INTERPHASE

It is the longest period of cell division and is also called the Intermitotic phase as it is the phase between two mitotic cycles.

Interphase: Animal Cell Diagram

Interphase Definition :

 Interphase is a stage between two mitotic cycles in an eukaryotic cell, during which various physical and chemical changes for the preparation of cell division take place.

There occur some complex changes in the newly formed cell before it is prepared for division. 

At this stage, the chromosomes are less visible as it is a highly hydrated condition. They appear as long and thin and form a diffused chromatin net.

Interphase: Condensation and division of chromatids in the cell cycle

Stages of interphase:

 The changes in cells during interphase can be marked and followed as three sub-phases or stages ( Howard and Pele 1953). They are the post-mitotic gap phase or G1 phase, synthetic phase or S- S-phase, and pre-mitotic gap phase or G2 phase.

 

1. Post- mitotic Gap phase ( G1 phase):

It is also called the first growth phase as it is the early stage of interphase. 

It occupies 30-50% of the cell cycle. Thus G1 is the longest phase of interphase. 

The events occurring are:

1. During interphase, the synthesis of RNA and various regulatory proteins occurs in the G1 phase.
2. Enzymes required for DNA synthesis like DNA polymerase are synthesised.
3. * Note: No synthesis of DNA takes place.
4. The nucleus elongates due to the accumulation of tubulin protein.

G0 phase:

 In terminally differentiated  (TD cells: The cells that, in the course of acquiring specialized function, have irreversibly lost their ability to proliferate and withdraw from the cell cycle)  somatic cells like neurons and muscle cells which stop dividing or rarely divide, the arrested G1 phase is known as the G0 phase. It is also called the resting phase, gap phase or quiescent phase.

The cells in this phase are not active in the preparation process. They are in the quiescent phase and exit the cell cycle.

 Some of the cells temporarily enter the G0 stage until they get external signals to go for the G1 stage. 

2. Synthetic phase (S-phase):

It constitutes 35- 45% of the cell cycle. It is the mid-interphase stage in which all major metabolic activities of dividing cells take place. The process is mediated by S-phase promoting factors including cyclin A (regulatory protein) and cdks (protein kinases). 

The major events occurring during this phase are:

1. *Note: DNA is synthesized in S-phase.
2. The volume of the nucleus is doubled due to DNA replication.
3. Histone proteins are synthesised.
4. The single chromatid of the chromosome gets duplicated forming two identical chromatids.

3. Pre-mitotic gap phase (G2 phase):

It makes up 10-20% of the cell cycle. It is the gap between the S-phase and prophase and is also referred to as the second growth phase.

The major events include:

1. Specific RNA and protein are synthesized.
2. Synthesis of DNA stops. 

Characteristics of Interphase:

The major characteristics of interphase are: 

1.  Replication of DNA and chromatids results in the formation of a double chromatid (diad) from a single chromatid (monad).
2. The nuclear membrane remains intact and distinct.
3. The nucleolus enlarges due to the accumulation of RNA and ribosomal protein.
4. Energy-rich ATP molecules and microtubules are synthesised which act as precursors of spindle fibres.
5. Cell size increases due to the accumulation of cytoplasm.
6. The chromatin network of thin and elongated chromatin threads appears diffused and less visible due to highly hydrated conditions.
7. In animal cells, the existing centrosome gives rise to a new centrosome with a pair of centrioles placed at right angles to each other. So on the whole, there are two pairs of centrioles within the cell.
8. Also in animal cells, the net membrane biosynthesis increases and gets stored at the cell surface in the form of "blebs" which help the cell to divide.

Importance of Interphase:

The intermediate stage between two successive divisions is known as interphase. The interphase occupies around two-thirds of the cell cycle and is of tremendous importance. 

1. During the G1 substage, the synthesis of RNA and protein occurs.
2.  The S phase is characterised by the replication of DNA thereby helping the chromosomes to divide at later stages.
3. The G2 phase is also of great importance as a synthesis of RNA, ribosomes and spindle fibre proteins occurs thus preparing for the division.
4. In animal cell, the centriole divide during this phase.
5.  In G1 G2 substages, there are two checkpoints which help in proper cell division.

Therefore during interphase, there occur many important changes which aid to a successful cell division.

Next related text:

Difference Between Mitosis and Meiosis

About the Author: This educational content on Zoology is written by Rekha Debnath, M.Sc. & M.Phil. in Zoology, with a focus on university-level academic topics. Read the full Author Credentials and Background here.