Cell Cycle: The Stages in Order and Why They Matter
The cell cycle is the ordered series of events in which a cell grows, copies its DNA and divides into two identical cells. It has two main parts: interphase (G1, S and G2 phases) and M phase, which is mitosis followed by cytokinesis. Checkpoints stop the cycle if something is wrong.
What are the stages of the cell cycle?
The cycle has two main parts. Interphase is the long preparation period, and M phase is the short division period.
| Stage | Part | What happens |
|---|---|---|
| G1 (first gap) | Interphase | The cell grows, makes proteins and organelles, and does its normal job |
| S (synthesis) | Interphase | DNA is copied, so each chromosome has two identical chromatids |
| G2 (second gap) | Interphase | The cell grows more, checks the copied DNA and makes spindle proteins |
| M phase | Division | Mitosis divides the nucleus, then cytokinesis divides the cytoplasm |
A simple way to remember the order: G1, S, G2, M. Think "Grow, Sync (copy), Get ready, Multiply".
Interphase takes up most of the time. In a 24-hour cycle, mitosis itself lasts only about an hour, which is why most cells you see down a microscope are not dividing.
What happens in the S phase?
The S phase is when the cell copies its DNA, a process called DNA replication. The double helix unzips and each strand is used as a template to build a new partner strand, following the pairing rules A with T and C with G. By the end, every chromosome consists of two identical sister chromatids joined at the centromere.
A human cell has 46 chromosomes. After S phase it still has 46, but each one is now a doubled structure, so the cell holds twice as much DNA as before. The amount of DNA halves again only when the cell divides.
What happens in M phase?
M phase has two steps:
- Mitosis divides the nucleus in four stages: prophase, metaphase, anaphase and telophase.
- Cytokinesis splits the cytoplasm. Animal cells pinch in the middle, while plant cells build a new cell wall called the cell plate.
The result is two daughter cells, each genetically identical to the parent. You can step through these stages on the mitosis diagram.
What are the cell cycle checkpoints?
The cell does not rush through the cycle. Cell cycle checkpoints are control points where the cell pauses to check that it is safe to continue.
- G1 checkpoint: is the cell big enough, are nutrients available and is the DNA undamaged? This is the main decision point. If conditions are poor, the cell can enter G0.
- G2 checkpoint: has all the DNA been copied correctly and without damage?
- Spindle (metaphase) checkpoint: are all chromosomes attached to the spindle fibres? Anaphase only starts when they are.
A protein called p53 helps halt the cycle when DNA is damaged, giving the cell time to repair itself or triggering controlled cell death if the damage is too severe.
What is the G0 phase?
G0 is a resting state outside the active cycle. Some cells, such as mature nerve cells, stay in G0 permanently. Others, such as liver cells, can leave G0 and re-enter the cycle when the body needs repair.
How does the cell cycle relate to cancer?
Cancer begins when the controls fail. Mutations in genes that speed up the cycle, or in genes like p53 that act as brakes, let damaged cells keep dividing. Many cancer drugs work by targeting rapidly dividing cells, which is also why they affect hair follicles and the gut lining.
Cell cycle vs mitosis
People often use the two words as if they were the same. Mitosis is only one part of the cell cycle: the division of the nucleus inside M phase. The cell cycle also includes the whole of interphase. For how mitosis differs from the type of division that makes gametes, read mitosis vs meiosis.
Common mistakes
- Treating interphase as a resting phase. The cell is very active, especially in S phase.
- Saying mitosis is the whole cell cycle. It is only part of M phase.
- Forgetting that cytokinesis is separate from mitosis.
- Thinking chromosome number doubles in S phase. The number of chromosomes stays the same, but the DNA content doubles.
Frequently asked questions
What is the cell cycle in simple words?
The cell cycle is the life cycle of a cell: it grows, copies its DNA, checks everything is correct and then divides into two cells. It repeats over and over in cells that need to divide, such as skin cells.
How long does the cell cycle take?
It varies a lot. A fast-dividing human cell grown in a lab takes about 24 hours, with mitosis lasting about an hour. Some cells divide every few hours, while others, like nerve cells, rarely divide at all.
What is the difference between interphase and mitosis?
Interphase is the growth and DNA-copying part of the cycle (G1, S and G2). Mitosis is the division of the nucleus. Interphase takes up most of the cycle, while mitosis is short.
Why are cell cycle checkpoints important?
They stop cells with damaged or incompletely copied DNA from dividing. Without checkpoints, mutations would pass to daughter cells and the risk of cancer would be much higher.
Try it yourself
Open the Cell Division simulation, watch a cell go through mitosis and cytokinesis, and pause at each stage. Then work out which part of the cell cycle comes just before the stage you are looking at, and which checkpoint would have checked it.
See it for yourself, in 3D and free.
Try it in the Cell Division: Mitosis & Meiosis