Cell Division by Mitosis

This lesson covers the cell cycle and mitosis as required by AQA GCSE Biology specification 4.1.1. You need to understand why cells divide, the stages of the cell cycle, and what happens during mitosis. You also need to know about the role of mitosis in growth, repair, and asexual reproduction.

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Why Do Cells Divide?

Cells need to divide for three main reasons:

1. Growth — multicellular organisms grow by increasing the number of cells.
2. Repair — damaged or worn-out cells need to be replaced (e.g. skin cells, red blood cells).
3. Asexual reproduction — some organisms reproduce by mitosis alone, producing genetically identical offspring (clones).

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The Cell Cycle

The cell cycle is the series of stages a cell goes through as it grows and divides. It consists of three main phases:

1. Interphase (Growth Phase)

This is the longest phase of the cell cycle — the cell spends most of its life in interphase. During interphase, the cell:

• Grows in size and produces more sub-cellular structures (organelles) such as mitochondria and ribosomes.
• Replicates its DNA — each chromosome is copied to produce two identical copies (called sister chromatids) joined at the centre by a structure called the centromere. This ensures that each new daughter cell will receive a complete set of genetic information.
• Carries out normal cell functions — producing proteins, respiring, etc.

2. Mitosis (Nuclear Division)

During mitosis, the nucleus divides. The replicated chromosomes are separated so that each new nucleus receives one copy of every chromosome. Mitosis is described in detail below.

3. Cytokinesis (Cell Division)

After mitosis, the cytoplasm and cell membrane divide to form two separate daughter cells. In animal cells, the membrane pinches inward (like tightening a drawstring). In plant cells, a new cell plate forms across the middle of the cell, which develops into a new cell wall.

graph LR
    A["INTERPHASE<br/>Cell grows<br/>DNA replicates<br/>Organelles increase"] --> B["MITOSIS<br/>Nucleus divides<br/>Chromosomes separate"]
    B --> C["CYTOKINESIS<br/>Cytoplasm divides<br/>Two daughter cells form"]
    C --> A

    style A fill:#3498db,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#27ae60,color:#fff

Exam Tip: Interphase is NOT a "resting phase" — the cell is very active during interphase, growing and replicating its DNA. This is a common misconception that examiners will test. Also remember that DNA replication occurs during interphase, NOT during mitosis itself.

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Stages of Mitosis

Although mitosis is a continuous process, it is divided into four stages for ease of description. You need to understand what happens at each stage:

Prophase

• The replicated chromosomes condense (coil up tightly) and become visible under a microscope. Each chromosome consists of two identical sister chromatids joined at the centromere.
• The nuclear membrane (nuclear envelope) begins to break down.
• Spindle fibres form from structures called centrioles (in animal cells) and extend across the cell.

Metaphase

• The chromosomes line up along the equator (middle) of the cell.
• Spindle fibres attach to the centromere of each chromosome.

Anaphase

• The spindle fibres contract (shorten), pulling the sister chromatids apart.
• One chromatid from each chromosome is pulled to each opposite pole (end) of the cell.
• The separated chromatids are now called chromosomes.

Telophase

• The chromosomes reach the poles of the cell and begin to uncoil (decondense).
• A new nuclear membrane forms around each set of chromosomes.
• Two new nuclei are formed, each containing a complete, identical set of chromosomes.

flowchart LR
    A["Prophase<br/>Chromosomes condense<br/>Nuclear membrane breaks down"] --> B["Metaphase<br/>Chromosomes line up<br/>at the equator"]
    B --> C["Anaphase<br/>Chromatids pulled<br/>to opposite poles"]
    C --> D["Telophase<br/>Nuclear membranes reform<br/>Chromosomes uncoil"]
    D --> E["Cytokinesis<br/>Cell divides into<br/>two daughter cells"]

    style A fill:#9b59b6,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#27ae60,color:#fff

Remembering the Stages

A useful mnemonic for the stages of mitosis is: PMAT — Prophase, Metaphase, Anaphase, Telophase.

Another mnemonic: Please Make Another Taco.

Exam Tip: You may be shown photographs or diagrams of cells in different stages of mitosis and asked to identify which stage is shown. Key clues: Prophase — chromosomes visible but scattered; Metaphase — chromosomes lined up in the middle; Anaphase — chromatids being pulled apart (V-shapes); Telophase — two groups of chromosomes at opposite ends with new nuclei forming.

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Key Features of Mitosis

Feature	Detail
Number of divisions	One
Number of daughter cells	Two
Chromosome number	Daughter cells have the same number of chromosomes as the parent cell (diploid — 46 in humans)
Genetic variation	None — daughter cells are genetically identical to each other and to the parent cell
Where it occurs	All body cells (somatic cells)
Purpose	Growth, repair, asexual reproduction

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The Role of Mitosis

Growth

In multicellular organisms, mitosis produces new cells for growth. As the organism develops from a single fertilised egg (zygote), mitosis creates the trillions of cells needed to form tissues and organs. A human body contains approximately 37 trillion cells, all produced by mitosis from the original zygote.

Repair and Replacement

When cells are damaged or die, mitosis produces new replacement cells. Examples include:

• Skin cells — the outer layer of skin is constantly being worn away and replaced. Skin cells are replaced every 2–3 weeks.
• Red blood cells — have no nucleus and a lifespan of about 120 days. New red blood cells are constantly produced by mitosis in the bone marrow.
• Gut lining cells — the cells lining the digestive system are replaced every 3–5 days due to exposure to acid and enzymes.

Asexual Reproduction

Some organisms reproduce by mitosis alone. This is called asexual reproduction and produces offspring that are genetically identical to the parent (clones). Examples include:

• Bacteria — divide by binary fission (a form of asexual reproduction).
• Strawberry plants — send out runners that grow into new plants.
• Hydra — reproduce by budding.
• Amoeba — divides into two by mitosis.

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Cancer: Uncontrolled Cell Division

Cancer occurs when cells divide uncontrollably by mitosis, forming a mass of abnormal cells called a tumour. This happens when the normal controls on the cell cycle break down, usually due to mutations in genes that regulate cell division.