Cancer Development

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•  Mutagens, oncogenes and metastasis are involved in the development of primary and secondary tumours 

Tumours are abnormal cell growths resulting from uncontrolled cell division and can occur in any tissue or organ

  • Diseases caused by the growth of tumours are collectively known as cancers


A mutagen is an agent that changes the genetic material of an organism (either acts on the DNA or the replicative machinery)

Mutagens may be physical, chemical or biological in origin:

  • Physical – Sources of radiation including X-rays (ionising), ultraviolet (UV) light and radioactive decay
  • Chemical – DNA interacting substances including reactive oxygen species (ROS) and metals (e.g. arsenic)
  • Biological – Viruses, certain bacteria and mobile genetic elements (transposons)

Mutagens that lead to the formation of cancer are further classified as carcinogens


An oncogene is a gene that has the potential to cause cancer

Most cancers are caused by mutations to two basic classes of genes – proto-oncogenes and tumour suppressor genes

  • Proto-oncogenes code for proteins that stimulate the cell cycle and promote cell growth and proliferation
  • Tumour suppressor genes code for proteins that repress cell cycle progression and promote apoptosis

When a proto-oncogene is mutated or subjected to increased expression it becomes a cancer-causing oncogene

Tumour suppressor genes are sometimes referred to as anti-oncogenes, as their normal function prevents cancer

Relationship between Proto-Oncogenes and Tumour Suppressor Genes



Tumour cells may either remain in their original location (benign) or spread and invade neighbouring tissue (malignant)

Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour

Secondary tumours are made up of the same type of cell as the primary tumour – this affects the type of treatment required

  • E.g. If breast cancer spread to the liver, the patient has secondary breast cancer of the liver (treat with breast cancer drugs)

Formation of Secondary Tumours via Metastasis