MEIOSIS

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MEIOSIS

1. Specialized kind of cell division that reduces the chromosome number by half . This results in the production of HAPLOID daughter cells.

2.occurs during gametogenesis in plants and animals.

3.involves 2 sequential cycles of nuclear and cell division : MEIOSIS I and MEIOSIS II .

4. Interphase occurs before meiosis which is similar to the interphase of mitosis except that S phase is prolonged.

5. 4 haploid daughter cells are formed.

MEIOSIS I :

Prophase I : subdivided in to 5 phases.

Leptotene: chromosomes become gradually visible under the microscope.

the chromosomes condense throughout this phase.

Zygotene : chromosomes start pairing together .....this type of association is called SYNAPSIS which is accompanied by the formation of SYNAPTONEMAL COMPLEX.

Such paired chromosomes are called HOMOLOGOUS chromosomes.

the complex formed by a pair of synapsed homologous chromosomes is called BIVALENT or a TETRAD.

Pachytene : crossing over occurs between non-sister chromatids of homologous chromosomes.

CROSSING OVER is the exchange of genetic material between 2 homologous chromosomes.

crossing over is an ENZYME MEDIATED process......and the enzyme is RECOMBINASE.

Diplotene: the synaptonemal complex DISSOLVES and the recombined homologous chromosomes separate from each other except at the sites of crossing over.

The X-shaped structures seen here are called CHIASMATA.

Diakinesis : The final stage of prophase I . terminalisation of chiasmata.

chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation.

nucleolus and nuclear envelope break down.

Metaphase I :

bivalent chromosomes align on the equatorial plate.

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

Anaphase I :

The homologous chromosomes separate but the sister chromatids remain associated at their centromeres.

Telophase I :

the nuclear membrane and nucleolus reappear and cytokinesis occurs .This is called as DIAD OF CELLS.

MEIOSIS II:

Prophase II : nuclear membrane disappears

chromosomes become compact.

Metaphase II : chromosomes align at the equator

the microtubules from opposite poles of the spindle get attached to the kinetochores of cister chromatids.

Anaphase II: simultaneous splitting of the centromere of each chromosome allowing them to move towards the opposite poles of the cell.

Telophase II : TWO groups of chromosomes once again get enclosed by nuclear membrane.

cytokinesis follows resulting in the formation of 4 haploid daughter cells.

SIGNIFICANCE OF MEIOSIS :

Formation of gametes in sexually reproducing organisms.

genetic variability

maintenance of chromosomal number.

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