Knowing When to Stop Suppose you had a paper cut on your finger. Although the cut may have bled and stung a little, after a few days, it will have disappeared, and your finger would be as good as new.

Bell Work 1.How do you think the body repairs an injury, such as a cut on a finger? 2.How long do you think this repair process continues? 3.What do you think causes the cells to stop the repair process?

Regulating the Cell Cycle

Cell Growth and Cell Division are carefully controlled Not all cells move through the cell cycle at the same rate  Example: Most muscle cells and nerve cells do not divide once they have developed; other cells, such as skin cells, divide rapidly throughout an organism’s life

Figure 10-7: Contact Inhibition The controls on cell growth can be turned on and off

Cell Cycle Regulators  The cell cycle is regulated by a specific protein o The amount of this protein in the cell rises and falls in time with the cell cycle  Cyclins are proteins that regulate the timing of the cell cycle in eukaryotic organisms.

Figure 10-8: Regulating the Cell Cycle A sample of cytoplasm is removed from a cell in mitosis. The sample is injected into a second cell in G 2 of interphase. As result, the second cell enters mitosis.

Internal Regulators Proteins that respond to events inside the cell are called internal regulators Internal regulators allow the cell cycle to proceed only when certain processes have happened inside the cell  Example: all chromosomes must be replicated

External Regulators Proteins that respond to events outside the cell are called external regulators External regulators direct cells to speed up or slow down the cell cycle  Example: healing wounds

Uncontrolled Cell Growth Why is cell growth regulated so carefully?  The consequences of uncontrolled cell growth in an organism are severe → it disrupts the normal functions of the body Cancer is a disorder in which some of the body’s own cells lose the ability to control growth

Cancer Cancer cells do not respond to the signals that regulate the growth of most cells  they divide uncontrollably and form masses of cells called tumors that can damage the surrounding tissues

Cancer Cancer is a disease that can be caused by: environmental factors, genetics (inherited), or viruses  they all have one thing in common: the loss of control over regulating the cell cycle