Types of Cell Division

Types of Cell Division
There are three types of cell division.
1. Mitosis
2. Meiosis
3. Amitosis
1. Mitosis (Somatic Or Vegetative Division)
The type of cell divides in which a parent cell divides into two daughter cell and each daughter cell gets same number of chromosomes from parent cell is called mitosis.
Phases of Mitosis
It can be divides into two phases.
1. Karyokinesis
2. Cytokinesis
1. Karyokinesis
Karyokinesis is the division of nucleus and occurs in four different phases.

• Prophase
• Meta phase
• Ana phase
• Telophase

1. Prophase
(Pro=before, phase=appearance)
Phrophase is the main phase of nuclear division it is the longest stage the following main change occur in nucleus and cytoplasm.
Changes in Nucleus

• The chromatin material in nucleus break into thread like structure called chromosome.
• As the prophase further proceeds Chromosomes become thick and short.
• Each chromosomes consist of two thread known as chromatids. These chromatids are attached together by a spherical body is known as Centromere or kinetochore.
• In the nucleus chromosome move to its peripheral, part and central part of nucleus remain empty.
• Nucleoli disappear.
• Nuclear membrane disappears and nuclear material is released in the cytoplasm.

Change in Cytoplasm

• The most important change which take place in cytoplasm is formation of mitotic apparatus.
• The formation of mitotic apparatus varies in plant cells and in animal cells.

In Animal Cell

• In animal cell two rod like structure found in cytoplasm cell as centrosome, each centrosome divides into two centriole.
• One centriole remain at its place while another one moves towards the opposite pole.
• Between the centriole thread like structure arise known as spindle fibre or nuclear spindle, which is of three types.

Continues Spindle Fibres
These fibers continuously running from one pole to other pole.
Half Or Discontinuous Spindle Fibers
These fibers running one pole to the broader region of cell.
Astral Fibers
These fibers arise from to Centriole and give the position to Centriole in the cytoplasm.
In Plant Cell
In higher plant and insects the Centriole is absent and spindle fibres are formed without it. In them the spindle appears with its fibers converging to piole regions comparable to the area occupied by Centriole in the animal cells.
2. Meta Phase
(Meta = after)

• At this stage centromere of chromosome attached with the equator of half or discontinuous fiber by kinetochore.
• All chromosomes arrange an alignment line and formed equatorial plane.
• Chromatids of chromosome attain their maximum thickness.
• The faces of chromatids are towards the opposite poles as they are ready for separation.

3. Ana Phase
(Ana = upper)

• Anaphase begins when centromere divide, separate the sister chromatids from each other.
• Two set of chromatids migrates toward the opposite pole due to the contraction of discontinuous spindle fibers.
• At the end of the anaphase, the two groups of chromatids reach the opposite pole.

4. Telo Phase
(Telo = end)

• When two sets of chromosome reach to opposite poles telophase begins.
• The daughter chromosomes become thin and long.
• The coil with each other forming network again.
• Spindle fibers start to disappear.
• Nuclear membranes reappear around each set of chromosome.
• Nucleolus reappears.
• Now at this time two daughter nuclei are formed in a cell.
• The process of karyokinesis completed.

2. Cytokinesis
The division of cytoplasm is called cytokinesis. There are two types of cytokinesis.
Cell – Plate Formation
In plant cell the division of cytoplasm begin with the formation of a structure called cell – plate at the equator. It grows outward dividing the mother cell into two daughter cells.
Cleavage
In animal cell construction or depression arise in the cell membrane from outer side to the inner side due to which cell divides into two daughter cell is known as cell-cleavage.
Important of Mitosis
i. No Change in Hereditary Materials
As there is no crossing over during this cell division, the genetic information remains unchanged generation after generation.
ii. Asexual Reproduction
All plants and some animals show asexual reproduction with the help of mitosis such as regeneration in starfish.
iii. Development
New organs are formed with the help of controlled and planned mitosis.
iv. Growth
Young individuals grow with the help of mitosis.
v. Healing of Wounds
Formation of lost organ (regeneration), healing of wounds and replacement of older cells takes place due to mitosis.
vi. Tissue Culture and Cloning
Tissue culture in plant and cloning in animal is carried out through mitosis.
2. Meiosis (Reduction Division)
(Meiosis => meioum => to reduce)
The type of cell division in which one cell divide into four daughter cells and each daughter cell gets half number of chromosome from parent cell called meiosis.
OR
Cell division in which one diploid cell divide into four haploid cells called meiosis.
Phases of Meiosis
Meiosis consists of two phases.
1. Karyokinesis
2. Cytokinesis
1. Karyokinesis
Karyokinesis is the divisions of nucleus. Karyokinesis in meiosis consist of two divisions.
i. First Meiotic Division
ii. Second Meiotic Division OR Mitotic Division
i. First Meiotic Division
In first meiotic division, one nucleus divides into two nuclei and number of chromosome reduce to half. It is consist of following stages.
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
a. Prophase I
Prophase I is the longest phase of meiosis consisting of a complicated chain of events. It further divide into following sub stages.

• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis

Leptotene (Slender)

• This is the first stage of meiosis.
• In this stage, cell is larger in size and has large nucleus.
• The chromosome become more uncoiled and seems as thin, long thread like shape.
• Beaded appearance found on chromosome at irregular interval is known as chromosome.

Zygotene (Joining)

• In this stage chromosome are shorter and thicker.
• The homologous chromosome come very close to each other and make their pair. This pairing of chromosome is called synapsis.
• The pair of homologous chromosome are called bivalents.

Pachytene (Thick)

• After pairing chromosome become thick and short.
• Each chromosomes of a bivalent forms two sister chromatids. Thsi called duplication.
• Now at this time, bivalent has four complete homologous chromatids called tetrad.

Diplotene (Double)

• The force of repulsion arise between the homologous chromosomes of bivalent and they start to separate from each other.
• But they remain attached at one or more point. Their point of contact are called chiasmata.
• By chiasmata process of crossing over occurs (crossing is a process by which transfered of genetic material occurs between non-sister chromatics of tetrad).

Diakinesis

• The separation processes of bivalent continue by a process called terminazation.
• In this process the chiasmala move from Centromere towards the end of the bivalent.

b. Metaphase I

• The discontinuous spindle fibers are attached to the Centromere of homologous chromosomes.
• All chromosomes arranged an alignment line and formed equatorial plane.

c. Ananphase I

• In this stage each chromosome of bivalent move toward the opposite pole due to contraction of half spindle fibers.
• At the end of this phase the chromosome are separated into haploid sets, on set being present at each pole.

d. Telophase I

• Nuclear membrane reorganized around each set at two poles.
• Nucleoli reappear thus two nuclei each with haploid number of chromosome are formed.

ii. Second Meiotic Division OR Mitotic Division
In second meiotic division, one nucleus further divide into two nuclei and number of chromosomes remain same. It consists of following stage.
a. Prophase II
b. Metaphase II
c. Anaphase II
d. Telophase II
a. Prophase II

• Nucleolus and nuclear membrane disappear.
• Centrioles duplicate and move towards the opposite pole.
• Spindle fibres are formed.

b. Metaphase II

• The discontinues spindle fibres get attached to centromere.
• Chromosomes are arranged at the equatorial plane.

c. Anaphase II

• The centromere divides and chromatide are separated from each other.
• Chromatids move to opposite poles on spindle fibre.
• Each chromatid is now called monad.

Importance of Meiosis
i. Crossing Over
Crossing over during meiosis brings about re-shuffling of genes. As a result new combination of character in the off spring occurs.
ii. Gamet Formation
Meiosis is responsible for the formation of haploid gametes and spores. Thus after fertilization, the constant diploid number of chromosomes is restored.
iii. Random Assortment
Separation of homologous chromosomes and their random distribution to different daughter cells, also bring about genetic variations, which are raw material for evolution.