Question

Compare and Contrast Mitosis and Meiosis

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Answer 1

MITOSIS AND MEIOSIS

Cell division is a series of events in which a cell divides to form daughter cells and they are of 2 types ;mitosis and meiosis .

MITOSIS

• Also known as Equational division.
• Occurs in somatic cell;hence known as somatic cell Division also.
• W Flemming coined the term mitosis,also he observed the mitosis in animal cell;while E.Strasburger observed mitosis in plant cells.
• In this type of cell division 2 daughter cells are formed.
• (Interphase ; resting phase between the two mitotic division ;also known as inter mitotic phase.Here the chromosomes do not divide and this is the longest phase of the mitotic cell cycle- divided into gap 1,S and gap 2 phases )
• MITOSIS can be divided into following stages ; namely - prophase ,Metaphase,Anaphase ,telophase.

Prophase

• Nuclear envelope ,Nucleolus - starts disappearing.
• DNA duplicated in the S phase so have 2 chromatids will be present in each chromosome,and this chromatids become thickened and shortened.
• Cell becomes spheroid,refractive and viscous
• Each of the centriole separates and starts moving towards the opposite poles of the cell.

Metaphase

• Nucleolus and nuclear membrane completely disappears.
• Presence of spindle apparatus.
• Unique feature is ;each chromosome reaches the equator and arrange themselves radially at the periphery of the spindle(metaphase plate); larger chromosome remain at the periphery while smaller towards the interior.

Anaphase

• Centromere of each chromosome divides into two; and the chromatids of each chromosome gets separated.
• Chromosomes becomes shorter ,thicker and starts moving towards the opposite poles of the cell.

Telophase

• Reverse events of prophase takes place in this stage.
• Everything reappears including nucleolus,nuclear membrane etc.
• Movement of chromosomes will be completed towards the opposite poles and nuclear membrane starts forming around these each group of chromosome .

Cytokinesis

• In plants separation of cytoplasm begins at the centre and extend outwards,while in animal cell it will begins with invagination / furrowing from the sides;resulting in the formation of 2 daughter cells exactly.

MEIOSIS

• J B Farmer and Moore ;coined the term Meiosis in 1905.
• Also known as reductional division.
• Completed in 2 rounds unlike in mitosis; meiosis I and meiosis II ; resulting 4 haploid cells.
• Occurs in sex cells ; thus results in the formation of sperm cells in males and egg cell in females.

Meiosis I

• Here this stage is exactly known as the reductional division because of the separation of homologous chromosomes.Also the chromosomal number is reduced to half.
• Divided into following stages ;

a. Prophase I - longest phase and divided again into following substages ;

• Leptotene ; (meaning - thin thread ) - chromosome becomes uncoiled and appears in thread shape ; centriole duplicates ; each daughter centriole moves to the opposite poles of the cell.
• Zygotene ;( meaning - paired threads) - Synapsis occurs(pairing of chromosomes) ; thus resulting in the formation of bivalents.Also characterized by the presence of synaptonemal complex.
• Pachytene ;( thick thread ) ; crossing over takes place ( exchange of hereditary material between 2 homologous chromosomes) ; accompanied by chiasmata formation
• Diplotene - (two thread ) - separation of the homologous chromosomes takes place except at one point called chiasma .
• Diakinesis - separation completes via terminalization of chiasmata .

b.Metaphase - I ; homologous chromosomal pairs (tetrads) align at equatorial plane and each homologous pair attaches to spindle Fibres produced by kinetochore.

c.Anaphase I ; Homologous pairs of chromosomes (not the sister chromatids ) are pulled to opposite poles by spindle Fibres .Thus each daughter cell has one chromosome having 2 sister chromatids.The sister chromatids remain together at meiosis I and separates in Meiosis II.

d.Telophase - I ; Nuclear membrane is formed around each Nucleus of the daughter cells.The cell thus divides through cytokinesis into 2 daughter cells; each receiving a Nucleus containing haploid number of chromosomes.

Meiosis II / Equatorial division

• Resembles mitosis .
• Here Division of haploid cells occurs unlike in mitosis (difference ).
• consists of following phases ;

a. Prophase II ; centrioles migrate to opposite poles and formation of spindle apparatus.

b.Metaphase II ; Chromosomes line up along the equatorial plate.

c.Anaphase II ; Shortest phase of meiosis II ; sister chromatids are separated and pulled to opposite poles by spindle Fibres.

d.Telophase II ; Nuclear membrane reformed around each new haploid Nucleus .Cytokinesis results in formation of 2 daughter cells ;thus totally 4 daughter cells - so from single diploid cell 4 haploid daughter cells are produced.