DETECTION OF ONCOGENES BY TRANSFORMATION
If DNA is extracted from cancer cells and is then inserted into healthy cells, it may change them into cancer cells. This is known as transformation. (Note that transformation has a related but different meaning in bacterial genetics, where it refers to the uptake and incorporation of any foreign DNA.) Thus, if the presence of an oncogene is suspected, this can be tested by adding a sample of the suspect DNA to suitable cells in culture.
Normal cultured animal cells usually grow as a thin monolayer on the surface of a culture dish. They do not normally crawl on top of each other or pile up into heaps (Fig. 18.8). Once the cells on all sides, they stop dividing, a phenomenon known as contact inhibition. In contrast, cancer cells are uninhibited and they continue to divide and to aggregate into heaps (see Fig. 18.8). Such miniature heaps of cells can be seen when DNA containing an oncogene is added to normal cells in culture. These heaps may be regarded as tiny tumors. If cells from these heaps are injected into an experimental animal such as a mouse, a real tumor will form (Fig. 18.9).
A related feature of cancer cells is anchorage independence. Unlike most normal cells, cancer cells can grow and divide in the absence of binding to proteins of the extracellular matrix. As a consequence, such cancer cells can move around and proliferate in “incorrect” locations in the body. This behavior is especially prominent in malignant (as opposed to benign) tumors.