Cell cycle

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;

• Interphase (Resting phase)
• M-phase (Dividing phase)

Interphase

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- phase or first growth phase
• S-phase or synthetic phase
• G₂-phase or second growth phase

G₁-phaseA slight increase in the size or volume of the nucleus.

• Pooling of amino acid, nucleotides(monomer of nucleic acid) and energy molecules ATP.
• Synthesis of RNA.
• Synthesis of carbohydrate, protein, lipid etc.

S-phase

• Replication of DNA and synthesis of histone protein.
• After replication DNA amount becomes double from 2n to 4n in a diploid cell.
• At this stage, each chromosome consists of two chromatids which are joined at the same centromere.

G₂-phase

• Synthesis of RNA and protein continue.
• Duplication of cell organelles like mitochondria, chloroplast, centrioles etc.
• Synthesis of spindle protein.

M-phase or Dividing phase

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.

• Prophase
• Metaphase
• Anaphase
• Telophase

Duration of cell cycle

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

• Bacterial cells divide in every 20 minutes
• The epithelial cells of intestinal wall divide in the period of 8- 10 hours.
• The onion root tip cells complete their cell cycle in 20 hours.

This indicates that the time required for every stage has been pre-set within the cell.

Types of cell division

Mainly there are three types of cell division they are;

• Amitosis(direct cell division)
• Mitosis(indirect cell division)
• Meiosis (reductional division)

Amitosis

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.