-> Different cells exhibit different patterns of division, i.e., skin cells divide frequently, liver cells retain ability to divide in appropriate situations, nerve and muscle cells do not divide

-> Some cells divide only in presence of specific regulatory substances called growth factors

-> Crowding inhibits cell division - density-dependent inhibition (lacking in cancer cells)

-> Whether a cell is destined to divide is determined at the restriction point occurring late in G1 phase of cell cycle (if cell not destined to divide it may exit cell cycle prior to restriction point and enter G0 phase

-> For actively dividing cells, the ratio of cytoplasmic volume to genome size is the most important indicator of whether a cell will pass the restriction point

-> Once a cell passes the restriction point it is destined to divide

-> A high concentration of MPF (maturation promoting factor) is required for a cell to progress from late interphase (G2) to mitosis; MPF is a protein kinase enzyme that activates other proteins

Summary of control of the cell cycle

1. The G1 phase is the most variable

2. Nutritional status, growth factors, cell density, and developmental state affect the length of G1 and whether a cell will pass the restriction point and divide

3. A cell that does not pass the restriction point will enter the G0 phase as a nondividing cell

4. When the cell acquires enough cytoplasm to reach the threshold volume-to-genome ratio, the cell will irreversibly be committed to divide

5. The cell duplicates its chromosomes in the S phase and continues to grow in the G2 phase

6. The transition from interphase to mitosis requires a threshold concentration of MPF to activate the proteins required for mitosis