Difficulty: Introductory | Prerequisites: Cell Cycle Regulation and Checkpoints notes (the companion document). You need to understand cyclins, CDKs, and the three checkpoints before this material makes sense.
The previous set of notes explained how the cell cycle is controlled. This set covers what happens when those controls break. Cancer is, at its core, a disease of unregulated cell division. Mutations in two classes of genes, proto-oncogenes (accelerators) and tumour suppressor genes (brakes), disrupt the normal checks on growth. Understanding these mechanisms is central to both introductory biology and to grasping how cancer treatments are designed. You should already be comfortable with checkpoints, cyclins, and CDKs before reading on.
Cancer arises when mutations turn proto-oncogenes into always-on oncogenes (stuck accelerator) or knock out tumour suppressor genes like p53 (broken brakes). Cancer cells ignore growth-factor requirements, evade checkpoints, resist apoptosis, and can eventually invade other tissues (metastasis). A single mutation is rarely enough; cancer typically requires an accumulation of several mutations.
Proto-oncogene
A normal gene whose protein product stimulates cell growth and division. Think of it as the cell's accelerator pedal. Proto-oncogenes are a necessary part of healthy cell function.
In simple terms, proto-oncogenes tell cells to grow, and that is perfectly normal, until they mutate.
Oncogene
A mutated version of a proto-oncogene that is permanently switched on, driving cell division even when it should not. An oncogene is a gain-of-function mutation: the accelerator is stuck down.
Tumour Suppressor Gene
A gene whose protein product inhibits cell division or promotes apoptosis. Tumour suppressors are the brakes of the cell cycle.
In simple terms, these genes say "stop dividing" or "self-destruct if something is wrong." Cancer requires these brakes to fail.
p53
The most studied tumour suppressor gene. Its protein product is a transcription factor activated by DNA damage. When active, p53 halts the cell cycle and upregulates DNA-repair genes. If the damage cannot be repaired, p53 triggers apoptosis.
Think of p53 as the cell's quality inspector: it stops the line when something is broken and scraps the product if it cannot be fixed.
Ras
A proto-oncogene whose protein product relays growth-factor signals from the cell membrane to the nucleus. When Ras is mutated, it is synthesised in a permanently active state, so it sends "grow" signals without any external growth factor.
Apoptosis
Programmed cell death. The cell dismantles itself in an orderly way and is recycled by neighbouring cells. Apoptosis is a normal, healthy process that removes damaged or unneeded cells.
In simple terms, it is the cell's self-destruct button.
Benign Tumour
An abnormal mass of cells that grows only at its original site and does not invade surrounding tissues or spread to other parts of the body. Benign tumours are not cancer.
Malignant Tumour
An abnormal mass of cells that can invade neighbouring tissues, enter blood vessels, and spread to distant sites. Having a malignant tumour is what defines cancer.
Metastasis
The process by which cancer cells spread from the original tumour to distant locations in the body, typically via the bloodstream or lymphatic system.
In simple terms, metastasis is cancer going on the move.
Angiogenesis
The formation of new blood vessels. Tumour cells release signalling molecules that stimulate capillaries to grow toward the tumour, supplying it with oxygen and nutrients.
Think of it as the tumour building its own supply line.
Dedifferentiation
The loss of specialised cell features. Cancer cells often revert to a less specialised state, losing the functions of the tissue they came from.
In simple terms, the cell "forgets" what it was supposed to be.
Proto-oncogenes code for proteins that stimulate cell growth and division. They are the gas pedal.
Tumour suppressor genes code for proteins that inhibit the cell cycle or promote apoptosis. They are the brakes.
Cancer requires disruption on both sides: an accelerator stuck on (oncogene) and brakes that no longer work (loss of tumour suppressor).
Mutations with opposite effects on their respective proteins can produce the same outcome in the organism: uncontrolled growth.
Normal cells divide only when an external growth factor binds to a receptor and triggers a signalling cascade. Cancer cells can sidestep this requirement in three ways:
They produce their own growth factor (autocrine signalling), so no outside signal is needed.
They have a mutant receptor or signalling protein (such as mutated Ras) that is permanently active, relaying the "grow" signal without any ligand binding.
They have an abnormal cell cycle control system that no longer requires a growth-factor input at all.
Ras is a signalling protein downstream of growth-factor receptors.
In its normal form, Ras toggles between active and inactive states.
When the Ras gene is mutated into an oncogene, the protein is synthesised in a permanently active state. The cell membrane receptor no longer needs a growth factor to start the signalling cascade, so the cell behaves as if it is constantly being told to divide.
p53 is a transcription factor activated by DNA damage.
When active, p53 binds to DNA and upregulates genes that halt the cell cycle, giving the cell time to repair the damage.
If the damage is irreparable, p53 triggers genes involved in apoptosis, destroying the cell before it can pass on faulty DNA.
Loss of functional p53 means damaged cells continue to replicate, and the damage accumulates with every division.
Uncontrolled proliferation. They divide without responding to normal stop signals.
Dedifferentiation. They lose specialised features and may not function like the tissue they originated from.
Abnormal appearance. Irregular shape, larger nuclei, and often aberrant chromosome numbers.
Loss of anchorage dependence. They can grow without being attached to a surface.
Loss of density-dependent inhibition. They pile up rather than forming a single layer.
Degraded extracellular attachments. They are not tethered to neighbouring cells, which allows them to detach and roam.
Angiogenesis. They release signalling molecules that stimulate new blood vessels to grow toward the tumour, supplying oxygen and nutrients.
Metastatic potential. They can enter the bloodstream and establish secondary tumours at distant sites.
A benign tumour grows locally and cannot spread by invasion or metastasis. It is not cancer.
A malignant tumour invades surrounding tissues, enters blood vessels, and can metastasise to distant sites. Cancer is defined as having a malignant tumour.
Metastasis is made possible by the loss of both density dependence and anchorage dependence.
As a tumour grows, its interior cells need oxygen and nutrients. Tumour cells release growth factors that stimulate nearby capillaries to sprout and grow toward the mass. This new blood supply sustains the tumour and, critically, provides a route for cancer cells to enter the bloodstream and metastasise.
"Proto-oncogenes are bad genes." They are not. Proto-oncogenes are normal, essential genes that regulate cell growth. They only become dangerous when mutated into oncogenes.
"A single mutation causes cancer." Cancer typically requires an accumulation of multiple mutations over time, often in both oncogenes and tumour suppressor genes. One mutation alone is rarely sufficient.
"All tumours are cancer." Benign tumours grow locally and do not spread. Only malignant tumours are classified as cancer.
"p53 directly repairs DNA." It does not. p53 is a transcription factor that activates other genes responsible for repair or apoptosis. It is the manager, not the repair crew.
⚠️ Know the car analogy cold: proto-oncogenes = gas pedal, tumour suppressor genes = brakes. Expect a question asking you to classify a gene or explain the analogy.
⚠️ Be able to list the three ways a cancer cell can divide without a growth factor (makes its own, has a constitutively active receptor/signal protein, has an abnormal control system).
⚠️ Understand mutated Ras: it is synthesised in a permanently active state, so no growth factor is needed to activate the membrane receptor.
⚠️ Know what p53 does step by step: DNA damage activates it, it binds DNA, it upregulates repair and cell-cycle-arrest genes, and if repair fails, it triggers apoptosis.
⚠️ Be able to distinguish benign from malignant tumours and explain how metastasis occurs (loss of anchorage and density dependence, entry into bloodstream, colonisation of distant sites).
⚠️ Angiogenesis is a commonly tested concept: know that tumour cells release growth factors that recruit new blood vessels.
True or False: Proto-oncogenes are harmful genes that should be silenced.
Fill in the blank: When a proto-oncogene mutates and becomes permanently active, it is called an _______.
True or False: p53 is a kinase that directly phosphorylates DNA-repair proteins.
Fill in the blank: A tumour that can invade surrounding tissues and spread to distant sites is called a _______ tumour.
True or False: A cancer cell typically has many mutations, not just one.
Answers: 1. False (they are normal, essential growth-regulating genes). 2. oncogene. 3. False (p53 is a transcription factor, not a kinase). 4. malignant. 5. True.
Q: Compare proto-oncogenes and tumour suppressor genes. What does each do, and how does a mutation in each contribute to cancer?
A: Proto-oncogenes code for proteins that stimulate cell growth and division. A gain-of-function mutation turns a proto-oncogene into an oncogene that is always active, pushing the cell to divide nonstop (stuck accelerator). Tumour suppressor genes code for proteins that inhibit division or promote apoptosis. A loss-of-function mutation disables these controls, removing the brakes. Cancer typically involves mutations in both types.
Q: Describe three mechanisms by which a cancer cell can divide without an external growth factor.
A: (1) The cell produces its own growth factor (autocrine signalling). (2) The cell has a mutant signalling protein, such as Ras, that is permanently active and relays the "grow" signal without any ligand. (3) The cell's control system is so abnormal that it no longer requires growth-factor input.
Q: What role does p53 play in preventing cancer, and what happens when it is lost?
A: p53 is a transcription factor activated by DNA damage. It halts the cell cycle so repair can occur, and if the damage is beyond repair, it triggers apoptosis. When p53 is lost or mutated, damaged cells continue to divide, accumulating further mutations with each round of replication.
Q: Explain why not all tumours are considered cancer.
A: Benign tumours grow only at their original site and cannot invade surrounding tissues or metastasise. They are abnormal growths but are not cancer. Malignant tumours can invade neighbouring tissues, enter the bloodstream, and spread to distant sites. Cancer is defined by the presence of a malignant tumour.
Q: What is angiogenesis, and why is it important for tumour growth?
A: Angiogenesis is the growth of new blood vessels. Tumour cells release growth factors that stimulate capillaries to grow toward the tumour, supplying it with oxygen and nutrients. Without angiogenesis, the tumour's interior would be starved. The new blood vessels also give cancer cells a route into the bloodstream, enabling metastasis.
This material is the direct continuation of cell cycle regulation and checkpoints (the companion notes). Every concept here, oncogenes, p53, growth-factor independence, traces back to a specific checkpoint or regulatory mechanism failing.
It connects to signal transduction (growth-factor signalling pathways, receptor tyrosine kinases, Ras–MAPK cascade). Understanding Ras as an oncogene requires knowing how growth-factor signals normally propagate.
It also links to DNA repair mechanisms. p53's role only makes sense if you understand how cells detect and repair DNA damage. Defects in mismatch repair, nucleotide excision repair, or double-strand break repair all feed into the accumulation of mutations that drives cancer.
Cancer biology, proto-oncogene, oncogene, tumour suppressor gene, tumor suppressor gene, p53, TP53, guardian of the genome, Ras, Ras protein, gain-of-function mutation, loss-of-function mutation, apoptosis, programmed cell death, benign tumour, malignant tumour, benign tumor, malignant tumor, metastasis, metastatic cancer, angiogenesis, tumour angiogenesis, dedifferentiation, growth factor independence, autocrine signalling, cell cycle and cancer, uncontrolled cell division, BIO Ch. 15