Cell Cycle Regulation

Factors that affect cell division Most animal cells divide only when stimulated, and others not at all In laboratory cultures, most normal cells divide only when Attached to a surface—anchorage dependent Have enough space Have enough growth factor

Cell Cycle Control System G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint

Growth factors signal the cell cycle control system Proteins within the cell control the cell cycle Signals affecting critical checkpoints determine whether the cell will go through a complete cycle and divide G1 checkpoint M checkpoint G2 checkpoint Control system Figure 8.9A Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

For many cells, the G1 checkpoint is the “restriction point” A go-ahead signal indicates that the cell will complete the cycle and divide In the absence of a go-ahead signal, the cell may exit the cell cycle and remain in the non-dividing state called G0 phase Many human cells are in the G0 phase until they die—muscle and nerve cells

Cell cycle control system G1 Checkpoint: The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division Growth factor Figure 8.8B Cell cycle control system Plasma membrane Receptor protein Signal transduction pathway G1 checkpoint Relay proteins Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

Cells must have Growth Factors Growth factors are proteins secreted by cells that stimulate other cells to divide After forming a single layer, cells have stopped dividing. Providing an additional supply of growth factors stimulates further cell division. Figure 8.8B

Cells are Density Dependent Cells continue dividing until they touch one another (density-dependent inhibition) Cells anchor to dish surface and divide. When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the dish with a single layer and then stop (density-dependent inhibition). Figure 8.8A Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings

G2 Checkpoint: Repair enzymes make sure DNA has been copied correctly There are plenty of proteins (growth factors) and organelles present Control system Figure 8.9A G2 checkpoint

Metaphase Anaphase M Checkpoint: Anaphase does not begin until all chromosomes are attached to spindle at metaphase plate Metaphase Anaphase

Cancer cells: Growing out of Control Cancer cells have abnormal cell cycles They divide excessively and can form abnormal masses called tumors Cancer cells do not respond normally to the body’s control mechanisms and divide excessively Density-independent—make their own growth factors and continue to divide uncontrolled (“immortal”) Anchorage-independent Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division

Abnormal cells that escape cell-cycle control are products of mutated or transformed normal cells May proliferate to form a tumor—an unregulated growing mass of cells within normal tissue Benign tumor—if cells remain at the original site Malignant tumor—if mass impairs normal function of one or more organs of the body Excessive proliferation Cells with unusual number of chromosomes Aberrant metabolism Detaches and migrates through body (metastasis)

Malignant tumors can invade other tissues and may kill the organism Lymph vessels Tumor Glandular tissue Metastasis 1 2 3 A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. Figure 8.10