Q.Describe the events taking place during interphase.

Interphase is the period between two successive divisions, when the cell prepares for its coming division by growing and copying its DNA. It is often wrongly called the resting phase. It has three sub-phases:

• G1 (Gap 1) — the interval between the end of mitosis and the start of DNA replication. The cell is metabolically active and grows, but does not copy its DNA.
• S (Synthesis) — DNA replication occurs and the DNA amount doubles (from 2C to 4C), while the chromosome number stays the same. In animal cells the centriole also duplicates in the cytoplasm.
• G2 (Gap 2) — the cell makes proteins needed for mitosis and continues to grow.

Interphase is the stretch of the cell cycle that lies between two successive divisions. It is often described as the resting phase, but this label is misleading — far from resting, the cell is hard at work, growing steadily and copying its DNA in an orderly way so that it is fully ready for the division to come. In a 24-hour human cell cycle, interphase makes up more than 95 per cent of the time.

Interphase is divided into three sub-phases:

• G1 phase (Gap 1) is the interval between the end of the previous mitosis and the beginning of DNA replication. Throughout G1 the cell is metabolically active and grows continuously, but it does not yet copy its DNA.
• S phase (Synthesis) is the period during which DNA synthesis, or replication, actually takes place. Here the amount of DNA per cell doubles — if the starting quantity is described as 2C, it rises to 4C. A crucial point is that the chromosome number does not change: a cell that was diploid (2n) before S phase still has 2n chromosomes afterwards; only the amount of DNA has increased. In animal cells, while DNA is being replicated in the nucleus, the centriole duplicates out in the cytoplasm.
• G2 phase (Gap 2) is when the cell manufactures the proteins it will need for mitosis, and cell growth continues.

Step 1 — Recall that interphase has three sub-phases, not one continuous inactive period — this immediately corrects the common misconception of 'resting phase.'

Step 2 — Go through each sub-phase and state what happens in it:

• G1: growth, no DNA synthesis yet.
• S: DNA replication, amount of DNA doubles (2C→4C), chromosome number unchanged; centriole also duplicates (animal cells).
• G2: further growth, synthesis of proteins needed for mitosis.

Step 3 — Assemble the three sub-phases in order into a single description of interphase's events.