The cell cycle and interphase

The big idea: A cell does not divide the instant it is made. First it spends a long time growing and copying its DNA, and only then does it divide into two.

This repeating sequence of events is called the cell cycle.

The cell cycle has two main parts: a long preparation phase called interphase, and a short dividing phase called the mitotic phase (M phase).

Cell cycle

The repeating sequence of events a cell goes through from when it is formed to when it divides into two new cells.

Interphase

The part of the cell cycle when the cell is NOT dividing — it grows and copies its DNA. It is made up of three stages: G1, S and G2.

G1 phase (first gap)

The first stage of interphase, when the cell grows larger and makes new proteins and organelles.

S phase (synthesis)

The stage of interphase when the DNA is replicated (copied), so the amount of DNA in the cell doubles.

G2 phase (second gap)

The last stage of interphase, when the cell keeps growing and prepares everything it needs to divide.

Mitotic phase (M phase)

The short part of the cell cycle when the nucleus divides (mitosis) and the cell splits into two (cytokinesis).

Interphase is NOT a 'resting' phase: It is easy to think interphase is the cell 'doing nothing' between divisions.

In fact interphase is the busiest, longest part of the cycle — the cell is growing the whole time and copying its entire set of DNA in the middle of it.

Division (M phase) is actually the quick part at the end.

Interphase is not one single step — it is split into three stages that always happen in the same order: G1, then S, then G2.

The order matters: the cell grows first (G1), then copies its DNA (S), then does final growth and checks before dividing (G2).

What happens in each stage of interphase: G1 (first gap): the cell grows larger and makes new proteins and organelles.

S (synthesis): the cell replicates its DNA — every chromosome is copied, so the amount of DNA doubles.

G2 (second gap): the cell keeps growing and prepares to divide, checking that the copied DNA is ready.

After G2 the cell leaves interphase and enters the M phase to divide.

The key change: DNA doubles in S phase: The single most important event of interphase is DNA replication in S phase.

Before S phase (at G1) the cell has its normal amount of DNA.

After S phase (at G2) every chromosome has been copied, so the cell has twice as much DNA.

This is why a cell at G2 has double the DNA of a cell at G1 — a favourite exam 'distinguish' point.

You can track this doubling as a set of numbers. If the amount of DNA at G1 is called 2 units, then it rises to 4 units during S phase and stays at 4 units through G2 — until mitosis shares it back out:

A memory hook for the order: G1 → S → G2 → M. The two Gaps sit either side of Synthesis (the DNA copy), and M (division) comes last.

Think: Grow → Synthesise (copy DNA) → Grow again → Mitosis.

How this is tested: On Paper 2 a 1-mark List question can ask you to name the stages that make up interphase — the answer is G1, S and G2 (M phase is NOT part of interphase).

A 1-mark Distinguish question often compares the amount of DNA in a cell at G1 versus G2 — because DNA is copied in S phase, a G2 cell has twice the DNA of a G1 cell.

On Paper 1 a data question may show a graph of DNA mass over two cell cycles and ask you to identify the stage at a labelled point — read it off the DNA amount: rising = S phase, a sudden drop = the cell dividing (mitosis).

✓ Why this scores the mark: The three correct stages — G1, S and G2 — are given, in order, and the M phase is left out because mitosis is not part of interphase.

A common slip is to include 'mitosis' in the list, which loses the mark.