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MEIOSIS

Updated: May 22

  • 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-

Meiocyte (2n) Meiotic products (n)

Spore Mother Cell Spores

Microspore Mother Cell Microspores

Megaspore Mother Cell Megaspores


  • 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:-

  1. Prophase I

  2. Metaphase I

  3. Anaphase I

  4. Telophase I

 

1. 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.

 

2. 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.

 

3. 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.

 

4. 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.

1. Prophase II

  • Chromatin condenses to form chromosomes.

  • Nuclear envelope disappears.

2. 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.

3. Anaphase II

  • Simultaneous splitting of centromere of each chromosome

  • Sister chromatids move towards opposite pole by shortening of Microtubules.

4. Telophase II

  • Nuclear envelope reforms

  • Chromatin decondenses.

  • Cytokinesis follows and as a result 4 haploid cells are formed.


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