MEIOSIS
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:-
Prophase I
Metaphase I
Anaphase I
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.
