CELL DIVISION ..................... MEIOSIS ( PART 3 )

What is meiosis ...................?

Meiosis is the process by which the nucleus divides in all sexually reproducing organisms during the production of spores or gametes. These cells have a single set of chromosomes and are called haploid, as opposed to diploid cells with two sets.

Image :- cell division

- Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.

- Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.

- Meiosis involves pairing of homologous chromosomes and recombination between them. Four haploid cells are formed at the end of meiosis II.

Meiosis I :-

It is a reduction division in which the number the number of the chromosome is reduced to half . It is studied in four stages.

A) Prophase I:-

Fig :- prophase

Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behaviour, i.e.

1) Leptotene,

2) Zygotene,

3) Pachytene,

4) Diplotene

5) Diakinesis.

leptotene :-

- During leptotene stage the chromosomes become gradually visible under the light microscope.

- The compaction of chromosomes continues throughout leptotene.

Zygotene:-

- During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes.

- Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex.

- The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. However, these are more clearly visible at the next stage.

Pachytene :-

- During this stage bivalent chromosomes now clearly appears as tetrads.

- This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes.

- Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase.

- Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.

Diplotene :-

- It is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other at the sites of crossovers.

- These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.

Diakinesis:-

- This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.

- By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down.

B) Metaphase I :-

- The bivalent chromosomes align on the equatorial plate.

Fig :- metaphase

The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.

C) Anaphase I:-

- The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

Fig :- anaphase

D) Telophase I & Cytokinesis :-

- The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as diad of cells .

Fig :- Telophase

- Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus.

- The stage between the two meiotic divisions is called interkinesis and is generally short lived.

Fig :- cytokinesis

- Interkinesis is followed by prophase II, a much simpler prophase than prophase I.

Fig :- meiosis I

Meiosis II :-

- During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes.

- Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.

Prophase II:-

- Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated.

Fig :- prophase II

- In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II .

- The chromosomes again become compact.

Metaphase II:-

- At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.

Fig :- Metaphase II

Anaphase II:-

- It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell.

Fig :- Anaphase II

Telophase II:-

- Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope, cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells.

Fig :- Telophase II

Cytokinesis :-

- The cytoplasm divide by forming a furrow in the animal cell and in cell plate in plant cell .

- After cytokinesis a single cell divide into two daughter cells.

Fig :- Cytokinesis II

- Each daughter cells of meiosis I divide to form two daughter cell hence at the end of meiosis , after meiosis I and meiosis II , a diploid cell gives rice to four haploid daughter cell.