Most cells in our body are continuously dividing, different types of cells at different rates. Each cell contains DNA, a long molecule that holds the genetic code necessary for the cell to function both as a single unit and as part of an entire organism.
Before a cell divides, it has complex mechanisms in place which check that all the DNA has been duplicated correctly. A mistake in the duplication of the DNA means that a gene has been 'mutated' and no longer contains the correct code. These mutations are normal and happen all the time in healthy cells. A healthy cell will recognise these mistakes and activate genes which make DNA repair 'kits.' These repair kits will literally go to the spot on the DNA that is mutated and correct the mistake. The cell then checks that the damage has been repaired correctly, and then goes on to divide into two daughter cells.
Genes are segments of DNA that 'spell out' the instructions for a cellular function. When there has been damage (or mutation) to a gene that regulates the cell cycle, we call this cancer.
These cell-cycle-control genes can be categorised as follows: genes that check that DNA has been copied correctly; genes that make DNA repair kits; genes that control growth promotion; genes that inhibit excess growth; genes that stimulate natural cell death, and genes that limit the number of times a cell can divide during its lifetime.
If the gene that is responsible for checking that DNA has been duplicated correctly is damaged, then the mechanism for checking will be faulty and will not pick up that there is damage in the DNA strand. As a result, the cell will go on to divide, thereby passing on these defunct genes to their daughter cells.
Subsequent generations of cells which inherit this damage are unable to regulate their cycle from the outset. They continue to accumulate mutations and divide irrespective of this damage to the DNA. In a nutshell, cancer is a cluster of dysfunctional cells that are dividing uncontrollably to form a mass. This is the tumour which will eventually compress surrounding structures and deplete surrounding tissues of nutrients, upsetting the balance of normally functioning organs.
Patients: A great explanation, with graphics and drawings can be found at the Khan Academy by clicking here
Practitioners: Published in 2000, Hanahan and Weinberg's original paper entitled 'The Hallmarks of Cancer' has been cited over 120,000 times. Twelve years later, they updated it. You can view Hanahan speaking about it on YouTube or read/download the updated article.
Next week: The biology of metastasis