Cell Division

MEIOSIS

Meiosis involves two sequential cycles of nuclear and cell division, but only a single cycle of DNA replication. Meiosis is divided into meiosis I and meiosis II.

• Meiosis I begins after the S phase, and meiosis II follows meiosis I.
• Pairing of homologous chromosomes happens during meiosis which results in recombination of genes.
• Four haploid daughter cells are formed at the end of meiosis.

MEIOSIS I

Prophase I:

Prophase in meiosis I is typically longer and more complex than the prophase in meiosis II. Prophase I is subdivided into five phases, viz. Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

Leptotene:

• During this stage, the chromosomes become gradually visible under light microscope.
• Compaction of chromosomes continues throughout this phase.

Zygotene:

• Chromosomes start pairing together. This process is called synapsis. The paired chromosomes are called homologous chromosomes.
• Formation of synapsis is accompanied by the formation of synaptonemal complex.
• The synaptonemal complex by a pair of homologous chromosomes is called a bivalent or a tetrad.

Pachytene:

• Bivalent chromosomes clearly appear as tetrads.
• Recombination nodules appear. These nodules are the sites at which crossing over takes place between non-sister chromatids of the homologous chromosomes.
• Exchange of genetic materials between two homologous chromosomes takes place during crossing over. This leads to recombination of genetic materials on the two chromosomes.

Diplotene:

• Synapotnemal complex is dissolved at this stage.
• The recombined homologous chromosomes of the bivalent separate from each other; except at the site of crossing over.
• The X-shaped structures; thus formed; are called chiasmata.

Diakinesis:

• Chiasmata is terminated at this stage.
• Meiotic spindles are formed to prepare the homologous chromosomes for separation.
• Nucleolus disappears and nuclear envelope breaks down by the end of diakinesis.

Metaphase I:

• The bivalent chromosomes are aligned on the equatorial plate.
• Spindle fibres from opposite poles attach to the pair of homologous chromosomes.

Anaphase I:

Homologous chromosomes separate, but sister chromatids remain attached at their centromeres.

Telophase I:

• Nuclear membrane and nucleolus reappear.
• This is followed by cytokinesis and this stage is called the diad of cells.

The stage between the two meiosis divisions is called interkinesis. Interkinesis is usually short lived.

MEIOSIS II

Prophase II: Meiosis II resembles the mitotic cell division. It begins immediately after cytokinesis. Nuclear membrane disappears. Chromosomes again become compact.

Metaphase II: The chromosomes align at the equator. Spindle fibres from the opposite poles get attached to the kinetochores of sister chromatids.

Anaphase II: Centromeres split and sister chromatids move towards the opposite poles.

Telophase II: The two groups of chromosomes get enclosed by nuclear envelope. This is followed by cytokinesis; resulting in the formation of four daughter cells.

Significance of Meiosis:

• Conservation of specific chromosome number of each species is achieved across successive generations in sexually reproducing organisms through meiosis.
• Meiosis helps in increasing the genetic variations in the population of organisms from one generation to the next.

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