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4.4.7Additional lessons

Uncontrolled Cell Division and Tumour Formation — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 46 of 101

📚 IB Biology HL Hub / Topical Revision
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Part 1 of 5How Control Is Lost

4.4.7 Uncontrolled Cell Division and Tumour Formation

A tumour is a mass of cells formed by uncontrolled division of one cell that no longer responds to normal signals.

Proto-oncogenes
  • Normal genes that stimulate division, such as those for growth factor receptors, are tightly regulated in healthy cells.
Oncogenes
  • A mutation can turn a proto-oncogene into an oncogene that drives division continuously, even without a signal.
Tumour suppressor genes
  • Genes such as p53 stop the cycle or trigger cell death when DNA is damaged;
    • if mutated they no longer protect the cell.
Accumulation
  • Several mutations in the same cell line are usually needed, which is why cancer risk rises with age.

Part 2 of 5Types of Tumour

Benign
  • The cells stay together in one mass, often surrounded by a capsule, and do not invade other tissues.
Malignant
  • The cells invade nearby tissue and may enter blood or lymph vessels, so the tumour is cancerous.
Primary tumour
  • The original site where the mutated cell started dividing.
Secondary tumour
  • Cells that spread from the primary tumour and establish new growths elsewhere, a process called metastasis.
FeatureBenignMalignant
Spread to other tissuesNoYes
Growth rateUsually slowOften rapid
Cell structureClose to normalAbnormal, less specialised

Part 3 of 5Causes and Risk Factors

Mutagens
  • Carcinogens such as tobacco smoke chemicals, UV light and ionising radiation increase the chance of mutations.
Viruses
  • Some viruses, for example human papillomavirus, are linked to cancers such as cervical cancer.
Inheritance
  • Inheriting a faulty copy of a tumour suppressor gene gives a higher chance that a second mutation will start a cancer.
Lifestyle
  • Smoking, diet and sun exposure change risk, which is why prevention advice targets these factors.

Part 4 of 5Stages of Tumour Development

1
A single cell acquires a mutation in a gene controlling division and starts to divide more often than normal.
2
Its descendants collect further mutations, some of which switch off checkpoints and cell death.
3
A mass forms; a malignant tumour then invades the surrounding tissue and gets its own blood supply.
4
Cells break away, travel in the blood or lymph, and form secondary tumours in other organs.

Part 5 of 5Exam Checklist & Tips

4.4.7 – Key Points for the Exam

  • Tumour An abnormal mass of cells formed by uncontrolled division.
  • Oncogene A mutated gene that promotes uncontrolled cell division.
  • Metastasis Spread of cancer cells from a primary tumour to form secondary tumours.
  • Carcinogen A substance or radiation that can cause cancer.

Common Mistakes

  • Saying all tumours are cancerous; benign tumours do not spread.
  • Claiming one mutation is always enough to cause cancer.
  • Describing metastasis as the first tumour forming; it is the spread to new sites.
  • Stating that cancer is caused only by inheritance.

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Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.