Interphase | Cell Cycle | Lecture 2

The cell cycle The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. It has three phases, namely interphase, nuclear division and cell division.Nuclear and cell division is Mitosis or M phase.
Interphase
During interphase, the cell grows to its normal size after cell division and carries out its normal functions, synthesising many substances, especially proteins, in the process. At some point during interphase, a signal may be received that the cell should divide again.
The interphase is divided into three further phases:
• G1 phase (Gap 1)
• S phase (Synthesis)
• G2 phase (Gap 2)
G1 phase (Gap 1)
During G1, cells make the RNA, enzymes and other proteins needed for growth. At the end of G1, the cell becomes committed to dividing or not dividing.
S phase (Synthesis)
S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n.
G2 phase (Gap 2)
During G2 the cell continues to grow and new DNA is checked and any errors are usually repaired. Preparations are also made to begin the process of division. For example, there is a sharp increase in production of the protein tubulin which is needed to make microtubules for the mitotic spindle. Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism. In animals, mitotic cell division is only seen in the diploid somatic cells. Against this, the plants can show mitotic divisions in both haploid and diploid cells.
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