560 Chapter 17 • Genetic Regulation of Cell Number: Normal and Cancer Cells • Mutations in tumor-suppressor genes that promote tumor formation are loss-of-function recessive mutations. That is, for cancer to occur, both alleles of the gene must encode gene products having reduced or no residual activity (that is, they are null mutations). either to increase the rate of cell proliferation or to pre-vent apoptosis. On the other hand, tumor-suppressor genes encode proteins that arrest the cell cycle or induce apoptosis; in these cases, the cell loses a brake that can stop cell proliferation. It is obvious why mutations that increase the rate of cell proliferation cause tumors. It is less obvious why mutations that decrease the chances that a cell will un-dergo apoptosis cause tumors. The reason seems to be twofold: (1) a cell that cannot undergo apoptosis has a much longer lifetime in which to accumulate proliferation-promoting mutations and (2) the sorts of damage and unusual physiological changes that take
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.