Cell cycle and cell division// mitosis, meiosis and their significance

 

CELL CYCLE

 

Definition of cell cycle:

Every cell has a fixed life span. During this period it grows, performs its activity and finally undergoes division to form new cells. This entire life span of the cell is called ‘Cell Cycle’.

 

Another words, all those changes which occur during cell growth and cell division collectively called ‘Cell Cycle’.

 

Duration of cell cycle:

Duration of cell cycle can vary from organism to organism and also from cell type to cell type.

Example:

               Duration of cell cycle is 20 minutes for bacterial cell, 20 hours for onion root tip cells.

 

The cell cycle is divided into two phases:-

A.   Interphase

B.   M phase or Mitosis phase

 

A.  Interphase:

                                 Interphase is the period between end of one cell division and beginning of the next.

It is a preparatory phase because in this phase cell prepare itself for the next division.

The interphase divided into following three periods-

 

a.     G₁or Gap1phase

b.     S phase or Synthetic phase

c.      G₂ or Gap 2 phase

 

a.     G₁ or Gap1phase:

·        Gap1 phase is also known as Post mitotic phase or Pre-synthetic phase or First growth phase.

·        It is the first phase of Interpahse .

·        It occurs immediately after the mitosis.

·        It is the most variable period of the cell cycle.

·        Gap phase is characterized by increase in size of the cell due to high rate of biosynthesis.

·        In this phase RNA synthesis increases (required for protein synthesis), Protein synthesis increases (required for DNA replication) and also nucleotides synthesized (required for DNA synthesis during S- Phase).

·        Some cells which do not divide at all after birth for example- mammalian nerve cells, skeletal muscles etc. pass their entire life in G₁ phase and do not enter S- phase.

·        Such cells are said to be in G₀ state or Quiescent stage (inactive stage).

 

b.    S phase or Synthetic phase:

·        This phase comes after G₁ phase and also it is a second phase of Interphase.

·        DNA replication takes place during this phase.

·        Each chromosomes is now formed of two sister chromatids held together at centromere.

·        During this time the amount of DNA per cell doubles.

 

c.      G₂ phase or Gap 2 phase:

·        Gap 2 phase is also known as Post-synthetic phase or Pre- mitotic phase or Second growth phase.

·        It is a third and last phase of the Interphase.

·        It is characterized by increase in nuclear volume.

·        Synthesis of RNA (required for protein synthesis) and synthesis of protein, like tubulin (required for spindle fibre formation) takes place.

·        Synthesis and storage of ATP required for M-phase also takes place during this phase.

·        Mitochondria, Plastids also duplicate during this phase.

·        Damaged DNA is required in G₂phase.

 

B.  M-phase or Mitotic phase-

·        It is also called as Equational division.

·        It is the phase in which cell divides or reproduce to form new daughter cells.

·        Two daughter cells are produced from one parental cell.

·        This phase is consists of 2 phases-

1.     Karyokinesis- It is division of nucleus, which is completed in to 4 steps- Prophase, Metaphase, Anaphase and Telophase.

2.     Cytokinesis- It is division of cytoplasm.