The Cell Cycle and Mitosis Revision Notes | A Level AQA Biology

1. Why Every Cell Comes From Another Cell

Every cell in your body — from a skin cell to a neuron — originated from a single fertilised egg. That egg divided, and its daughters divided, and so on, billions of times over. The process that makes this possible is the cell cycle, and at its heart is mitosis.

Not all eukaryotic cells retain the ability to divide. Mature neurones, for example, are largely non-dividing. Those that do retain this ability show a repeating cell cycle — a tightly regulated sequence of growth, DNA copying, and division. Understanding this cycle is central to understanding both normal development and what goes wrong in cancer.

2. The Cell Cycle: An Overview

Picture the cell cycle as a clock face. The vast majority of the time — roughly 90% in many actively dividing cells — the cell is in interphase. This is not a resting phase; it is a period of intense activity. The remaining time is spent in mitotic division followed by cytokinesis.

The AQA specification is explicit: DNA replication occurs during the interphase of the cell cycle, specifically during S phase. By the end of interphase, the cell has doubled its DNA content and is ready to divide. The replicated chromosomes each consist of two identical sister chromatids — this is the raw material that mitosis will separate.

• G1 phase: cell grows, synthesises proteins, and increases organelle number
• S phase: DNA replication — each chromosome is copied to form two sister chromatids joined at the centromere
• G2 phase: further growth, organelles replicate, cell checks DNA integrity before division begins

3. Mitosis: Stage by Stage

With DNA replicated, the cell enters mitosis. Mitosis is the part of the cell cycle in which a eukaryotic cell divides to produce two daughter cells, each with identical copies of DNA to the parent cell. It is divided into four named stages: prophase, metaphase, anaphase, and telophase. Knowing the appearance of cells at each stage is a core AQA requirement.

3a. Prophase

At the start of prophase, the chromosomes condense — they coil and shorten, becoming visible under a light microscope as distinct, thick structures. Each chromosome at this point consists of two sister chromatids joined at the centromere. The spindle begins forming before the nuclear envelope fully breaks down towards the end of prophase. The nuclear envelope breaks down, and the spindle apparatus begins to form from opposite poles of the cell.

Spindle fibres extend from structures called centrioles (in animal cells) toward the middle of the cell. The nucleolus disappears. In an exam question asking you to describe a cell in prophase, look for: condensed chromosomes visible, no nuclear envelope, spindle forming.

3b. Metaphase

During metaphase, the chromosomes are moved to the equator (middle) of the cell — this imaginary line is called the metaphase plate. Spindle fibres attach to the centromere of each chromosome, with fibres from opposite poles attaching to the same centromere. The chromosomes are held under tension, lined up precisely.

This is the stage where chromosomes are most condensed and straightforward to count under a microscope. If you see chromosomes arranged in a line across the middle of the cell, that is metaphase. The role of spindle fibres attached to centromeres is critical here — they are what will pull the chromatids apart in the next stage.

3c. Anaphase

Anaphase is the stage of separation. The spindle fibres shorten, pulling the sister chromatids apart at the centromere. Each chromatid is now pulled to an opposite pole of the cell. Because the chromatids were identical copies (produced during S phase), each pole receives a genetically identical set of chromosomes.

The key visual feature of anaphase: chromosomes appear V-shaped or J-shaped as they are dragged centromere-first toward the poles, with the arms trailing behind. The cell also begins to elongate. This is the stage that directly demonstrates the role of spindle fibres attached to centromeres in the separation of chromatids.

3d. Telophase and Cytokinesis

In telophase, a full set of chromosomes has arrived at each pole. Nuclear envelopes reform around each set, and the chromosomes begin to decondense (uncoil). The spindle fibres break down. The cell now contains two nuclei, each genetically identical to the original parent nucleus.

Telophase is followed by cytokinesis — the division of the cytoplasm. In animal cells, the cell membrane pinches inward (cleavage furrow) to produce two separate cells. In plant cells, a cell plate forms across the middle, which develops into a new cell wall. Cytokinesis usually occurs alongside or immediately after telophase, producing two new daughter cells.

4. Chromosome Behaviour: The Full Picture

It helps to track a single chromosome through the entire cycle to see how the behaviour at each stage connects. The table below summarises what is happening to chromosomes and the spindle at each stage — this is exactly the kind of comparison an AQA examiner may ask you to produce.

Notice the logic running through this sequence: DNA is replicated in interphase so that there is enough genetic material to share; condensation in prophase makes chromosomes manageable; alignment in metaphase ensures each pole gets one of each; separation in anaphase distributes them; and telophase plus cytokinesis packages them into two new cells. Every step serves the overarching goal — two genetically identical daughter cells.

5. Why the Daughter Cells Are Genetically Identical

The genetic identity of daughter cells is not accidental — it is the direct result of two tightly linked events. First, during S phase of interphase, DNA replication produces an exact copy of every chromosome (semi-conservative replication ensures each new double helix contains one original and one new strand). Second, the spindle mechanism in anaphase ensures that one copy of each chromatid goes to each pole.

If either step fails — for example, if replication introduces errors, or if spindle fibres fail to separate chromatids correctly — the daughter cells will not be genetically identical. This is why the cell has checkpoint mechanisms at G1, G2, and the metaphase-to-anaphase transition. These checkpoints detect problems and can halt the cycle before errors are passed on.

6. Uncontrolled Division: Tumours and Cancer

Mitosis is a controlled process. Under normal conditions, the cell cycle is regulated by proteins that act at checkpoints, ensuring division only occurs when conditions are appropriate. When this control breaks down — due to mutations in genes that regulate the cycle — cells can divide repeatedly without stopping. This is the basis of tumour formation.

A tumour is a mass of cells produced by uncontrolled cell division. Benign tumours remain localised and do not invade surrounding tissue. Cancers are malignant tumours: the cells can invade neighbouring tissues and spread via the blood or lymphatic system to other parts of the body (metastasis). The distinction matters in exam answers — not all tumours are cancers.

Many cancer treatments are directed at controlling the rate of cell division. Chemotherapy drugs often target rapidly dividing cells by interfering with DNA replication or spindle fibre formation. For example, some drugs prevent spindle fibres from forming, so chromosomes cannot be separated in anaphase and division is halted. Radiotherapy damages DNA in tumour cells, triggering cell death. Both approaches exploit the fact that cancer cells divide far more frequently than most normal cells.

7. Identifying Mitotic Stages: A Worked Approach

AQA exam questions frequently present a diagram or micrograph of dividing cells and ask you to identify the stage or describe what is happening. Here is a reliable method for working through such questions systematically.

Worked example: Identifying the Stage of Mitosis from a Cell Image

A student views a cell under a microscope. The cell shows chromosomes that are condensed and clearly visible, but they are scattered throughout the cell rather than aligned. There is no nuclear envelope, and a spindle is forming. Identify the stage and justify your answer.

1. No nuclear envelope is present. — The nuclear envelope breaks down at the end of prophase. Its absence rules out interphase (where the envelope is intact) and telophase (where it reforms).
2. Chromosomes are scattered — not aligned at the equator, not at the poles. — Alignment at the equator indicates metaphase. Movement to the poles indicates anaphase. Scattered chromosomes with no alignment points to prophase.
3. Spindle is forming but not yet fully assembled. — The spindle forms during prophase. A fully assembled spindle with fibres attached to centromeres is characteristic of metaphase.
4. Chromosomes are condensed and each consists of two sister chromatids — they have not yet separated. — Chromatid separation occurs in anaphase. Condensed but unseparated chromatids confirm this is before anaphase.
5. No nuclear envelope + scattered condensed chromosomes + spindle forming + chromatids not separated = prophase. — All features are consistent with prophase. State this clearly in your answer and quote the specific features as evidence.

Answer: The cell is in prophase. Evidence: chromosomes are condensed and visible, the nuclear envelope has broken down, the spindle is forming, and the chromosomes are not yet aligned at the equator.

8. Exam Technique and Common Mistakes

Mitosis questions appear across all three AQA Biology papers and can carry anywhere from one to six marks. The most common errors are not factual — they are about precision of language. Here is what separates a full-mark answer from a near-miss.

Taken together, the cell cycle — from DNA replication in interphase through the four stages of mitosis to cytokinesis — is a precisely controlled sequence that underpins growth, repair, and asexual reproduction in all eukaryotic organisms. When that control fails, the consequences range from benign tumours to life-threatening cancers, which is why so much cancer research focuses on the molecular machinery of the cell cycle itself.