MITOSIS: CELL CYCLE CONTROL

OVERVIEW: Has 4 phases – G 1, S, G 2, and M Driven by specific molecular signals present in the cytoplasm Controlled by a number of checkpoints – at G 1, G 2, and M phases Checkpoint = a critical control point where stop and go signals regulate the cycle Signals from within the cell tell it whether crucial cellular processes have been completed correctly and whether or not the cell should proceed with division. Signals from outside the cell are also registered at checkpoints.

OVERVIEW: In mammalian cells, G1 (the “restriction point”) is the most important.  If “go,” the cell will usually complete the rest of the cell cycle and divide  If “stop,” the cell will switch into G 0 phase (non-dividing state)

REGULATORY MOLECULES: Two main types: Kinases and Cyclins Kinase – enzymes that activate or inactivate other proteins by phosphorylating them  Often called Cdks Cyclins are proteins that attach to a kinase to activate them; they fluctuate in concentration within the cell. For cell division to proceed, two important factors must be present: essential nutrients and growth factors.  Growth Factor = a protein released by certain cells that stimulates other cell to divide

OTHER MEANS OF REGULATION Density-dependent inhibition = a phenomenon in which crowded cells stop dividing  Primarily determined by amount of required growth factors and nutrients available to each cell Anchorage dependence = cells must be attached to a substrate to divide

CANCER What causes it?  Cellular transformation always involves the alteration of genes that somehow influence the cell cycle control system Why does chemotherapy target cancer cells?  Radiation damages DNA in cancer cells more because they have lost the ability to repair such damage  Drugs are toxic to actively dividing cells Why are there side affects?  Normal cells such as intestinal cells, hair follicle cells, and immune system cells are affected too.

CANCER Terms:  Benign tumor – when abnormal cells remain at the original site  Malignant tumor – when abnormal cells become invasive enough to impair the functions of one or more organs  Metastasis – when cancer cells spread to locations distant from their original site  Happens when some tumor cells separate from the original tumor, enter blood vessels and lymph vessels, and travel to other parts of the body