The cell cycle is the ordered sequence through which a proliferating eukaryotic cell grows, duplicates its DNA and divides.

The canonical cycle is

G1 → S → G2 → M → G1 ...

The first three phases together are called interphase.

G1: growth before DNA replication

During G1 phase, the cell carries out ordinary cellular functions, grows and prepares the molecular machinery needed for DNA synthesis.

A cell can also leave the actively proliferating cycle and enter a nondividing state commonly called G0. G0 is not simply a synonym for G1: a G0 cell is not currently committed to progressing toward DNA replication, although some cells can later re-enter the cycle.

S: genome duplication

During S phase, DNA synthesis replicates each chromosome. The chromosome number is not doubled merely because DNA has been copied: after replication, each chromosome consists of two newly related DNA copies that remain physically associated until chromosome segregation.

G2: preparation after replication

During G2 phase, the genome has been replicated but chromosome segregation has not yet occurred. The cell continues to grow and prepares the structures and proteins required for division.

M: chromosome segregation and cell division

The M phase includes nuclear chromosome segregation by mitosis and is commonly followed or overlapped by cytokinesis, the physical division of the cell body.

The ordering matters:

grow → copy genome once → verify/prep → segregate copies → divide

A second round of DNA replication before segregation would create too many genome copies, while segregation before replication would leave insufficient genetic material for two equivalent daughter nuclei.

The cell cycle is a controlled sequence, not a clock face

The durations of G1, S, G2 and M vary greatly among cell types and conditions. Some cells remain in G0 for long periods; rapidly dividing embryonic cells can have unusually short gap phases.

What defines the cell cycle is therefore not equal timing but causal ordering: DNA must be duplicated before duplicated chromosomes can be segregated, and key transitions are regulated so that later events normally wait until prerequisite events are complete.