Cell cycle

Cell  cycle refers to all  the changes associated with cell growth and development. It is the series of changes through which cell duplicates it's genome ( a complete set of genetic information) , synthesizes the other components of cell and eventually divides into two daughter cells. The cell cycle is controlled by protein cyclins, Cyclin Dependent Kinases (CDKs). The interval between two cell cycles is called generation time.

Cell cycle consists of three main stages:

1) Interphase

2) M- phase or Mitotic phase 

The cell spends 75% to 95% of time in Interphase (Intermitosis or resting phase) while it spends rest of it's time in M- phase.

Interphase

It is the stage between end of one Cell division to the beginning of next cell division. Though cell seems to be resting in this stage but actually metabolically very active as cell prepares itself for next cell division by a number of biosynthetic activities. It consists of three sub- phases.

i) G₁ phase (Gap 1 or first growth phase)

• Longest of interphase.


• Growth and general metabolic activities of cell takes place


• Cell nucleus enlarges.


• Cell organelles multiply in number


• Energy rich compounds (carbohydrates, lipid and proteins) and RNA formed.

ii) Synthetic phase or S- phase

• Replication of DNA and histone protiens synthesis occur.


• Chromosomes with two chromatids are held together by centromere.

iii)G₂ phase ( Gap-2 phase or second growth phase)

• Replication of DNA stops.


• RNA and protein synthesis takes place.


• Cell organelles multiply in number. 

To be noted
At the end of G₁ phase, the cell has two options either to continue the cell cycle and enter into S phase or to stop cell cycle and enter G_° phase ( where cell cycle is arrested due to scarcity of mitogens and energy rich compounds). The deciding factor is availability of mitogens and energy rich compounds and the deciding point is called Check point. The next check point lies between G₂ phase and M phase where the abundance of RNA and proteins required for spindle formation and cell growth is checked.

 M- phase ( Mitotic phase)

• Phase of actual division


• Sub divided into

I) Karyokinesis : Karyokinesis is the division of nucleus and completes in 4 phases : Prophase, Metaphase, Anaphase and Telophase. 

II) Cytokinesis: Cytokinesis is the division of cytoplasm. In plant cells cytokinesis takes place by cell plate method whereas in animals it takes place by cleavage or constriction method.

Duration of cell cycle 

The duration of Cell cycle depends upon the type of Cell and external factors such as temperature, food and oxygen supply. Normally human cells divide in every 24 hours ( 10 hrs in G₁ phase, 9 hours in S phase, 4 hrs in G₂ phase and 1 hr in M phase) .