Cell Cycle Phases
o Every cell has a life cycle that begins with the birth of the cell and ends with the death or division of the cell.
Cell Cycle Diagram
o M ⇒ Mitosis
o G1 ⇒ Gap 1
o G2 ⇒ Gap 2
o S ⇒ Synthesis
o G0 ⇒ Gap 0/Resting.
Gap 0 / G0
o The cell has left the cycle and has stopped dividing.
o In humans, liver cells spend a great deal of time in G0.
o Mature neurons and muscle cells are in G0 permanently.
o Liver (hepatic) and pancreatic cells are normally in G0 but can reenter normal division
Gap / G1 Phase
o G1 is normally the longest phase.
o Cells increase in size in Gap 1, regions of heterochromatin has been unwound and decondensed.
o RNA synthesis and protein synthesis is very active.
o The G1 checkpoint control mechanism ensures that everything is ready for DNA synthesis.
o The main factor in triggering the beginning of S is cell size based upon the ratio of cytoplasm to DNA.
Cell Cycle Checkpoints
Synthesis / S Phase
o DNA replication occurs during this phase.
o Organelles and proteins are produced more slowly.
o Each chromosome is exactly duplicated, but the cell is still considered to have the same number of chromosomes, only now, each chromosome is made of two identical sister chromatids.
Gap 2 / G2 Phase
o During the gap between DNA synthesis and mitosis, the cell will continue to grow.
o Cellular organelles continue to duplicate.
o RNA and protein (especially tubulin for microtubules) are actively synthesized.
o The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis/meiosis) phase and divide.
o It checks for mitosis promoting factor (MPF), when the level is high enough, mitosis is triggered.
Cell Division / Mitosis / Meiosis / M Phase
o Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells.
o A checkpoint in the middle of mitosis (Metaphase Checkpoint) ensures that the cell is ready to complete cell division.
Cell Cycle Citations