Interphase: G1, S and G2

Interphase is often called the resting phase, but the cell is far from idle: it is getting ready to divide by growing and by replicating its DNA in an orderly way.

Interphase has three parts in order: G1 (Gap 1), S (Synthesis) and G2 (Gap 2).

G1 is the interval between the end of mitosis and the start of DNA replication. The cell is metabolically active and keeps growing, but it does not copy its DNA. Most organelle duplication also happens in this phase.

S phase is when DNA is synthesised (replicated). The DNA per cell doubles: if the starting amount is called 2C, it becomes 4C.

The chromosome number does not rise in S phase. A cell that is diploid (2n) in G1 is still 2n after S; each chromosome now simply has two copies of its DNA.

In animal cells, DNA replication starts in the nucleus during S phase while the centriole duplicates in the cytoplasm.

In G2 the cell makes proteins in preparation for mitosis, and growth continues.

Onion root tip cells have 16 chromosomes. They have 16 in G1, still 16 after S, and 16 in each daughter after M. If the DNA content after M phase is 2C, it is 2C in G1 and 4C after S and in G2.