Cells division, Mitosis ,karyokinesis, cytokinesis

Cells is the structural and functional unit of living organism and all cells originate division of pre-existing cells.

Cell division is the division of parents cells into two daughters cells is known as cell division.

Function of cell division.

1. growth and development of somatic tissue of organisms.
2.  regeneration of damage tissue.
3.  production of new tissue.
4.  reproduction.
5.  keeping the size of cells within a limited range.

The division of pre - existing cell into daughter cells involves mainly two broad events -

 a. Karyokinesis i.e. division of chromosomes of a cell into their daughter cells.

 b. Cytokinesis: division of cytoplasm of a cell into their daughter cells.

 The combined process Karyokinesis and Cytokinesis is known as cell division.

 In Eukaryotic cells there are two types of cell division: 1. Mitosis and 2. Meiosis In addition, bacterial cells divide by fission (similar to mitosis).

 Mitosis:

 The term mitosis was coined by Flemming in 1882. It occurs in somatic cells hence also called as somatic cell division. It involves nuclear division into two daughter nuclei each of which contain the same number and kind of chromosome in such a way that they are genetically identical to the parent cell. 

Important features of mitosis:

 1. It lead to production of two daughter cells from a mother cell in each cell cycle

 2. The daughter cells are similar to mother cell in shape, size and chromosome complements so this is also called homotypic or equational division

 3 In plants mitosis take part in somatic organs like root tip, stem tip and leaf base which lead to growth of vegetative parts on the basis fo chages in the morphology of necleus and the chromosomes,

 the events in a mitotic cell division are grouped into five stages;

 i. Interphase

 ii. Prophase

 iii. Metaphase

 iv. Anaphase

 V. Telophase.

 INTERPHASE

 in this stage of cell after the telophase of previous division and before oneset of prophase of the next one. During interphase, chromosomes are fully extented and uncoiled so that they do not take up sufficient stain. Interphase in the longest stage. In a cell undegoing mitosis every 24 hours i.e. having a cell cycle of 24 hr. , interphase may occupy upto 23 hours, while the division or mitotic phase may take up onluy 1 hour.

 DNA replication occurs during the middle part of interphase. 

This provides the basis for classifying interphase into three substages.

 1. G1 (first gap)

 2. S (Synthesis of DNA)

 3. G2 (Second gap)

 G1, G2 - Protein + RNA synthesis

 G2 (12 hrs) S - DNA synthesis

 M - Chromosome movement