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18 Cancer 2009 - Biol 61 Cancer Biology(Ch 18 p.373-377...

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Biol 61 – Cancer Biology (Ch. 18 p.373-377) 3/18/09 Cancer Biology A person who has cancer has a tumor, and a tumor is just a collection of out-of-control cells that forms because of unregulated cell growth/cell division. Cancer is caused by mutations in the DNA that lead to alterations in the expression of certain genes in a cell. A normal cell will only divide when: 1. it is told to do so 2. it decides it is healthy enough to do so Cancer cells are cells that no longer listen to instructions about whether or not they should divide, and they also ignore proteins that tell them they are damaged. Obviously, when you are a little kid many of the cells in your body are busy dividing to help you grow. Even in adults the cells in some tissues turn over very rapidly – for example, the cells lining the intestines are replaced every few days by new ones that are formed from cell division. However, most cells in the adult body are NOT actively dividing, and they might only begin to divide to replace damaged tissue (this cell division is actually carried out by ‘stem cells’ that are found within many tissues). “Resting” cells are said to be in the G0 state, and cells enter the G1 phase of the cell cycle only when they are going to divide. One of the key genes that helps move a cell into and through G1 makes a protein called cyclin D, and activation of cyclin D expression helps a cell enter the G1 phase of the cell cycle. Cells normally have many different genes whose protein products make a cell divide, but only when it is told to (due to growth, damage, or normal turnover, etc.). An example of a gene like this is ras (see Fig. 18.21).
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