Chromosomes, Mitosis and the Cell Cycle

This lesson covers the cell cycle, chromosomes and the process of mitosis as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to describe the stages of the cell cycle, explain the importance of mitosis and understand what chromosomes are.

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Chromosomes

Chromosomes are thread-like structures found in the nucleus of eukaryotic cells. They are made of tightly coiled DNA (deoxyribonucleic acid).

Key Facts About Chromosomes

• Each chromosome consists of a single, very long molecule of DNA wrapped around proteins.
• Human body cells contain 46 chromosomes, arranged in 23 pairs. Each pair consists of one chromosome from the mother and one from the father.
• The number of chromosomes in a body cell is called the diploid number (represented as 2n). For humans, 2n = 46.
• Sex cells (gametes — sperm and egg cells) contain half the normal number: 23 chromosomes. This is the haploid number (represented as n).

Cell Type	Number of Chromosomes	Ploidy
Human body cell	46 (23 pairs)	Diploid (2n)
Human gamete (sperm or egg)	23	Haploid (n)

Exam Tip: A common exam question asks why gametes must be haploid. The answer: when two haploid gametes fuse at fertilisation, the resulting zygote has the full diploid number (23 + 23 = 46). If gametes were diploid, the chromosome number would double every generation.

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The Cell Cycle

The cell cycle is the series of events that takes place in a cell, leading to cell growth and division. It consists of three main stages:

Stage 1: Interphase (Growth Phase)

Interphase is the longest stage of the cell cycle (typically about 90% of the total time). During interphase:

• The cell grows and increases in size.
• The number of subcellular structures (organelles) increases — for example, the number of mitochondria and ribosomes increases.
• The cell's DNA replicates — each chromosome makes an identical copy of itself. The two copies remain joined at a point called the centromere, forming an X-shaped structure.

Stage 2: Mitosis (Nuclear Division)

During mitosis, the nucleus divides. The replicated chromosomes are separated so that each new nucleus receives an identical and complete set of chromosomes.

Stage 3: Cytokinesis (Cell Division)

During cytokinesis, the cytoplasm and cell membrane divide, producing two identical daughter cells. In plant cells, a new cell wall also forms between the two cells.

graph LR
    A["Interphase<br/>• Cell grows<br/>• Organelles increase<br/>• DNA replicates"] --> B["Mitosis<br/>• Chromosomes separate<br/>• Two nuclei form"]
    B --> C["Cytokinesis<br/>• Cytoplasm divides<br/>• Two daughter cells"]
    C --> A

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What Happens During Mitosis

Although the AQA Combined Science specification does not require you to name the individual sub-stages (prophase, metaphase, anaphase, telophase), you should understand the overall process:

Step	What Happens
1	The chromosomes condense (become shorter and thicker) and become visible under a microscope
2	The nuclear membrane (nuclear envelope) breaks down
3	Spindle fibres form and attach to the centromeres of the chromosomes
4	Chromosomes line up along the centre (equator) of the cell
5	The spindle fibres pull the two copies of each chromosome apart to opposite ends (poles) of the cell
6	New nuclear membranes form around each set of chromosomes
7	The cell now has two nuclei, each with an identical and complete set of chromosomes

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Key Features of Mitosis

Feature	Detail
Number of cell divisions	1
Number of daughter cells	2
Genetic identity	Daughter cells are genetically identical to the parent cell and to each other (they are clones)
Chromosome number	Maintained — each daughter cell has the same number as the parent cell (diploid)
Purpose	Growth, repair of damaged tissue, replacement of worn-out cells, asexual reproduction

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Importance of Mitosis

Mitosis is essential for:

1. Growth — a multicellular organism grows by producing more cells through mitosis.
2. Repair — when tissues are damaged, mitosis produces new cells to replace the damaged ones.
3. Replacement — some cells have a limited lifespan and must be continually replaced (e.g. skin cells are replaced approximately every 2–3 weeks; red blood cells last about 120 days).
4. Asexual reproduction — some organisms reproduce by mitosis alone (e.g. strawberry plants sending out runners, bacteria undergoing binary fission).

Exam Tip: Never say mitosis produces "four cells" — that is meiosis. Mitosis produces two genetically identical daughter cells. This is one of the most common errors in biology exams.

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Comparing Mitosis and Meiosis

Although meiosis is covered in more detail in the genetics topic, it is useful to compare:

Feature	Mitosis	Meiosis
Where it occurs	Body cells (somatic cells)	Reproductive organs (ovaries and testes)
Number of divisions	1	2
Number of daughter cells	2	4
Genetic variation	None — cells are identical	Yes — cells are genetically different
Chromosome number	Diploid (2n) → Diploid (2n)	Diploid (2n) → Haploid (n)
Purpose	Growth, repair, asexual reproduction	Production of gametes (sex cells)

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Cell Growth — Why Cells Divide

As a cell grows, its volume increases faster than its surface area. This is a problem because:

• Substances enter and leave the cell through its surface (the cell membrane).
• If the cell becomes too large, the surface area is no longer big enough to supply the volume with the nutrients it needs or to remove waste quickly enough.
• Dividing into smaller cells maintains a large surface area to volume ratio, which is more efficient for exchange of substances.

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Cancer — Uncontrolled Cell Division

Cancer is the result of changes (mutations) in the DNA that lead to uncontrolled cell division. Cells divide by mitosis much faster than normal, forming a mass of abnormal cells called a tumour.