Cell Division | What is Meiosis? Process and Different Stages Complete

Cell Division | Meiosis, Process and Different Stages Complete

Meiosis important facts:- 

• Two spindle using divisions which reduce chromosome number from diploid to haploid constitute meiosis.
• Main function of meiosis is to produce gametes in an organism.
• 4 daughter cells (from a mother cell) identical to mother cell in shape and size but different in chromosome composition.
• Neurosis occurs in reproductive organs like anther and ovary.

Interphase:-

• Interphase consists G1, S and G2 phases.
• Interphase occurs only in once in the entire process of meiosis. (Not after meiosis 1)
• During S phase 99.7% DNA is synthesized present in nucleus rest of the 0.3% DNA synthesized in zygotene in stage.
• G2 phase is very short duration. (replication of mitochondria and chloroplast)

(A). First Meiotic Division:-

It results in reduction of chromosome number in each new cell to just half of the mother cell, so it is referred to as reductional division.

(1). Prophase 1:- 

It consists of 5 sub stages:-

(a). Leptotene:- 

• Chromosomes look like in thin threads under light microscope.
• Chromosomes are scattered throughout the nucleus in random manner.
• RNA and Protein synthesis also takes place in this stage.

(b). Zygotene:-

• Homologous chromosomes begin to pair.
• Chromosomes becomes shorter and thicker.
• Synthesis of remaining DNA 0.3% takes place in this stage.
• Synaptonemal complex also develops during this stage.

(c). Pachytene:- 

• Chromosomes look like bivalent. Each bivalent has two chromatids. Thus, each pair has four chromatids, which is known as Tetrad.
• Chromosome look like haploid number of chromosomes in the cell.
• Nucleolus is present and attached to a chromosome.
• Formation of chiasma and crossing over takes place.
• Crossing over takes place between non-sister chromatids.

(d). Diplotene:-

• Separation of homologous chromosomes begin.
• It starts at centromere and moves towards the end.
• Separating chromosome are attached at some points, these points are called chiasmata.
• Chromosome condensed and become shorter and thicker.
• Nucleolus decreases in size.

(e). Diakinensis:-

• This stage begins after terminalisation of chiasmata.
• Chromosomes are condensed.
• Bivalents are distributed throughout the cell.
• Nucleolus and nuclear membrane disappear and of diakinensis.

(2). Metaphase 1:-

• Spindle apparatus gradually organises.
• Bivalents arranged on equatorial plate.
• Centromere of each chromosome divides longitudinally.

(3). Anaphase 1:-

• From each chromosome, one chromosome move towards one pole and another one opposite pole.
• Sister chromatids of each chromosome remain attached at the centromere.
• Homologous chromosomes reach to the opposite pole at the end of this stage.

(4). Telophase 1:-

• Chromosomes uncoiled, relax and regrouping of chromosome occurs.
• Nucleolus and nuclear membrane reappears.
• Two haploid daughter nuclei are formed.

Cytokinensis:- 

Division of cytoplasm takes place by cell plate method in plants and by furrow method in animals.

(B). Meiosis 2:-

• Meiosis 1 results in reduction of chromosome number (haploid).
• Interface is very short. 
• It does not have a synthesis phase because each chromosome already contain two chromatids.
• Chromatids have alternate segments of non-sister chromatids due to recombination.

(a). Prophase 2:- 

• This stage is quite similar to mitosis, however there are several differences between them.
• There is no relational coiling sister chromatids.
• Chromosomes are much condensed and shorter and thicker than mitosis.
• At the end of prophase 2, nucleolus and nuclear membrane disappear and spindle apparatus organised.

(b). Metaphase 2:- 

• Chromosomesme arranged on equatorial plate.
• Two sister chromatids of each chromosome are distinctly separated from each other.
• Chromosome become more thicker and shorter.

(c). Anaphase 2:-

• Centromere of chromosome of observably divides longitudinally.
• Two sister chromatids of each chromosome begin to separate.
• This stage occurs when sister chromatids reach opposite poles.

(d). Telophase 2:-

• Chromatids and uncoiled, so that they assume the appearance of a loose ball of thread.
• Nuclear envelope is reorganized from the elements of endoplasmic reticulum and nucleolus reappears.

Cytokinensis:-

• Cytoplasm of each of two cells of a dyad divides into two daughter cells.
• One mother cell produce for haploid cells.

Thank You

Vikas Kashyap:)