16 Mar

Usually a cancer begins when a single cell within the body undergoes essential changes in its DNA so that the ‘switches’ which control the processes of proliferation and of differentiation are altered. That cell multiplies by dividing into two cells and the cancer characteristics are transmitted in the DNA to each new ‘daughter’ cell. Hence each cell that is the product of cellular proliferation has the same version of the genetic code. In this way the characteristics of a cancer are inherited from cell to cell arid retained by all the cells within that cancer. Cancer is thus a generic disease, because the genetic information is passed from cell to cell within the cancer. It is not usually a genetic disease in the sense that its occurrence is passed from parents to their children.
So what are the essential switches that control how the cell behaves and how are they altered to turn that cell into a cancer cell? By no means all the answers to these questions are yet available. However, the new biology has provided a number of them and yielded glimpses of several more.

The first point to make is that the change into a cancer cell does not occur as a result of an alteration in a single controlling gene. A series of changes, as many as half a dozen, will usually be necessary before the full characteristics of a cancer cell develop. One or two changes may produce an abnormal cell, but without all of those characteristics of growth, invasion and metastasis that we considered in the last section.
The characteristic of abnormal growth is passed on to ‘daughter’ cells but does not yet form a cancer. The second change increases the abnormal growth pattern and it may give rise to a lump, but this does not spread and is not yet cancer. A further change occurs and now, as we look down the microscope, the growth appears like a cancer but it is growing slowly and does not spread. After a fourth change the cancer starts to grow more rapidly, but a fifth change is necessary before it has all the features of growth and spread which are the hallmarks of the disease. In no single cancer are all these stages, or the exact number of stages, clear, but for some cancers a great deal of information exists. Two important kinds of gene in which changes occur to promote the development of a cancer have now been discovered and there are many examples of each.

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