Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as parts of a larger cell cycle. These are two types.
a. Mitosis
b. Meiosis
a. Mitosis: Mitosis or mitotic cell division or equational cell division is meant for multiplication of cells. Development of complete organism from zygote is by means of mitotic cell division. So mitosis is necessary for maintenance, growth, repair and continuity of life. It takes place in vegetative or somatic cells.
Function of Mitosis:
Surface Volume Ratio: Smaller cells have higher surface volume ratio and are, therefore, more efficient in exchange of materials. Increase in size decreases surface volume ratio and makes the cells less efficient. Mitosis restores the surface volume ratio.
Nucleo cytoplasmic index: For efficient control, there is a particular ratio between size of nucleus and that of cytoplasm. Increase in size of cell due to growth reduces this index.
Growth of multicellular organisms: It occurs by increasing the number of cells, rather than increasing cell size. This is because small sized cells are more efficient. A single cell zygote grows into full blown baby (6 x 1022) by repeated mitosis.
Maintenance of genetic constitution: Mitosis maintains a similar genetic constitution in all the cells of an organism through qualitative and quantitative equitable distribution of chromosomes.
Asexual reproduction: It helps in vegetative propagation and asexual reproduction of organisms. In unicellular organisms every mitotic division leads to asexual reproduction.
Maintenance of somatic variations: Somatic variations develop in somatic or body cells. Useful somatic variations can be maintained through vegetative propagation which involves in mitotic cell divisions.
Regeneration: Cells of upper layers of skin epidermis, lining of gut, old RBCs and WBCs regularly die. They are replaced by equal number of new cells. It is estimated that a young baby losses 5 x 109 cells every day.
* Mitosis is similar in different organisms providing that they are related to one another.
Healing of wounds: Wounds heal due to mitotic divisions of healthy cells surrounding the area of injury.
Differentiation: Production of multicellularity by mitosis opens an opportunity for differentiation or specialization of cells and division of labour.
Maintenance of Haploidy-Diploidy conditions: Mitosis maintains the diploid and haploid genetic constitution of parent cells in their daughters.
Evidence of basic relationship: The mechanisms of mitosis are similar in the majority of organisms, showing basic similarity and relationship among them.
b. Meiosis: Meiosis is a type of cell division occurring in the reproductive cell of sexually reproducing organisms. It was described by Strasburger (1888). As a result of meiosis, the parent cell with diploid (2n) number of chromosomes is reduced to the haploid number (n) in daughter cells. So meiosis is also called reductional division.
Functions of Meiosis:
* Meiosis results in the reduction of chromosome number to half in gametes.
* Meiosis is essential in the life cycle of all sexually reproducing organisms.
Maintenance of Chromosome number: Fertilization doubles the chromosome number during the fusion of two gametes. Meiosis halves the number of chromosomes and thus help in maintaining the same chromosome number generation after generation.
Crossing Over: Every pair of homologous chromosomes undergoes at least one crossing over or exchange of chromatid segments at the time of meiosis. Longer chromosomes may have several crossing overs. It produces recombination or new recombination of traits. This is highly useful for breeders in development of new improved varieties.
Gene Mutations: The chance of gene mutations increase during meiosis due to action of multienzyme complex of recombination nodules over the DNA strands. Mutations are fountain head of all variations.
Chromosome variations: Chromosomal aberrations, aneuploidy and polyploidy are caused by problems in disjunction and distribution of chromosomes during anaphase I. Such variations have helped in evolution of many forms of life through in majority of the cases they are extremely harmful.
Gametophytic Expression: Meiosis switches off the sporophytic information and switches on the gametophytic expression. The latter is essential for production of gametes and fertilization.
Basic similarity: Meiosis is essentially similar in all sexually reproducing organisms showing that they are related to one another.
