Cell Division (science)

# Cell division
Cell division is a process of multiplication of cell by its division. It is of two major types. They are mitosis and meiosis.
1. Mitosis
Mitosis cell division is that in which one cell divides into two diploid daughter cells. It is also called educational cell division and mitotic cell division. It causes growth.
2. Meiosis
Meiosis cell division is that in which one cell divides into four haploid daughter cell. It is also called reductional cell division and meiotic cell division. It forms gamete.

1. Mitosis Cell Division
a. Cytokinesis
Cytokinesis is division of cytoplasm and karyokinesis is division of nucleus.
b. Karyokinesis
Karyokinesis of mitosis completes in the following four main phases:
Prophase
Metaphase
Anaphase
Telophase

Prophase
In prophase, nucleolus disappears, nuclear membrane is disappearing, centrosome fibres is started between the centrosomes. At the last of this phase, chromosomes make their duplicates.
Metaphase
In metaphase, nuclear membrane disappears, formation of spindle fibre is completed, chromosomes come at equator and attach and their spindle fibres by the help of their centromeres.
Anaphase
In anaphase, centromere decomposes into two parts and chromatids. Now the chromosomes move towards opposite poles due to contraction of spindle fibres towards the poles.
Telophase
Telophase, chromosomes reach at poles and form two daughter nuclei, the cell is rehydrated, finally the cell goes for cytokinesis and one cell divides into two diploid daughter cells.

# Significance of mitosis
   Significance of mitosis cell division are as the followings:
a. Helps in growth                          c. Replacement of cell and regeneration
b. Does asexual reproduction      d. Maintains genetic stability



# Meiosis Cell Division
   Karyokinesis
   Karyokinesis of meiosis cell division complete in 8 phases. They are-
Meiotic first division
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiotic second division
Prophase II
Metaphase II
Anaphase II
Telophase II

Prophase I
Prophase I of meiosis completes in five sub-phases. The changes that take place in the nucleus of cells in these sub-phases are mentioned below:
Leptotene [Thread stage]: Chromatin divides in particular number of chromosomes. The chromosomes are so thin that chromosomes are seen in the form of beads.
Zygotene [Paring stage]: Homologous pairs or bivalents are formed. The process of forming the pairs is called synapsis.
Pachytene [Thickening stage]: Chromosomes shorten and form tetrads. Crossing over between non-sister chromatids is started.
Diplotene [Duplication stage]: Crossing over is completed. The two homologous chromosomes tend to separate from each other but both remain attached at chiasmata.
Diakinesis [Moving apart stage]: Overlapped chromo;somes move away from each other.


# Significances of meiosis are:
It forms gametes for sexual reproduction.
It causes variation, which is the main unit of evolution. Variation is caused by crossing over.
It helps to make chromosomes’ number constant.

# Chiasmata
Chiasmata are the points at which paired homologous chromosomes remain in contact during the prophase I of meiosis cell division.

# Homologous chromosomes
Homologous chromosomes or bivalents are the pairs of identical chromosomes formed in the beginning of meiosis. One member of each pair comes from the female parent and other from the male.

# Synapsis
Synapsis is the process of pairing chromosomes in the first phase of meiosis in order to make homologous pairs.
# Crossing over
Crossing over is defined as an exchange of portions of chromatids between homologous chromosomes.

# Chromatids
Chromatids are the thread like strands formed from a chromosome during the early stages of cell division. Each chromosome divides along its length into two chromatids, which are held together at the centromere and separate completely at a later stage.

# Chromosomes
Chromosomes are the thread like structures, which are found in the nucleus of plant and animal cells (eukaryotic cells). They are made of protein and nucleic acid.

# Tetrads
Tetrads are the paired chromosomes in meiosis, after each chromosome has duplicated itself, and  the pair is visibly four stranded.


# Differences between mitosis and meiosis cell division


Mitosis cell division    Meiosis cell division (Meiotic First)      
1.  In Prophase, Chromosomes do not form bivalents.    1. Bivalents are formed.      
2. Crossing over does not take place.    2. Crossing over takes place.      
3. It is the longest phase mitosis cell division.    3. It is longer than prophase of mitosis cell division.      
4. In Metaphas, Centromeres are found equator    4. Centromeres are faced towards the poles.      
5. In C- Anaphas, Centromere divides to separate chromosome from a chromatid.    5. Centromere does not divide but chromosomes separate from their bivalents.      
6. Cytokinesis  starts.    6. Cytokinesis may or may not start.      
7. In Telophas, Cytokinesis takes place.    7. Cytokinesis may or may not take palce.      
8. Diploid nuclei are formed.    8. Haploid nuclei are formed.      
9. One cell divides into two daughter cells.    9. One cell divides into four daughter cells.      
10. Causes growth.    10. Causes formation of gametes.      
11. Completes in less phases (4 phases).    11. Completes in more phases (8 phases).      
12. Forms diploid cells.    12. Forms haploid cells.