• Meiosis is a cell division in which four haploid cells are formed from a single diploid cell.
• It usually occurs in reproductive organs or gonads of the organisms.
• Meiosis is also known as reductional cell division because four daughter cells produced contain half the number of chromosomes than that of their parent cell.

Meiosis has two nuclear division phases:

1. Meiosis-I (Reductional or Heterotypic division)
2. Meiosis-II (Equational or Homotypic division)

Meiosis-I (heterolytic or Reductional division)

Meiosis-I has four different phases or stages:

1. Prophase-I
2. Metaphase-I
3. Anaphase-I
4. Telophase-I

1. Prophase-I

• It occupies the longest duration in Meiosis-I.
• It is divided into five sub-stages or sub-phases.

i. Leptotene

• This phase starts immediately after interphase.
• The size of cell and nucleus increases
• The chromosomes appear long, uncoiled thread-like in structure bearing many bead-like structures called chromomeres.
• The nuclear membrane and nucleolus remain as it is.

 ii. Zygotene

• Homologous chromosomes come closer and starts to pair up along their length.
• The pairing of homologous chromosomes is called Synapsis and the paired homologous chromosomes are referred as bivalents.
• The homologous chromosomes are held together by ribonuclear protein between them.

 iii. Pachytene

• The chromosome become shorter and thicker.
• Each chromosome of the bivalents splits longitudinally to form two chromatids such that bivalents is composed of four strands and is known as a tetrad.
• The process of crossing over starts (crossing over; a small fragment of chromosome exchange between two non-sister chromatids of bivalent by breakage and rejoining).
• Crossing over is the most important genetic phenomenon of meiosis which causes variation in genetic characters in offspring.

iv. Diplotene

• In this stage crossing over takes place.
• Bivalents (chromatids) repel each other.
• Homologous chromosome (two non-sister chromatids) begins to separates but separation is not complete, they remains attached to a point with a knot like structure called chiasmata (singular – chiasma).
• The number of chiasmata varies. Depending upon the number of chiasmata, chromosome appears different shape.
  • 1 chiasmata: cross like
  • 2 chiasmata: ring like
  • Many chiasmata: series of loop
• Nuclear membrane and nucleolus begins to disappear.

v. Diakinesis

• The chiasma moves towards the end of the chromosomes (tetrad) due to contraction of chromosomelastly slips over separating the homologous chromosome. This movement of the chiasmata towards the end of chromosome is called terminalization.
• By the end of diakinesis the nuclear membrane and nuleolus get completely disappeared and the chromosomes are free in the cytoplasm.
• Spindle fibres begin to form

2. Metaphase-I

• The spindle fibres organized between two poles and get attached to the centromere of chromosomes.
• Chromosome moves to equator
• The bivalent chromosomes are arranged in the equatorial plate in such a way that 2 metaphasic plates are formed.

3. Anaphase-I

• Spindle fibres contracts and pulls the whole chromosomes to the polar region.
• The separated chromosome is known as dyads
• No splitting of chromosomes occurs so the centromere of each homologous chromosome does not divide. Thus, the chromosome number of the daughter nuclei is reduced to half.
• Now the separated chromosome moves toward opposite poles.

 4. Telophase-I

• Two groups of chromosome formed at each pole and organized into nuclei.
• The nuclear membrane and nucleolus reappears.
• The chromosomes get uncoiled into chromatin thread.
• The spindle fibres disappear totally.

Cytokinesis I

• Cytokinesis may or may not follow nuclear division (meiosis-I Cytokinesis occurs by cell plate formation method in plant cell and furrowing method in animal cells.

Interphase II or Interkinesis

• The two cells or nuclei thus formed pass through a short stage called interphase-II. Sometimes, interphase-II is absent.
• It is the resting phase between meiosis-I and meiosis-II.
• It is either very short or may be absent
• No DNA synthesis occurs.

Meiosis-II ( Homolytic or equational division)

• Meiosis-II is exactly similar to mitosis, so it is also known as meiotic mitosis.
• In this division, two haploid chromosome splits longitudinally and distributed equally to form 4 haploid cells.
• It completes in 4 stages.

1. Prophase-II
2. Metaphase-II
3. Anaphase-II
4. Telophase-II

1. Prophase-II:

• The dyads chromosome becomes thicker and shorter
• Nuclear membrane and nucleolus disappear
• Spindle fibre starts to form

2. Metaphase-II:

• The dyads chromosomes comes to equatorial plane
• Spindle fibres organize between poles and attaches to centromere of chromosome.

3. Anaphase-II:

• Centromere of each chromosome divides and sister chromatids separates to form two daughter chromosome
• Spindle fibre contracts and pull the daughter chromosome apart towards opposite pole.

4. Telophase-II:

• Chromosome become organize at respective pole into nuclei
• Chromosome elongates to form thin networks of chromatin
• Nuclear membrane and nucleolus reappears

Cytokinesis-II:

• The result of cytokinesis is four haploid daughter cells (gametes or spores).
• Cytokinesis takes place by cell plate formation in plant cell
• Successive methods: cytokinesis followed by each nuclear division resulting in 4 haploid cells. Eg. Monocot plants
• Simultaneous methods: cytokinesis occurs only after meiosis-II to form 4 haploid cells. Eg. Dicot plants
• In animal cells, cytokinesis occurs by furrow formation or depression.

SIGNIFICANCE OF MEIOSIS

1. Meiosis helps to maintain a constant number of chromosomes by reducing the chromosome number in the gametes
2. Essential for sexual reproduction in higher animals and plants
3. Meiosis helps in the formation haploid gametes and spores for sexual reproduction.
4. Number of chromosome remain fixed in a species from generation to generation
5. Crossing over occurring brings genetic variations in offspring which helps in evolution of organisms.
6. Failure disjunction in Meiosis leads mutation to the formation of polypoid forms.
7. The random distribution of maternal and paternal chromosomes takes place into daughter cells during meiosis and it is a sort of independent assortment which leads to variation.

Meiosis: Meiotic cell division, stages and significance

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• Mitosis is a type of cell division in which single haploid cell (n) or diploid cell (2n) divides into two haploid or diploid daughter cells that are same as parent.
• Mitosis occurs in somatic cells of plants and animals. In this cell division, the two daughter cells have same number of chromosomes as that in the parent cells.

The process of mitosis consists of the following stages or phases:

1. Interphase or Interkinesis
2. Karyokinesis
3. Cytokinesis

1. Interphase or interkinesis

• Interphase is the phase between two successive cell division (end of one cell division to the beginning of next cell division).
• It is the longest phase in the cell cycle.
• Interphase looks dormant but it is metabolically active stage.

It is divided into 3 sub-stages viz. G₁-phase, S-phase and G₂-phase.

i. G₁-Phase or Gap-1 phase

• The cell grows in size due to active biosynthesis.
• Formation of structural and functional proteins.
• Synthesis of mRNA, tRNA and rRNA takes place.

ii. S-Phase or Synthetic phase

• Replication of DNA takes place.
• Synthesis of histone proteins takes place which covers DNA.

iii. G₂-Phase or Gap-two phase or Second growth phase

• RNA and protein is synthesize.
• Centrioles get replicate (in case of animal cell)
• Synthesis of spindle proteins takes place.

 2. Karyokinesis

• Karyokinesis is the division of the nucleus.
• It consists of the following four phases.

i. Prophase

• It is the first visible stage in karyokinesis.
• The chromosomes appear as long coiled threads called chromatids.
• The chromatin becomes shorter, thicker and visible due to the condensation of DNA.
• The chromatins are now called chromosomes.
• Stainability of nucleus increase.
• Each chromosome starts to splits longitudinally into two sister chromatids. These sister chromatids are attached with each other at centromere.
• The nuclear membrane and nucleolus starts to disappear and by the end it will completely disappeared.

ii. Metaphase

• Nuclear membrane and nucleolus completely disappears and simultaneously appearance of spindle fibres
• Spindle fibres attached to the centromere of chromosome.
• The chromosomes are arranged on the equatorial plane.
• The process of gathering of chromosomes in equator is called congression and plate formed is called metaphasic plate.

iii. Anaphase

• The centromere of each chromosome splits into two sister chromatids and forms two daughter chromosomes.
• The daughter chromosomes are pulled towards the poles due to the contraction of spindle fibres and stretching of inter zonal fibres.
• During polar movement, the chromosomes shows different shapes i.e. J,U,V,L or I shaped in appearance.
• At the end of anaphase, each pole will get one set of daughter chromosomes.
• It is shortest phase and is also known as migratory phase.

iv. Telophase

• The daughter chromosomes reach respective poles and uncoil and become thin, long and visible.
• The spindle fibres start disappearing and finally disappear.
• The nuclear membrane and the nucleolus reappear.
• Two nuclei are formed at the end of telophase. Both the nuclei have the same number of chromosome as parent cell.
• It is the last visible stage of karyokinesis and is also known as reorganization phase.

3.  Cytokinesis

• Cytokinesis is the division of the cytoplasm.
• In plant cells, cytokinesis occurs by cell plate formation.
• During cytokinesis, many granular matrix formed by the golgibody and endoplasmic reticulum accumulates in the equatorial region. These granular matrix form cell plate. This plate divides the cell and by the end of telophase, cytokinesis is completed.
• In animal cells, cytokinesis occurs by cleavage or furrow formation.

SIGNIFICANCE OF MITOSIS:

• Mitosis produces 2 genetically identical cells, so mitosis maintains the genetic stability of organisms.
• DNA remains constant, so mitosis keeps the chromosomes number constant in a species.
• Mitosis helps in the development of multicellular organism.
• Mitosis helps to replacement of old, dead or damaged cells by new one.
• It helps in the recovery of wounds and injury of the body by formation of new cells.
• In unicellular organisms like Yeast, Paramecium , mitosis is a means of asexual reproduction.
• Mitosis causes maturation and multiplication of germ cells and makes them ready for meiosis.

Mitosis: mitotic cell division, stages and significance

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