• Meiosis is a special type of division which occurs in both plants and animals during different stages of sexual reproduction.
• Meiosis always occurs in specific diploid (2n) cells and results in reduction of number of chromosomes to half. So it is also called Reductional division.
• The cell undergoing meiosis is called Meiocyte and the haploid cells formed after completion are called Meiotic products.
• Examples-
  Spore Mother Cell (2n) produces Spores (n)
  Microspore Mother Cell (2n) produces Microspores (n)
  Megaspore Mother Cell (2n) produces Megaspores (n)
• Meiosis occurs after completion of DNA replication during S-phase of Interphase and involves 2 sequential cycles of nuclear and cell divisions (Meiosis I and Meiosis II) but only single cycle of DNA replication.

TYPES OF MEIOSIS

• Zygotic Meiosis – Meiocyte is Zygote eg. Algae
• Sporic Meiosis- Occurs in Spore Mother Cells.
• Gametic Meiosis- Occurs during Gametogenesis

MEIOSIS I

Meiosis I is further divided into:-

• Prophase I
• Metaphase I
• Anaphase I
• Telophase I

PROPHASE I

• Longest and the most complex phase of Meiosis.
• For the ease of study, it is divided into 5 sub-stages based on chromosomal behaviour:-
  • Leptotene
  • Zygotene
  • Pachytene
  • Diplotene
  • Diakinesis

LEPTOTENE

• Condensation of chromatin starts.
• All the condensing chromatin align together on the nuclear lamina resulting in formation of bouquet like structure. So it is also called Bouquet stage.

ZYGOTENE

• Pairing of homologous chromosomes starts and the process is called Synapsis.
• Formation of Synaptonemal complex to stabilize Synapsis.
• Complex formed by a pair of synapsed homologous chromosome is called Bivalent or Tetrad.

PACHYTENE

• 4 chromatids of each bivalent becomes distinct and clearly appear as tetrads.
• Appearance of Recombination Nodule at sites of Crossing over.
• Recombination Nodule consist of enzymes like Recombinase, Endonuclease and Ligase.
• CROSSING OVER- Exchange of genetic material between non-sister chromatids of homologous pair of chromosomes.
• Crossing over completes in Pachytene but chromosomes remains attached at the site of crossing over.

DIPLOTENE

• Dissolution of synaptonemal complex.
• Site of crossing over becomes visible in the form of X-shaped structures called Chiasmata.
• In oocytes of some vertebrates, diplotene stage lasts for several moths or years.

DIAKINESIS

• Marked by terminalisation of chiasmata.
• Chromosomes are completely condensed.
• Meiotic spindles are assembled.
• Nucleolus disappears and nuclear envelope also breaks down.

METAPHASE I

• Spindle fibres from opposite poles of the cell attach to kinetochore of homologous chromosome.
• Bivalent chromosomes align at he equator of the cell resulting in formation of Double Metaphasic Plate.

ANAPHASE I

• Homologous chromosomes (bivalents) separates while sister chromatids remains associated at centromere.
• Spitting of centromere do not occur during Anaphase I
• Anaphase I results in reduction of number of chromosomes to half.

TELOPHASE I

• Nuclear envelopes and nucleolus reappears.
• Chromosomes do not decondense completely.
• Cytokinesis follows and results in formation of Dyad of cells.

INTERKINESIS/ Intra-meiotic interphase

• Short lived stage between 2 Meiosis I and Meiosis II in which doubling of centrosomes occurs but DNA replication is absent.

MEIOSIS II

• Events of Meiosis II are exactly similar to Mitosis.
• Mitosis can occur in both haploid and diploid cells but Meiosis II always occurs in cells formed by Meiosis I.

PROPHASE II

• Chromatin condenses to form chromosomes.
• Nuclear envelope disappears.

METAPHASE II

• Chromosomes align at equator of the cell resulting in formation of a single metaphasic plate
• Microtubules from opposite poles of spindles get attached to kinetochore of sister chromatids.

ANAPHASE II

• Simultaneous splitting of centromere of each chromosome
• Sister chromatids move towards opposite pole by shortening of Microtubules.

TELOPHASE II

• Nuclear envelope reforms
• Chromatin decondenses.
• Cytokinesis follows and as a result 4 haploid cells are formed.

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