All the series of changes that occurs during cellular growth and cell division are collectively called cell cycle. It consists of two distinct phases they are;
It is the period between the end of one cell division to the beginning of the next cell division. It is also called resting phase because no visible changes occur during this phase. It is metabolically or synthetically active phase in which there are cellular growth and cell division. It is a long phase and is divided into three sub-phases;
G1-phaseA slight increase in the size or volume of the nucleus.
In this phase, the cellular materials already synthesized during interphase are equally distributed into daughter cells. This occurs through karyokinesis ( the division of nucleus) and cytokinesis ( the division of cytoplasm). The karyokinesis occurs in the four stages i.e.
The duration of cell cycle depends on upon the type of the cell and external factors like temperature, food and oxygen supply. The time period for the completion of G1- phase is 30-40 percent, S-phase is 30-50 percent and G2phase is 10-20 percent and M-phase is 5-10 percent. The duration of cell cycle for some cells are given below
This indicates that the time required for every stage has been pre-set within the cell.
Mainly there are three types of cell division they are;
It is a primitive type of cell division in which a mother cell directly divides into two daughter cell without the formation of chromosomes and spindle fibres. It occurs in organisms like protozoans, yeast, bacteria.
Process of amitosis
This division occurs by nuclear and cytoplasmic division. The nucleus of the mother cell constricts in the middle and appears dumbbell shaped. Gradually the construction grows dipper and divides nucleus into two parts. This is followed by cytokinesis which occurs by construction method.
Drawbacks of amitosis
This division results in the unequal distribution of genetic materials into the daughter cells.