Sample Page from Chapter 14
Cancer and Aging

Normal Role of Proto-Oncogenes

The pathway for activating cell growth and division has several stages. The proto-oncogenes encode the proteins taking part in this scheme (Fig. 14.14). Not surprisingly, mutations that result in hyperactivation of any of the components involved in this can turn proto-oncogenes into oncogenes.

The major components involved are:

1. Growth Factors: These are proteins circulating in the blood and bringing messages from outside to the cell surface.
2. Cell Surface Receptors: These proteins are found in the cell membrane where they receive messages from outside and pass the signal on.
3. Transcription Factors: These proteins bind to and switch on genes in the cell nucleus (see Ch. 11). This results in the synthesis of new proteins, as opposed to the activation of those which are already present (Fig. 14.14).
4. Signal transmission proteins: These pass on the signal from outside the cell to enzymes or genes needed in cell division. Protein kinases are signal transmission proteins that act by adding a phosphate group to enzymes that are part of the machinery for growth and division. Some of these enzymes are already hanging around, but in an inactive form. Addition of the phosphate group switches needed enzymes on, and also turns others that are no longer required, off (Fig. 14.15).