by meiosis I and meiosis II

Meiosis I

In meiosis I, the homologous chromosomes separate band two haploid daughter cells form.

It is the strep which form genetic variations.

Two steps

1: Karyokinesis

2: Cytokinesis

The Karyokinesis is sub-divided into Prophase I, Metaphase I,

 Ana-phase I, and Telo-phase I.

Pro-phase I

ØProphase I is the longest phase in meiosis. During this phase

       chromatin condense into chromosome.

ØThe homologous chromosomes line up with each other and form

       pairs by Synapsis.

ØEach pair of Chromosomes is called Bivalent.

Ø Each bivalent has four chromatids, so it is also called Tetrad.

ØThe two non-sister chromatids join each other at certain points along their length. These points called Chaismata.

Ø They exchange their segments and the phenomenon is know as Crossing over.

The exchange of segments result in the recombination  of genetic information. After crossing over, each pair of homologous chromosomes remain as a Bivalent.

The nucleoli disappear ,Nuclear envelop disintegrates.
Centrioles migrate toward poles and form spindle fibers.

Two kinetochore fibers attach with a pair of chromosomes. In mitosis, we have seen that two kinetochore fibers attach with one chromosomes

Meta-phase I

The pairs of homologous chromosomes align along equatorial plane forming the metaphase plate.

                                       

Ana-phase I

ØKinetochore spindle fibers shorten.

ØIt results in pulling apart the chromosomes of each pair.

ØSince one chromosomes is pulled toward one pole, two haploid sets are formed.

Each chromosomes still contains a pair of sister chromatids

                               

Telo-phase I

ØChromosomes arrive at the poles. Each pole now has the half number of chromosomes.

ØSpindle network disappear and nuclear envelop is formed around each haploid set.

ØChromosomes uncoil back into chromatin 

                                    

Cytokinesis

Cytokinesis occur and the creation of two haploid daughter cells is completed.

The pitching of cell membrane in animal cell

and formation of the cell wall in plants cell.

Meiosis II

ØIt is the second part of meiosis and similar to mitosis.

ØIt sub-divided into Prophase II, Metaphase II, Anaphase II, and

        Telophase II.

ØProphase II, takes much lesser time compared to prophase I. In prophase nucleoli and nuclear envelop disappear and chromatin condenses. Centrioles move to the polar region and make spindle fibers.

ØIn metaphase II, chromosomes attach with kinetochore spindle fibers and align at the equator of cell.

ØIn anaphase II, centrosomes are cleaved and chromatids are pulled apart. Sister-chromatids are now sister-chromosomes.

ØTelophase II, is marked with uncoiling of chromosomes into chromatin. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of 4 daughter cell, each with a haploid set of chromosomes.

This is all about Meiosis


JazakAllah