3 main types.

(1) Mitosis (2) Meiosis (3) Amitosis

Term mitosis was proposed by Flemming & its detail study was given by A. Schneider. Mitosis produced genetically identical cells, which are similar to mother cell.

(I) Kern plasm theory: Hertwig proposed kern plasm theory. According to this theory mitosis occurs due to disturbance in Karyoplasmic Index (KI) of cell.

Karyoplasmic Index :

`KI= (V_n) / (V_(c) - V_(n) )` `V_(n) =` Volume of nucleus

`V_c= ` Volume of cell

`V_(c) - V_(n) =` Volume of cytoplasm

Karyoplasmic Index of small cell is high as they have less cytoplasm. Nucleus efficiently controls the acitivity of cytoplasm in small cells.


In a large cell nucleus fail to control the activity of cytoplasm. To attain the control of nucleus on metabolism a large cell divides into two cells.

`->` Surface-volume ratio of a cell plays an important role in starting cell division.

`->` A cell draws all the materials needed for its maintenance & growth from its surface. When a cell grows in size its volumes increases more than its surface. So a stage will reach when the surface area becomes insufficient to draw the material. At such critical stage, division of cell started.

`->` Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place .

`->` All these processes, i.e., cell division, DNA replication, and cell growth, hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes .

`->` The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle .

`->` Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle .

`->` The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control. Complete life cycle of a cell is called as cell cycle.

`->` A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours.

`->` Yeast can progress through the cell cycle in only about `90` minutes. The time period of cell cycle is
varied from organism to organism and also from cell type to cell type.

Cell cycle involves two stages.

(1) Interphase

(2) Division phase/M-phase

This is phase between two successive M-phase. In interphase cell grows in size and prepares itself for next division. Interphase is most active phase of cell cycle.

The interphase laste more than 95lX> of the duration of cell cycle.

`->` In interphase, metabolism of cell increases. A series of rnetabolic changes occurs during interphase in These changes are not visible under microscope, So some scientist termed interphase as resting phase. But it's known that it is the most active phase of cell cycle.

`->` Howard and Pelc classified interphase into three sub stages :-

(i) `G_1` -phase or Pre DNA synthesis phase (Ist Gap phase)

`G_1` phase corresponds to the interval between mitosis and initiation of DNA replication. During `G_1` phase the cell is metabolically active and continuously grows

During `G_1`-most of cell organelles increases in cell and cell rapidly synthesizes different types of RNA proteins. Due to availability of protein, synthesis of new protoplasm takes place in cell and it starts growing in size. Cell grows maximum in `G_1` stage.

(ii) S-phase (DNA synthesis phase) :

`->` Replication of nuclear DNA and synthesis of histone protein takes place ins-phase. Replication of cytoplasmic DNA may occur in any stage of cell cycle.

`->` During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then l!
increases to `4`C. However, there is no increase in the chromosome number; if the cell had diploid or 2n
number of chromosomes at `G_1`
, even after S phase the number of chromosomes remains the same, i.e., `2n`


`->` S-phase marks the phase of DNA replication and chromosome duplication (DNA content in a chromosome become double).
`->` In animal cells, during the S phase. DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. Centriole replicates in late S-phase.

`->` Actual preparation (Final preparation) of M-phase occurs during this phase. Special materials required for M phase are synthesized in `G_2` phase. eg. Tubulin protein. -{Required for formation of spindle fibres) .

`->` During this phase proteins are synthesised in preparation for mitosis while cell growth continues.

`->` After `G_2` phase cell enters in division or M-phase .

`->` `G_0` phase - Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit `G_1` phase to enter an inactive stage called quiescent stage (`G_0`) of the cell cycle .

`->` Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.