The Cell Cycle and Mitosis

This lesson covers the cell cycle and mitosis as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to understand the stages of the cell cycle, describe what happens during mitosis, and explain why mitosis is important for growth and repair.

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Why Do Cells Divide?

Cells divide for three main reasons:

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

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The Cell Cycle

The cell cycle is the series of events that take place in a cell, leading to cell division and the production of two new daughter cells. It is a continuous process divided into three main stages:

1. Interphase (the longest stage)

Interphase is the period of the cell cycle before mitosis begins. The cell prepares for division. Interphase occupies approximately 90% of the total cell cycle time.

During interphase:

• The cell grows in size.
• The number of organelles increases (e.g. mitochondria and ribosomes are duplicated).
• The cell carries out its normal functions (metabolic reactions, protein synthesis).
• DNA replicates — each chromosome is copied to produce two identical copies joined at the centromere. These are called sister chromatids.

At the end of interphase, the cell has:

• Twice the normal amount of DNA.
• Enough organelles for two cells.
• Sufficient nutrients and energy to proceed with division.

Exam Tip: Interphase is NOT a "resting phase". The cell is very active — it is growing, replicating DNA, and preparing for division. A common mistake is to say the cell is "doing nothing" during interphase.

2. Mitosis (nuclear division)

Mitosis is the stage where the nucleus divides, producing two genetically identical nuclei. It is a continuous process but is described in four phases for convenience.

3. Cytokinesis (cell division)

After mitosis, the cytoplasm divides to produce two separate daughter cells, each with its own nucleus. In animal cells, the cell membrane pinches inward (cleavage furrow). In plant cells, a new cell plate forms across the middle of the cell, which develops into a new cell wall.

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Stages of Mitosis

Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase.

Exam Tip: Remember the order using the mnemonic: PMAT — Prophase, Metaphase, Anaphase, Telophase. Some students add "I" for Interphase at the beginning: IPMAT.

Prophase

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

Metaphase

• Chromosomes line up along the equator (middle) of the cell.
• Spindle fibres attach to the centromere of each chromosome.
• The chromosomes are aligned in a single plane across the centre of the cell.

Anaphase

• The centromeres divide (split).
• The sister chromatids are pulled apart to opposite poles (ends) of the cell by the shortening spindle fibres.
• Each chromatid is now considered an individual chromosome.
• The cell appears elongated as the chromatids move apart.

Telophase

• The chromatids (now called chromosomes) reach the opposite poles of the cell.
• A new nuclear membrane forms around each set of chromosomes.
• The chromosomes begin to uncoil and become less visible.
• The spindle fibres break down.
• Two nuclei have now formed — each with the same number of chromosomes as the original cell.

Cytokinesis

• The cytoplasm divides to form two separate daughter cells.
• In animal cells: the cell membrane constricts inward (cleavage furrow).
• In plant cells: a cell plate forms across the middle, which develops into a new cell wall and cell membrane.

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Summary Table: Stages of the Cell Cycle

Stage	Key Events
Interphase	Cell grows; organelles replicate; DNA replicates (each chromosome becomes two sister chromatids joined at centromere)
Prophase	Chromosomes condense and become visible; nuclear membrane breaks down; spindle fibres form
Metaphase	Chromosomes line up at the equator; spindle fibres attach to centromeres
Anaphase	Centromeres split; sister chromatids pulled to opposite poles by spindle fibres
Telophase	New nuclear membranes form; chromosomes uncoil; spindle breaks down; two nuclei formed
Cytokinesis	Cytoplasm divides; two genetically identical daughter cells produced

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Key Features of Mitosis

• Mitosis produces two daughter cells.
• The daughter cells are genetically identical to each other and to the parent cell.
• The daughter cells have the same number of chromosomes as the parent cell — they are diploid (in humans, 2n = 46 chromosomes).
• Mitosis does not produce genetic variation — the daughter cells are clones.

Chromosome Numbers

• Human body cells (somatic cells) are diploid — they have 46 chromosomes (23 pairs).
• During interphase, DNA replicates so the cell temporarily has 92 chromatids (joined in 46 pairs of sister chromatids).
• After mitosis, each daughter cell has 46 chromosomes — the original diploid number.

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Importance of Mitosis

Purpose	Explanation
Growth	Multicellular organisms grow by adding more cells through mitosis
Repair and replacement	Damaged or dead cells are replaced by genetically identical cells (e.g. skin wound healing, replacing red blood cells)
Asexual reproduction	Some organisms (e.g. bacteria by binary fission, strawberry plants by runners, yeast by budding) reproduce using cell division to create genetically identical offspring

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Mitosis vs Meiosis — Brief Comparison

Although meiosis is covered in detail in Topic 6 (Inheritance), it is useful to understand the key differences:

Feature	Mitosis	Meiosis
Number of divisions	One	Two
Number of daughter cells	Two	Four
Chromosome number in daughter cells	Diploid (same as parent — 46 in humans)	Haploid (half — 23 in humans)
Genetically identical?	Yes (clones)	No (genetically different)
Where it occurs	All body cells (somatic cells)	Reproductive organs (ovaries and testes)
Purpose	Growth, repair, asexual reproduction	Production of gametes (sex cells)
Genetic variation?	No	Yes (crossing over and independent assortment)

Exam Tip: At GCSE level, you need to know the basic comparison between mitosis and meiosis but the detailed mechanism of meiosis is in Topic 6. For this topic, focus on mitosis. The key phrase for mitosis is: "produces two genetically identical diploid daughter cells."

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The Cell Cycle Diagram

The cell cycle can be represented as a circular diagram:

graph TD
    A["INTERPHASE ~90% of cycle"] --> B["Cell grows in size"]
    B --> C["DNA replicates"]
    C --> D["Organelles duplicate"]
    D --> E["MITOSIS"]
    E --> F["Prophase - chromosomes condense"]
    F --> G["Metaphase - chromosomes line up"]
    G --> H["Anaphase - chromatids pulled apart"]
    H --> I["Telophase - nuclear membranes form"]
    I --> J["CYTOKINESIS - cytoplasm divides"]
    J --> K["Two genetically identical daughter cells"]
    K --> A

Interphase occupies the largest portion of the cycle (approximately 90%).

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Controlling Cell Division

Cells normally divide in a controlled way. The cell cycle is regulated by a series of checkpoints and chemical signals.