Cell Cycle control

The cell cycle consists of four main stages, G1 phase, S-phase, G2-phase, and M-phase. G1 is considered the start of the cell cycle and know as the growth or gap stage. In addition, G0 is the phase in which the cell stays in for the majority of its life, in a nondividing state. During G1, the cell prepares all of the various nutrients and materials needed to replicate its DNA. It can also encompass the synthesis of organelle copies and check for growth signals. At the end of G1, there is a checkpoint that checks for damaged DNA, enough nutrients, and correct growth factor signals. Following G1, the cell enters S-phase. During this phase, the cell copies all of its DNA and DNA replication can be halted to fix damaged DNA. If it cannot be repaired, then the cell undergoes apoptosis.  Next G2 phase begins and further growth proceeds as the cell prepares for chromosome segregation. The G2 checkpoint ensures that the copied DNA is not damaged or incomplete. Finally, the cell enters M-phase. Spindles, which are made up of microtubules and other proteins attach to the chromosomes and pull them apart, eventually resulting in cytokinesis and two copies of the same cell. The M-phase checkpoint ensures that chromosomes are all properly attached to the mitotic spindle. If not, CDKI prevent the function of CDKs and effectively stop the M-phase. Again, if the cell cannot fix the issue it undergoes programmed cell death.