⨵ Meiosis

Learn about Meiosis.

Meiosis

Meiosis has two successive divisions (viz. meiosis I and meiosis II) with a short interphase in between them.
Before undergoing meiosis I during the interphase genetic materials are duplicated due to active DNA replication.
- Karyokinesis I (Meiosis I): Reductional division. It produces two haploid cells from a single diploid cell.
- Interphase (Interkinesis): No Duplication of DNA, short period.
- Karyokinesis II (Meiosis II): Equational division/simple mitosis.

Meiosis I/Karyokinesis I

It has a long prophase, metaphase, anaphase and telophase.

Prophase I

Prophase I is of longer duration than the mitotic prophase and most of the cytogenetically events such as synapsis, crossing over etc. take place during this prophase. It is the longest stage and has five distinct phases.

(a) Leptotene / Leptonema / Bouquet stage

Name stands for Thin Thread / Thin Thread
Each chromosome is longitudinally single with a large number of chromomeres. Chromomeres are the beads on the chromosome.
Chromomeres are discovered by Balbiani.
Chromosome appears as long thin (lepto) thread like structure hence the name leptotene or leptonema.
In some cells chromosomes appear and look like a bouquet hence the name Bouquet stage.
A definite type of orientation and polarization of chromosomes towards the centrioles takes place.

(b) Zygotene/Zygonema/stage of synapsis or syndesis

Name stands for Coupled yoked
Pairing of homologous chromosomes (i.e. synapsis/ syndesis) takes place called synaptonimal complex.
Each pair of homologous chromosomes is called bivalent/Tetrad. Bivalent is formed as a result of synapsis. If pairing takes place from terminal region, it is called polarization.
Synaptonemal complex appears and controls pairing, Synaptonemal complex is in between two homologous chromosomes (one is maternal and other is paternal chromosome) and discovered by Mosses at zygotene.
Nuclear Membrane and Nucleolus are present.

emoji="👀"

👉🏻 Homologues chromosomes means chromosomes with same size, shape and sequence of genes but alleles may differ.

👉🏻 Pairing at meiosis between two or more non-homologous chromosomes whether it be complete or partial from in an allopolyploid individual is called `allosyndesis`. RRB SO 2020

(c) Pachytene/Pachynema

Name stands for Thick
Longitudinal splitting of paired chromonemata along the length of chromonemata. The coiled filament of the chromosome which bears genes is called chromonema (chromonemata are plural) but there is no splitting of centromere.
Splitting of chromonemata into chromatids takes place and bivalent has four chromatids.
Chromatid + its own centromere = chromosome. As a result Tetrad stage (4 chromatids) of homologous chromosomes is found.
Dehydration and condensation of chromosomes occurs.
The exchange of chromosomal or genetic material between non-sister chromatids of each tetrad takes place. Such exchange of genetic materials between non-sister chromatids is called crossing over which is the cause of hereditary variations.
Crossing over leads to recombination of genes.
Stern and Hotta (1969) reported that endonuclease enzyme breaks the chromatid and ligase enzyme unites the broken non-sister chromatids.
A small amount of DNA is synthesised to repair the broken chromatids.
Percentage of crossing over helps in the preparation of genetic map or chromosome map. The unit of genetic map is centimorgan. 1 centimorgan means 1% crossing over. First genetic map was prepared by Sturtevant (of drosophila). Drosophila is the queen of genetics.
Nuclear membrane and Nucleolus are present.

(d) Diplotene/Diplonema

Name stands for Double
Repulsion between homologous chromosomes begins.
Terminalization of chiasma starts.
Synaptonemal complex disappears.
The point of crossing over (inter change) is called chiasma (plural-chiasmata) which is visible as characteristic X-shaped configuration.
Nuclear membrane and Nucleolus present.

(e) Diakinesis

It is the end of both diplotene and prophase I.
Terminalization is the characteristic feature.
At the end, Nuclear membrane and nucleolus disappear.
It is the best sub stage for counting the no. of bivalents.

Metaphase I

Nuclear Membrane and Nucleolus are absent.
Bivalents are at equator. Chromosomes are exactly in equatorial plane but not necessarily centromere.
Chiasmata are completely terminalized means absent.
Spindle fiber is connected with centromere only at one side.

Anaphase I

Centromere does not divide means no separation of centromere; no splitting of centromere.
Spindle fiber is connected with centromere at one side only.
V-shaped chromosomes move to their poles.
Reduction in number of chromosomes i.e. n of 2n.
Nuclear membrane and nucleolus are absent.
A tetrad is separated into diad and each diad has only one pair of chromatids.
Here is simply separation of chromosomes.
Every nucleus is in n-phase (half no. of chromosomes)

Telophase I

Chromosomes reached to the respective pole.
Formation of nuclear membrane and nucleolus after some period.
2 daughter nuclei are formed.

Interkinesis

Nuclear membrane and nucleolus become prominent.
No duplication of DNA.

Meiosis II (simple mitosis)

👉🏻 There is no prophase II. After certain period all the chromosomes lie on the equatorial plane immediately forming.

Metaphase II

Centromere are exactly on the equatorial plane.
Centromere are connected by spindle fibers on both sides.
Nuclear membrane and nucleolus absent.

Anaphase II

Longitudinal splitting of centromere.

Telophase II

Nuclear membrane & nucleolus reappear.
👉🏻 Cytokinesis separates each nucleus from the others and thus the process of meiotic nuclear division completes.