Interphase – Definition and Stages with Examples
It is a long, non dividing, growing phase of the cell cycle, where the cell prepares itself for division. It represent the most active stage of the cell cycle where both cell growth and DNA replication occur in an orderly manner. It is the period of intense growth and synthesis in which a cell stocks all the biomolecules required during cell division. It is called the resting phase because there is no apparent activity related to cell division rather, it deals with the changes that occur in a cell and the nucleus, before it enters into cell division.
It is further divided into three stages:
(i). G1 Phase
G1 phase is the interval between mitosis and initiation of DNA replication. In this phase, cell does not occur synthesize or replicate DNA but remains metabolically active. The cell grows in size continuously and synthesizes nucleotides, ATP, proteins, amino acids, RNA etc. (required during S phase). Most of the organelles duplication occurs in this phase. The length of the G1 phase varies from one cell to another. If a cell divides frequently, then it has a shorter G1 phase and if a cell does not divide frequently, then it has a longer G1 phase.
Some cells in an adult animal do not divide, e.g., nerve cell, whereas many cells divide occasionally just to replace the cell lost due to injury or Cell death. Such type of cells remains in G0 phase.
The non-dividing cell enters the quiescent stage G0 stage of the cell cycle. In this phase, the cell remains metabolically active but do not proliferate unless they are called on to do so.
(ii). S Phase
‘S’ or synthesis phase is a phase in which synthesis or replication of DNA takes place. It is the phase in which cell double its DNA. For example, if the initial amount of DNA is denoted by 2C, then after the S phase the amount of DNA would become 4C. The amount of nucleic acid, i.e., DNA (genetic material) Doubles but the number of chromosomes remains the same. For example, if a diploid cell has 2n number of chromosome at G1 Phase, then even after S phase, the number of chromosome would remain to 2n. The existing DNA molecules act as template to synthesise new DNA molecules. The new DNA molecules forms the new chromatin fibres which remain attached in pair, the chromatin material condenses to form two sister chromatid which remain attached to each other through a centromere. It holds the two sister chromatid together.
In animal cell, during the S phase, DNA replication begins in the nucleus, and the centrioles duplicate in the cytoplasm. Centrioles are required during the M phase. The centrioles are absent in the plant cell.
(iii). G2 Phase
G2 phase or Gap2 phase is the second phase present between S and M phase. Tubulin Protein synthesis take place. Mitochondria, chloroplast and Golgi bodies duplicate in this phase.
The DNA synthesis stops in this phase but cell synthesizes RNA, proteins etc. require during the next phase. In animals, mitotic division is only seen in a diploid somatic cells, whereas in plant cells, mitotic division is seen in both haploid and diploid cells.