1. Regulation of the Cell Cycle The cell cycle can be regulated at any of the phases, but typically, variability in the length of the cell cycle is based on cells exiting the cell cycle at the G1 and G2 phases. In multicellular organisms, cells divide at different rates. Cells in the intestine for example live only three days and must be constantly replaced, whereas cells of the liver live for more than a year. Brain cells on the other hand may never reproduce. The tow phases of arrested growth are referred to as “G 0 ” and “G2 arrest”.

2. Mammalian cell growth is often studied in cultures Cells stop growing when when there is no more surface area for them. This is called Contact inhibition of growth, and the cells are said to have anchorage dependence.

3. Regulation of the Cell Cycle Some cell lines lose the ability to control growth. These cell lines are referred to as transformed, and they have become anchorage independent. Mutations in two types of genes can cause this transformation: Proto-oncogenes: These are genes associated with proteins that signal growth stimulation. Tumor suppressor genes: These are genes associated with proteins that act as cell brakes that inhibit cell proliferation.

4. Cancer Cancer affects many aspects of cellular physiology/biology, but one of the major factors involved is a loss of ability to control cellular proliferation. Typically, about five independent mutations are required to produce malignant cancer cells. This is why older people are more likely to get cancer.

5. Cancer Potential causes: 1. mutations 2. retroviruses 3. exposure to DNA from transformed cells 4. heredity 5. any environmental factor that can induce 1, 2, and/or 3. 6. other