The nucleus of every body cell contains chromosomes — long, tightly coiled molecules of DNA.
Each chromosome is one very long DNA molecule associated with proteins called histones that help package it.
Each chromosome carries many GENES — a gene is a specific section of DNA coding for one protein.
Humans have 46 chromosomes in most body cells, arranged as 23 PAIRS of homologous chromosomes.
'Homologous' means matching — each pair carries genes for the same characteristics, but may carry different alleles (versions) of those genes.
One chromosome in each pair came from the mother (via the egg) and one from the father (via the sperm).
BODY CELLS: 46 chromosomes (diploid — 2n). All cells in your body except gametes.
GAMETES (sperm and eggs): 23 chromosomes (haploid — n). When two gametes fuse at fertilisation: 23 + 23 = 46 — the full number is restored.
The Cell Cycle
The cell cycle is the ordered series of events that a cell goes through from its formation to its division into two new cells.
The cycle has three main phases:
PHASE 1 — INTERPHASE (growth and preparation):
This is the longest phase — the cell spends most of its life here.
• G1 phase: cell grows in size. Proteins are synthesised. Number of organelles increases (more ribosomes, mitochondria etc).
• S phase (DNA synthesis): each chromosome is REPLICATED — a new copy of every DNA molecule is made. The cell now has 92 chromatids (2 copies of each of the 46 chromosomes, held together).
• G2 phase: further growth. Cell checks that DNA has been copied accurately.
PHASE 2 — MITOSIS (nuclear division):
The duplicated chromosomes are separated into two identical sets, each with 46 chromosomes.
PHASE 3 — CYTOKINESIS (cytoplasm division):
The cytoplasm divides to form two separate daughter cells, each containing a complete nucleus with 46 chromosomes.
Mitosis — What Happens
Mitosis is the type of nuclear division that produces two genetically IDENTICAL nuclei.
You do NOT need to know the names of the specific phases (prophase, metaphase, anaphase, telophase) for Foundation GCSE.
What you DO need to know is the overall process:
1. Chromosomes condense and become visible (they have already been duplicated during interphase)
2. The duplicated chromosomes line up along the middle of the cell
3. The two copies of each chromosome are pulled to OPPOSITE ENDS of the cell
4. The nuclear membrane reforms around each set — two new nuclei form
5. The cytoplasm divides (cytokinesis) → two daughter cells, each with 46 chromosomes
RESULT: two cells genetically identical to each other AND to the original parent cell.
Why Mitosis is Important
Mitosis is used for:
• GROWTH: from a single fertilised egg, all the trillions of body cells are produced by repeated mitosis.
• REPAIR: damaged tissues are repaired by producing new identical cells — e.g. healing a cut skin wound.
• REPLACEMENT: some cells wear out quickly and must be continuously replaced — red blood cells (~120 days), gut lining cells (~5 days), skin cells (~2–4 weeks).
• ASEXUAL REPRODUCTION: some organisms reproduce entirely through mitosis — all offspring are clones of the parent.
CANCER — when the cell cycle goes wrong:
Normally, the cell cycle is tightly controlled by regulatory genes.
If a MUTATION occurs in these regulatory genes, the control is lost and cells divide uncontrollably.
This produces a mass of cells called a TUMOUR.
Benign tumour: grows in one place, does not invade surrounding tissue, usually not life-threatening.
Malignant tumour (cancer): cells break away, travel through blood or lymph, and form NEW tumours elsewhere in the body — this spreading is called METASTASIS.
Treatments: surgery (remove the tumour), radiotherapy (gamma rays damage tumour cell DNA), chemotherapy (drugs that kill rapidly dividing cells).
⚠️ Common Mistake
Mitosis produces TWO daughter cells that are genetically IDENTICAL to each other and to the parent — same number of chromosomes (46 in humans). Do NOT confuse with meiosis (not required at Foundation but you will hear the word). Meiosis produces FOUR cells each with HALF the chromosomes — used only for making gametes. Mitosis = for the BODY. Meiosis = for GAMETES.
📌 Key Note
Mitosis: growth, repair, replacement. Two identical daughter cells. 46 → 46. DNA replicates BEFORE division. Cancer = uncontrolled mitosis caused by mutation in regulatory genes.
🎯 Matching Activity — Cell Cycle — Match the Phase to What Happens
Match each description to the correct phase of the cell cycle. — drag the symbols on the right to match the component names on the left.
Interphase — growth (G1)
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Interphase — DNA replication (S phase)
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Mitosis
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Cytokinesis
Drop here
Cell grows in size and produces more organelles and proteins
Duplicated chromosomes separate to opposite ends of the cell — two new nuclei form
Every chromosome is copied — cell now has double the DNA
Cytoplasm divides — two genetically identical daughter cells produced
⭐ Higher Tier Only
The cell cycle has three stages: (1) growth/interphase — cell grows, DNA replicates, organelles increase; (2) mitosis — chromosomes line up and are pulled to opposite ends of the cell; (3) cytokinesis — cytoplasm divides to form two identical daughter cells. Cancer results from uncontrolled cell division caused by mutations in genes controlling the cell cycle. Mitosis produces two genetically identical diploid daughter cells.
🎯 Test Yourself
Question 1 of 5
1. A human body cell has 46 chromosomes. How many chromosomes will each daughter cell have after mitosis?
2. What must happen to DNA before a cell can divide by mitosis?
3. What is the difference between a benign and a malignant tumour?
4. Which of the following best describes the purpose of mitosis?
5. Cancer is caused by...
⭐ How Well Do You Understand This Topic?
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