The Biology of Cancer
Normally cells in our body only undergo mitosis for growth and repair. There are genes that control cell division so that mitosis does not occur too often. However, sometimes these processes that regulate cell division go wrong. When this happens, a tumour may result.
Types of Tumour
- A tumour is a mass of cells that have divided too many times.
- Some tumours may be benign.
- Benign tumours are slow-growing and stay within 1 tissue. They do NOT spread to other parts of the body and are NOT usually life-threatening. However they may cause damage e.g. by pressing on an important nerve or blood vessel. When benign tumours are removed they do NOT usually grow back.
- The other types of tumour are malignant.
- Malignant tumours usually grow rapidly. They invade surrounding tissues and they also have cells that break off from the main tumour. These can spread around the body in the blood or lymph vessels and form a new tumour somewhere else. Malignant tumours are life-threatening unless they are treated at an early stage.
Control of Cell Division
Genes called proto-oncogenes stop cells dividing by mitosis too often. They work in one of two ways.
- Some proto-oncogenes code for receptor proteins which are present on cell membranes. When the receptor is activated by a specific growth factor it then activates the genes that stimulate cell division.
- E.g. a normal cell receives signals from growth factors that tell it when to divide: Growth factor fits into the receptor protein in the cell membrane. Relay proteins take it the signal through the cytoplasm. The nuclear protein then switches on genes needed for DNA replication inside the nucleus.
- Other proto-oncogenes produce the growth factors that stimulate cell division.
- However a change in the base sequence of these proto-oncogenes occurs – a mutation. When a photo-oncogene mutates it can become an oncogene. Oncogenes cause uncontrolled cell division in one of two ways.
- Some oncogenes produce a different form of the receptor proteins that stimulate cell division, even when the growth factor is not present.
- Other oncogenes produce uncontrolled amounts of growth factors.
Cells also contain tumour suppressor genes.
- These genes code for proteins that stop cells dividing. They also cause cells with damaged DNA to die.
- If a tumour suppressor gene mutates, the cell may carry on dividing uncontrollably and passing on its damaged DNA to its daughter cells.
Our cells have many proto-oncogenes and tumour suppressor genes. Usually several of these genes must mutate before cancer develops.
Primary and Secondary Tumours
An early primary tumour can develop and spread to form secondary tumours.
- An early tumour begins as an abnormal mass in one tissue of the body.
- As it grows larger, it develops its own blood and lymphatic vessels BUT it is still in one place. This is a primary tumour.
- If a primary tumour is detected and removed at this stage it is very likely it will not return.
- However, as the primary tumour grows some of its tumour cells may break off and squeeze into blood and lymph vessels.
- These cells travel around the body and can set up new tumours in other parts of the body. E.g. tumour cells adhere to blood vessel walls and squeeze through to other parts of the body. These tumours are called secondary tumours.
- The spreading of tumours is called metastasis. Another name for a secondary tumour is a metastasis.
Causes of Cancer
There are many different kinds of cancer, each caused by different factors. Some kinds of cancer are genetic but others are caused by environmental factors. For example:
1. Genetic Factors
Some people have proto-oncogenes that are more likely to mutate then others. This is why some kinds of cancer are more common in certain families e.g. there are 2 genes called BRCA1 and BRCA2 that increase the chance that a person will get breast cancer.
People are more likely to suffer from cancer as they get older. Partly because they have had more years of exposure to environmental factors that cause cancer, such as ionising radiation and cancer-causing chemicals. Also because they have had more chance to accumulate damage to their DNA.
3. Ionising Radiation
This includes X-rays, alpha- and beta-radiation. Ionising radiation contains a great deal of energy that can penetrate into our body cells and break bonds in DNA molecules.
4. Ultraviolet Radiation
This has less energy than ionising radiation but has enough energy to penetrate skin cells. It breaks bonds in the DNA of skin cells and can cause skin cancer.
Many chemicals can cause cancer. These are called carcinogens. Cigarette smoke contains several different carcinogens. Asbestos fibres, some pesticides and diesel exhaust can also cause cancer.
Some viruses can cause cancer. The best known example is Human Papilloma Virus (HPV), some kinds of which can cause cervical cancer. This is because these types of HPV carry the code for a protein that interferes with a tumour suppressor gene. HPV is passed on during sexual intercourse.