MITOSIS

– The term mitosis was first used by Flemming in 1882.

– Definition : The cell division in which each nuclear division is preceded by one round of DNA replication, during anaphase sister chromatids separate and pass to the opposite poles.

– This is the cell division in which one parent cell undergoes division to produce two daughter cells which are identical to each other and to the parent cell.
– Mitosis occurs in all somatic cells.

Importance of Mitosis

– Embryo develops from zygote through repeated mitotic divisions.
– In plants mitosis is confined to the merisiematic tissues of root and shoot tips, young leaves and cambium.
– Mitosis is responsible for healing of wounds and replacement of damaged organs in plants as well as animals.
– Mitosis leads for production of new organs e.g. root and shoot branches in plants.
– Production of progenies identical to the parent plant through asexual reproduction is possible through mitosis.
– Mitosis helps in production of male and fermale gamete as microspores and megaspores.

Stages of Mitosis

The mitotic cell division is grouped into five stages
i) Interphase, ii) Prophase, iii) Metaphase, iv) Anaphase, v) Telophase

1) Interphase

– The chromosomes are fully extended and uncoiled so that they do not take up sufficient stain.
– Nucleus is surrounded by a well defined nuclear membrane.
– Nucleolous is also present.
– Since chromosomes do not undergo any cytologically observable change during interphase it was believed that this is resting stage.
– But this is not resting stage since chromosome replication, protein and RNA synthesis occur during this stage.
– If a cell undergoes mitosis for every 24 hours, interphase may occupy upto 23 huurs while the division or mitotic phase may occur only for 1 hour.
–  The interphase is subdivided into three stages.
– 1) G¹ phase 2) S phase 3) G² phase

G1 phase : This is the beginning of interphone
– Protein and RNA synthesis takes place.
– The enzymes and substrates necessary for DNA synthesis during S phase ore syntbesized

S phase :
– It is the period of interphase during which DNA synthesis occurs as a result of which chromosome replicates.
– Each chromosome consists of a single chromatid during telophase and G, stage of interphase.
– DNA synthesis during S stage leads to the production of two nister chromatids which makes up a full chromosome.
– Since the two chromatids of a chromosome are produced by senticonservative replication a single parent Chromatid, they are known as sister chromatids.

G2 phase
– RNA and protein synthesis occurs during G2 phase
– The components of mierotubules are synthesized during this stage.
– Some proteins synthesized during this period are essential for the entry of cells into mitosis.

2) Prophase

– The chromcsomes appear like thread like structure in the nucleus is the most important event of prophase.
– In the beginning chromosomes appear as a loose ball of thin wool.
– As prophase progresses chromosomes become increasingly shorter and thicker due to increased condensation.
– By mid prophase the two chromatids of each chromosome become visible.
– At the end of prophase all the chromosomes become considerable shorter and thicker.
– During prophase nucleolus and nucleolar membrane remain present.

3) Metaphase

– Nucleolus gets disappear.
– Breakdown of nuclear membrane and distribution of its components into the endoplasmic reticulum occurs.
– Spindle fibres appear and attached to the centromere of each chromosome.
– Chromosomes are arranged on a single plane on equatorial plate.
– The chromosomes are shortest and thickest so they are clearly visible under compound microscope.
– At metaphase centromeres of all the chromosomes lie on the equatorial plate, while their arms may extend outside this plate.
– The movement of chromosomes and their orientation on the equatorial plate is known as metakinesis.

4) Anaphase

– During anaphase the two sister chromalids of each chromosome separate and migrate towards the opposite poles of the cell.
– The centromere of each chromosome appear to divide longitudinally 30 that tbs sister chromatids separate from each other.
– The centromere is the first part of each chromosome which begins to move to respective pole.
– The spindle fibres originate at two poles (South pole and north pole)
– In animal cells centrioles are found at both the poles and responsible for organization of spindle fillers.

5) Telophase

– Telophase begins when sister chrematids of all the chromosomes of a cell reach the opposite poles.
– The chromosomes undergo uncoiling so that they become very long and thin and appear as loose ball of fine thread.
– Nucleolous gets reorganize.
– Nuclear membrane is reorgnised around each group of chromosomes.
– At the end of telophase phragmoplast appears at the equatorial plate of the cell.

Cytokincsis

– Generally cytokinesis is completed by the end of telophase.
– At the equatorial plate three different kinds of membranous emements of endoplasmic retimulum and products of golgi bodies organize into the phragmoplası which gives rise to cell plate formation.
– In plants the separation of cytoplasm begins in the centre of the cell and gradually extends outwords in plane perpendicular to the axis of spindle.
– The two daughter cells thus produced by mitosis contain one nucleus each and has same number of chromosomes as the parent cell.
– Each daughter cell enlarges in size till it becomes comparable to the parent cell.

MEIOSIS

Defination : This is the cell division in which one parent cell (2n) undergoes division 10 give rise to four haploid daughter cells.

– Generally meiosis takes place during gamete formation and hence it is confined to reproductive cells only.

– Importance of meiosis :

– Meiosis leads to production of gametes with half of the somatic chromosome number (n)
– Therefore union of one male gamete (n) with one female gamete (n) during fertilization restores the chromosome number of diploid state.
– Thus the chromosome number of a species remain constant generation after generation produced by sexual reproduction.
– The sexual reproduction in plants and animals could be possible only because of cvolution of meiosis.
– In absence of meiotic cell division the chromosomo number of a species will be doubled in every next generation and there would be very complex biological status of the species.

– Diploid : An individual, organism or cell containing two sets of chromosomes.
– Haploid : An individual, organism or cell containing single sets of chromosomes.

Function of meiosis

1) Production of haploid gametes (n) so that fertilization restores the normal somatic chromosome number (2n) of a species.
2) Segregation of the two alleles of a gene due to pairing of homologous chromosomes.
3) Independent segregation of alleles of different genes located on the different homologous chromosomes.
4) Recombination between linked genes is possible due to crossing over during pachytene stage.
5) Generation of genetic variation through segregation and recombination is possible due to meiosis.

Stages of Meiosis

– During meiosis the nucleus of parent cell undergoes two successive divisions.
– The two nuclear divisions referred as meiotic first and meiotic second divisions.
1) Interphase -I,
2) Prophase -I,
3) Metaphase -I,
4) Anaphase -I,
5) Telophase -I.

1) Interphase – I
– G@ phase – RNA and protein synthesis take place.
– S phase – The chromosome replication takes place.
– But approximately 0.03% of total DNA present in the nucleus does not replicate during S phase.
– This DNA is replicated during the zygotene substage of prophase I
– G2 – The entry of cells into meiosis also depends on some substances synthesized during S phase.

2) Prophase – I
– This stage of meiotic division is longest in duration.
– On the basis of chromosome morphology and their movement this stage is subdivided in to.
a) Leptotene, b) Zygotene, c) Pachytene, d) Diplotene, e) Diakinesis.

a) Leptotene
– There is narked increase in the nuclear volume.
– Chromosome condensation occurs so that they become visible as loose ball of thin wool
– Each chromosome consists of two chromatids.
– Proteins are synthesized which are required for chromosome condensation

b) Zygotene
– This stage begins with the initiation of pairing between homologous chromosomes.
– The remaining 0.03% DNA replication occurs which is referred as Z-DNA synthesis.
– Specific nuclear protein is synthesized.
– There is development of synptonemal complex.
– There is progressive condensation of chromosome takes place.
– Pairing of homologous chromosomes is extremally precise.
– Synapsis : the paring of homologous chromosomes is called as synapsis.

c) Pachytene
– It beigns when synapsis comes to an end.
– There is further condensation of chromosome occurs.
– The chromosome pairs become shorter and thicker.
– As a consequence of pairing (Synapsis) of homologous chromosomes the number of observable chromosomes is only half of the somatic chromosome number.
– They are known as bivalents.
– Since each of the two chromosomes of bivalent consist of two sister chromatids cach bivalent has four chromatids therefore they are said to be in tetrad stage.
– The chromosomes are easily observed during this stage.
– The crossing over between homologous chromosomes occurs during this stage only.

d) Diploten

e- The homologous chromosomes of each bivalent begin to move away from each other.

– The two homologous chromosomes appear to be attached to each other at one or more points, these points are known as chaismata.
– As diplotene progresses chaismata slowly move towards the ends of homologous chromosomes this movement is called as chaismata terminalization.
– There is further condensation of chromosomes occur so that they become progressively shorter and thicker.

e) Diakinesis

– The end of chaisma terminalization marks the beginning of this stage
– The two homologous chromosomes of each bivalent are now attached at or close to one or both the telomeres only.
– Chromosomes become shorter and thicker due to further condensation.
– The bivalents move away from cach other and spread lowards the periphery of cells
– Nucleolous and nuclear envelope disappear.
– The spindle fibres get organized.

3) Metaphase I:

– Nucleous disappears and nuclear membrane distengrates and its elements become parts of endopasmic reticulum.
– All the bivalents migrate to the equatorial plate.
– Centromeres of the two homologous chromosomes of each bivalent become attached to the chromosomal fibres originating from the opposite poles.

4) Anaphase -I

– Separation of two homologous chromosomes of each bivalent marks the beginning of anaphase.
– One chromosome from each bivalent begins to migrate to one pole while the other migrate to the opposite pole.
– As a result of which each pole receives half of the somatic chromosomes.
– The homologous chromosomes reach the opposite poles.

5) Telophase -I

– The chromosomes uncoil only partially.
– Nuclear envelope becomes organized around the two groups of chromosomes.
– Nucleoulus also reappears.
– In some specius e.g. Trillium cells enter directly into the second prophase from the first anaphase.

Cytokinesis I

– The cytoplasm of each cell divides into two halves by the end of first telophasc.
– In cach of the two halves of cell a single haploid nucleus is present.
– The two halves of each cell do not separate.
– They stay together and this two celled structure is called as dyad.
– In some cases cytokinesis occurs only at the end of second meiotic division.

Interphase – II

– In many species the interphase after first meiotic division is absent.
– If it is present it is of shorter duration.
– Chromosome do not uncoil any further than at telophase I

Second Meiotie division

– During second meiotic division the two sister chromatids of each chromosome separate and migrate to the opposite poles.
– As a result the number of chromosomes in each of the two haploid nuclei remains the same at the end of this division.
– Therefore it is also known as equational division.
– The second meiotic division is divided into four stages.
1) Prophase II, 2) Metaphase II, 3) Anaphase II, 4) Telophase II.

1) Prophase II

– The two sister chromatids of each chromosome are clearly visible.
– The chromosomes are much more condensed, hence they appear shorter and thicker.
– At the end of this stage nucleolous and nuclear envelope disappear.

2) Metaphase – II

– Spindle fibres get organized.
– The centromere of each chromosome is arranged in single plane at the equatorial plate.
– The two sister chromatids of each chromosome are separated from each other except at centromere due to repulsion between them.
– The chromosomes are shortest in length and thicker in diameter.

3) Anaphase II

– The cenromeres of each chromosome divide longitudinally.
– So that the two sister chromatids of cach chromosome begin to separate and move to the opposite poles.
– This movement of sister chromatids occur due to the force of spindle apparutus.

4) Telophase – II

– The sister chromatids of all the chromosome reach to the opposite poles.
– The chromatids undergo uncoiling so that they appear as loose ball of thin wool.
– Nuclear envelope get reorganized from the elements of Endophasmic Reticulum.
– Nucleolous also reappears.

Cytokinesis – II

– This stage begins when the cytoplasm of each of the two cells of a dyad divides into two parts.
– As a result, one parent cell produces four haploid daughter cells after completion of meiotic cell division.
– These four daughter cells present together are known as tetrad.
– The four haploid cells produced by meiotic division may differentiate into gametes (animals) or spores (plants).