Interphase: G1, S and G2 Phases

Interphase, the interval between one M phase and the next, is not a single uniform state but is itself divided into three successive sub-phases, conventionally labelled G1, S and G2, each dedicated to a distinct set of preparatory tasks that must be completed, in order, before the cell is ready to divide.

G1 phase (Gap 1) is the interval immediately following the completion of the previous M phase. It is typically the longest of the three sub-phases and the one whose duration varies most between different cell types, since it is during G1 that a cell effectively 'decides' whether conditions are favourable enough to commit to another round of division. Throughout G1 the cell grows in size, synthesises the RNA and proteins it needs for its normal metabolic activities, and duplicates most of its cytoplasmic organelles, but the nuclear DNA itself is not yet copied -- each chromosome is still present as a single, unreplicated DNA molecule. A cell that, for whatever reason, does not proceed further into the cycle may instead shift out of G1 into the quiescent G0 phase described in the previous section.

S phase (Synthesis phase) follows G1 and is defined by a single, defining event: the replication of the entire nuclear DNA content, so that by the end of S phase the amount of DNA in the cell has exactly doubled. Each chromosome, which entered S phase as a single DNA double helix, emerges from S phase duplicated into two identical sister chromatids, held together at a shared region called the centromere -- though this duplication is not yet visible under the light microscope, since the chromatin remains in its loosely dispersed, uncondensed form throughout interphase. In animal cells, S phase is also the period during which the centriole, the organelle that will organise the mitotic spindle, duplicates, so that the cell enters G2 with two pairs of centrioles rather than one. …