# The Cell Cycle and Mitosis

> How a cell copies its DNA, divides it equally between two daughters, and what happens when the controls fail.

Level: High school | Reading time: 13 min | Last reviewed: 2026-10-01

Web page: https://cellnaut.com/learn/cell-division

**Quick answer:** The cell cycle runs from interphase (G1, S and G2), when the cell grows and copies its DNA, through mitosis and cytokinesis, which split it into two identical daughter cells. Mitosis has four stages in order: prophase, metaphase, anaphase and telophase.

## What you will learn

- List the phases of the cell cycle and say what happens in each.
- Name the four stages of mitosis in order.
- Compare cytokinesis in animal and plant cells.
- Explain how mutations in cell-cycle control can lead to cancer.

## Key takeaways

- Interphase has three parts: G1 growth, S phase DNA replication and G2 preparation for division.
- The four stages of mitosis are prophase, metaphase, anaphase and telophase.
- Animal cells divide by pinching in a cleavage furrow; plant cells build a cell plate.
- Checkpoints stop damaged cells from dividing, and mutations in the genes that control them can lead to cancer.

## Why cells divide

Your body makes millions of new cells every second. They replace those that wear out, repair wounds, and, in a growing child, increase the total number. Each new cell must receive a full and accurate copy of the DNA. The cell does this through a repeating sequence of events called the cell cycle.

## The phases of the cell cycle

Most of a cell's life is spent in interphase, the long stretch between divisions. Interphase has three parts.

| Phase | Name       | What happens                                                        |
| ----- | ---------- | ------------------------------------------------------------------- |
| G1    | First gap  | The cell grows and does its normal work.                            |
| S     | Synthesis  | The DNA is copied, so each chromosome is duplicated.                |
| G2    | Second gap | The cell grows more, checks the copied DNA, and prepares to divide. |

After interphase comes the M phase, in which the cell divides. M phase has two jobs: mitosis, which splits up the nucleus, and cytokinesis, which splits up the rest of the cell. A fast-dividing human cell in culture may complete the full cycle in about a day, with mitosis itself taking around an hour. Other cells, such as most neurons, stop cycling altogether.

> **Common misconception**
>
> **"The DNA is copied during mitosis."** DNA copying happens earlier, in the S phase of interphase. By the time mitosis begins, each chromosome already consists of two identical copies, called sister chromatids, joined together.

## The stages of mitosis

Mitosis is a continuous process, but it is easier to understand as four stages.

1. **Prophase.** The chromatin condenses into visible, X-shaped chromosomes. The nuclear envelope breaks down, and a spindle made of microtubules begins to form.
2. **Metaphase.** The chromosomes line up along the middle of the cell, each attached to spindle fibers from both poles.
3. **Anaphase.** The sister chromatids are pulled apart toward opposite ends of the cell. Each is now an independent chromosome.
4. **Telophase.** Two new nuclear envelopes form around the two sets of chromosomes, and the chromosomes relax back into chromatin.

The outcome: two nuclei with identical sets of chromosomes. A human cell starts with 46 chromosomes, and each of the two daughter nuclei ends with 46.

![Stylized 3D illustration of a plant root tip in which many cells show chromosomes at different stages of mitosis.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/root-tip-mitosis-teaching.jpg)

*A stylized plant root tip, a classic place to find cells in every stage of mitosis at once. Credit: Project-generated teaching illustration (AI-generated; simplified)*

[Open the interactive, labelled version](https://cellnaut.com/topics/root-tip-mitosis)

**Check your understanding:** During which phase of the cell cycle is DNA copied?

- A. G1
- B. S phase
- C. Metaphase
- D. Cytokinesis

**Answer:** B. S phase

The S (synthesis) phase of interphase is when each chromosome is duplicated.

**Check your understanding:** What happens during anaphase?

- A. DNA is copied
- B. Sister chromatids are pulled to opposite poles
- C. The nuclear envelope reforms
- D. The cell wall forms

**Answer:** B. Sister chromatids are pulled to opposite poles

In anaphase the spindle pulls the sister chromatids apart toward opposite ends of the cell.

## Dividing the cytoplasm

Cytokinesis differs between plants and animals.

- In an **animal cell**, a ring of actin and myosin around the middle tightens like a drawstring, pinching the cell in two. The dent is called the cleavage furrow.
- In a **plant cell**, the rigid wall prevents pinching. Instead, vesicles gather in the middle and fuse to build a new wall from the inside out. This is the cell plate.

Interactive 3D model: [Plant Cell](https://cellnaut.com/studio/plant/cellWall) - A plant cell builds a new wall between its daughters.

Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - An animal cell pinches in two. Its nucleus has to divide first.

Interactive 3D model: [Stem Cell](https://cellnaut.com/studio/stem/nucleus) - Stem cells keep dividing, and can also leave the cycle to become specialized.

Interactive 3D model: [Yeast Cell](https://cellnaut.com/studio/yeast/bud) - Yeast divides by budding, an unequal division that makes a small daughter cell.

## Checkpoints and cancer

Division is risky, so the cycle has built-in checkpoints, where proteins check whether the cell is big enough, the DNA is undamaged, and the chromosomes are properly attached to the spindle. If something is wrong, the cycle halts for repair or the cell is instructed to self-destruct.

Cancer develops when mutations break these controls. Two kinds of genes are especially important. Genes that act like an accelerator pedal can become stuck in the "on" position, and genes that act like a brake, such as the well-known p53 gene, can be lost. A cell that has accumulated several such changes ignores the stop signals and divides without control. It is not a single change, but a build-up over time, which is why the risk rises with age.

## Meiosis in one paragraph

Cells that make eggs and sperm divide differently, through a process called meiosis. It has two rounds of division and produces four cells, each with half the number of chromosomes. When egg and sperm fuse, the full number of 46 is restored in the fertilized egg. Mitosis keeps the chromosome number the same; meiosis halves it.

**Check your understanding:** Why is cancer described as a disease of lost cell-cycle control?

- A. Cancer cells have no DNA
- B. Mutations let cells ignore checkpoints and keep dividing
- C. Cancer cells skip interphase and never grow
- D. Cancer cells only divide by meiosis

**Answer:** B. Mutations let cells ignore checkpoints and keep dividing

Mutations in genes that regulate the cell cycle let cells bypass the normal checkpoints and divide when they should not.

## Sources

- [Biology 2e, Chapter 10: Cell Reproduction (OpenStax)](https://openstax.org/books/biology-2e/pages/10-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim.
- [Biology 2e, Chapter 11: Meiosis and Sexual Reproduction (OpenStax)](https://openstax.org/books/biology-2e/pages/11-introduction) - CC BY-NC-SA 4.0. Used for the short comparison with meiosis.
- [Project teaching illustration: root-tip mitosis (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset)