Meiosis or meiotic division//Cell division

 

Cell division part:2

Meiosis or meiotic division

 

Meiosis:

Meiosis is also called Reductional Division or Meiotic Division.

The term meiosis was given by Farmer and Moore in 1905.

Meiosis occurs in special cells called Meiocytes. For example, germ cells of sex organ, Pollen mother cell etc.

Meiosis is a special type of division in which number of chromosome becomes the half of the parental cell.

From one diploid parental cell, four (4) haploid daughter cells are formed.

 

Meiosis involves two divisions-

1. Meiosis I or Reductional Division or Heterotypic Division

2. Meiosis II or  Homotypic Division

 

1.  Meiosis I or Reductional division or Heterotypic division-

It is also called Reductional Division because it involves formation of two daughter cells each with half the number of chromosomes as compared to the parent cell.

It is completed into two phases-

A.   Karyokinesis I

B.   Cytokinesis I

 

A.  Karyokinesis I

Karyokinesis involves division of the nucleus.

It is completed into four phases-

a.     Prophase I

b.     Metaphase I

c.      Anaphase I

d.     Telophase I

 

a.    Prophase I

                Prophase I is the longest phase of the Meiosis.

                It is the divided into 5 sub-phases-

                               i.            Leptotene or Leptonema

                             ii.            Zygotene or Zygonema

                          iii.            Pachytene or Pachynema

                          iv.            Diplotene or Diplonema

                             v.            Diakinesis

 

i.                   Leptotene or Leptonema-

Leptotene is the first phase of prophase I.

It is also called Thin threaded stage.

Volume of nucleus increases.

Chromatin material gets highly condensed to form thread like chromosomes.

In this stage, chromosomes are not clearly visible they appear like bouquet, so this stage is also called as Bouquet stage.

Nuclear membrane starts disappearing.

 

ii.                 Zygotene or Zygonema-

This sub-phase is also called Yoked threaded stage.

In zygotene pairing of homologous chromosomes takes place.

The pairing of chromosomes is known as Synapsis and the pair is known as Bivalent.

Due to this pairing two chromosomes get associated with each other by a complex, called Synaptonemal complex.

 

iii.              Pachytene or Pachynema-

In pachytene, further condensation of chromosomes takes place.

Each chromosomes is now visibly formed of two sister chromatids joined at a centromere and is called a Dyad.

Each bivalent that is one homologous pair has two dyad and called a Tetrad.

This stage is characterized by appearance by Chiasmata which is the point where non- sister chromatids cross over.

Now exchange of genetic material between homologous chromosomes takes place. This exchange is called Crossing over.

 

iv.              Diplotene or Diplonema-

Separation of homologous chromosomes takes place due to which synaptonemal complex start disappeared.

It is the longest and the most active sub-phase of meiosis I.

 

v.                 Diakinesis-

Chromosomes become more compact and contracted.

Complete dissolution of nuclear membrane and nucleoli takes place.

This sub-phase represents transition to metaphase I.

 

b.    Metaphase I

The chromosomes arranged on the equator.

Kinetochore or centromere of one chromosome of the homologous pair is attached to the chromosomal spindle fibre of one pole of the same pole.

 

c.     Anaphase I

It involves separation of homologous chromosomes which start moving towards opposite poles.

The shape of separating chromosomes may appear like U, L, J or I shaped.

 

d.    Telophase I

In telophase I, chromosomes reach the opposite poles.

At each pole a daughter nucleus with nuclear membrane and nucleolus is formed.

 

B.  Cytokinesis I-

In animal cell it takes place by furrow formation and in plant cell by cell plate formation.

Cytokinesis does not necessary in the each species.

                                   Various phases of meiosis I in plant cell

 Note: Interkinesis-

It is the period between Meiosis I and Meiosis II.

It involves only protein and RNA synthesis.

There is no ‘s” phase and no DNA synthesis.

2. Meiosis II

Meiosis II is also known as Equational division because the number of chromosomes remains the same as after meiosis I.

In meiosis II, each cell which is formed by meiosis I is divided into two daughter cells.

Finally four haploid cells are formed.

It is also completed into two phases-

A.   Karyokinesis  II

B.   Cytokinesis II

A.  Karyokinesis II

It is completed into four phases-

a.     Prophase II

b.     Metaphase II

c.      Anaphase II

d.     Telophase II

 

a.     Prophase II

Disappearance of nuclear membrane and nucleolus takes place.

Chromatids have widely separated arms.

Spindle of prophase II is perpendicular to that of prophase I.

b.    Metaphase II

Chromosomes get arranged at the equator.

They get attached to the spindle fibres with their centromere.

c.      Anaphase II

In anaphase, splitting of centromere of each chromosome takes place resulting in formation of two daughter chromosome.

Daughter chromosomes are pulled towards the pole by chromosomal spindle fibres.

d.    Telophase II

Daughter nuclei appear at each pole due to reappearance of nuclear membrane and nucleoli.

Spindle fibres also disappear.

B.  Cytokinesis II

It involves the division of the cytoplasm.

In animal cell, it takes place by furrow formation and in plant cell by cell plate formation.

                              Various phases of meiosis II in plant cell

Significance of meiosis-

Meiosis is results in genetic variation which are essential for evolution.

It is a reductional division but is essential for maintaining species specific diploidy.

By meiosis a diploid parent cell divides two times and formed four haploid gametes.