Course 9.3Lesson 2 of 3
Cancer is cell division that has escaped from its controls. A cell that grows a tumour has usually collected several mistakes: accelerators that stay on and brakes that no longer work. This lesson shows what the mistakes are, how a tumour spreads, and why the treatments can harm healthy cells.
Try the code and the simulations beside the lesson.
Cancer is not one disease. It is a group of diseases that share one cause: cells divide when they should not. A cancer cell ignores the checkpoints, and it does not die when it should. It makes a mass of cells called a tumour.
NCERT discusses cancer in Class 12, in the chapter on human health and disease. A-Level, IB and AP teach it with the cell cycle. This lesson joins the two.
A normal cell divides only when it gets a growth signal. It stops when it touches its neighbours. This is contact inhibition. A cancer cell has lost it, and goes on dividing in a heap.
| A normal cell | A cancer cell |
|---|---|
| divides only when a growth signal arrives | divides without the signal |
| stops at the checkpoints if the DNA is damaged | passes the checkpoints with damaged DNA |
| dies by apoptosis if the damage cannot be repaired | escapes apoptosis |
| stops dividing when it touches other cells | has lost contact inhibition |
| stays in its own tissue | invades other tissues |
Proto-oncogenes are normal genes that code for the accelerators of the cycle: growth factor receptors, cyclins and other positive regulators. A mutation can turn a proto-oncogene into an oncogene. An oncogene makes the accelerator work too much, or work all the time. It is a gain of function. One changed copy is often enough.
Tumour suppressor genes code for the brakes: Rb, p53 and p21 are examples. A mutation can stop them from working. It is a loss of function. The brake is cut, and the cycle goes on when it should stop. Mutated p53 is found in more than half of all human tumours.
A single mutation is usually not enough. Several changes, in both kinds of gene, collect in one cell and its descendants, often over years. The cell cycle gets shorter, the repair gets weaker, and the cells outgrow the normal ones.
Substances and agents that cause cancer are carcinogens. NCERT groups them as physical, chemical and biological.
Mistakes in copying the DNA in S phase also happen by chance. They are rare, but they add up over a life.
Doctors find cancer by a biopsy, in which a piece of the tissue is examined, and by blood tests and by imaging such as radiography, CT and MRI. Treatment may be surgery, radiation, chemotherapy (drugs) or immunotherapy, in which the immune system is helped to attack the tumour. NCERT names α-interferon as an example.
Many chemotherapy drugs harm cells that are copying their DNA or dividing. Some block the copying of DNA in S phase. Others stop the spindle from forming. They hit cancer cells hard, because these divide often. But they also hit healthy cells that divide often: the hair follicles, the lining of the gut and the bone marrow. That is why hair loss, sickness and a low blood count are side effects.
You should see that a longer cycle slows the tumour, and that a larger share of dying cells slows it too. Even a slow tumour reaches a billion cells in the end, because the number doubles again and again.
The program works out the doubling time of a tumour and the days it needs to reach a billion cells.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Every lesson is open to read. Enrolling also lets you:
Log in to join the discussion.
Loading…
This lesson is free to read. Enroll free to keep your place and take the tests.