MEIOSIS :
- Specialized kind of cell division that reduces the chromosome number by half, resulting in formation of 4 haploid daughter cells.
- Occurs during gametogenesis in plants and animals.
- Involves two sequential cycles of nuclear and cell division called Meiosis I and Meiosis II.
- Interphase occurs prior to meiosis which is similar to interphase of mitosis except the S phase is prolonged.
- 4 haploid daughter cells are formed.
Meiosis I -
Prophase I : Subdivided into 5 phases.
Leptotene :
- Chromosomes make their as single stranded structures.
- Compaction of chromosomes continues.
Zygotene :
- Homologous chromosomes start pairing and this process of association is called synapsis.
- Chromosomal synapsis is accompanied by formation of synaptonemal complex.
- Complex formed by a pair of synapsed homologous chromosomes is called bivalent or tetrad.
Pachytene :
- Crossing over occurs between non-sister chromatids of homologous chromosomes.
Diplotene :
- Dissolution of synaptonemal complex occurs and the recombined chromosomes separate from each other except at the sites of crossing over. These X-shaped structures are called chaismata.
Diakinesis :
- Terminalisation of chaismata.
- Chromosomes are fully condensed and meiotic spindles assembled.
- Nucleolus disappears and nuclear envelope breaks down.
Metaphase I :
- Bivalent chromosomes align on the equatorial plate.
- Microtubules from opposite poles of the spindle attach to the pair of homologous chromosomes.
Anaphase I:
- Homologous chromosomes separate while chromatids remain associated at their centromeres.
Telophase I :
- Nuclear membrane and nucleolus reappear.
- Cytokinesis follows (diad of cells).
Interkinesis : Stage between two meiotic divisions. (Meiosis I and meiosis II)
Meiosis II -
Prophase II
- Nuclear membrane disappears.
- Chromosomes become compact.
Metaphase II
- Chromosomes align at the equator.
- Microtubules from opposite poles of spindle get attached to kinetochores of sister chromatids.
Anaphase II
- Simultaneous splitting of the centromere of each chromosome, allowing them to move towards opposite poles of the cell.
Telophase II
- Two groups of chromosomes get enclosed by a nuclear envelope.
- Cytokinesis follows resulting in the formation of tetrad of cells i.e., 4 haploid cells.
Significance of Meiosis
- Formation of gametes: In sexually reproducing organisms.
- Genetic variability
- Maintenance of chromosomal number: By reducing the chromosome number in gametes. Chromosomal number is restored by fertilisation of gametes.
