Controlling cell division The cells within an organisms

Controlling cell division • The cells within an organism’s body divide and develop at different rates: • • • Ex. Skin cells(continue to divide), muscle and nerve cells (stop dividing) liver cells (stop and restart after injury) Cell division is controlled by: 1. Molecular Control a. the presence of essential nutrients, b. Regulatory molecules c. growth factors - proteins that stimulate division 2. Physical Control a. density-dependent inhibition b. anchorage dependence

Molecular Control System presence of certain chemicals triggers next phase § There are three major checkpoints in the cell cycle. (G 1, G 2, M) 1. G 1 checkpoint a. allows entry into the S phase or b. causes the cell to leave the cycle, entering a nondividing G 0 phase 2. G 2 checkpoint 3. M - mitosis

G 0 G 1 S G 2 M checkpoint G 2 checkpoint

1. G 1 – Restriction Point • 1. most important – A. If receives go ahead will divide – B. No go ahead –> G 0 (non-dividing state) Involves Chemical triggers i. e. A. Growth Factors – - proteins secreted by body cells that trigger cells to divide Ex. PDGF Most work at G 1 checkpoint through a Signal Transduction Pathway

Figure 8. 8 B Growth factor EXTRACELLULAR FLUID Plasma membrane Relay proteins Receptor protein Signal transduction pathway G 1 checkpoint G 1 S Control system M G 2 CYTOPLASM

Cultured cells suspended in liquid The addition of growth factor

G 2 Checkpoint • Involves Regulatory molecules (chemical triggers) – A. proteins involved – kinases and cyclins – Kinases - Enzymes that phosphorylate other proteins (activate or inactivate) – Cyclins – proteins that fluctuate in cell – Cyclin + kinases – cyclin-dependent kinases (cdks)

Figure 8. 8 A 1. As cell grows (G 1, S, G 2) cyclin levels increase 2. At G 2 –high level of cyclins 3. Cdks form and become mitosis promoting factors (MPF) 4. MPF fragments nuclear envelope (phosphorylation) 5. MPF destroys cyclin – levels Cyclins increase G 1 checkpoint G 0 S G 1 M G 2 checkpoint

Physical Control 1. Density dependent inhibition a. crowded cells stop dividing b. Cells reach high density have insufficient growth factors and nutrients Anchorage Single layer of cells 2. Anchorage dependence a. Cells must be in contact with a solid surface Removal of cells Restoration of single layer by cell division

Cancer • • • Abnormal cells: 1. Do not exhibit: -density –dependent inhibition - anchorage dependence 2. Cell undergoes transformation - tumor results - 2 types • Benign/ malignant