Control of the Cell Cycle
Instead of proceeding in lockstep fashion through the cell cycle, normal cells halt cell division at what are called checkpoints. During this stoppage, proteins survey the cell to ensure that conditions for a favorable cellular division have been met .
Before moving from of interphase to the S phase, proteins check that the cell is large enough to divide and that all the nutrients the cell will require during division are available. At the checkpoint, the cell environment is also assessed for the presence of the proteins required to stimulate cells to divide, called growth factors—when growth factors are limited, cell division is discontinued.
There is also a checkpoint at Proteins involved in regulating the cell cycle at ensure that the DNA was replicated properly and double-check the cell size, again making sure the cell is large enough to divide.
The third and final checkpoint that cells must proceed through occurs during metaphase. Proteins present at metaphase verify that all of the chromosomes are attached to microtubules so that cell division can proceed properly.
If the proteins surveying the cell at any of the three checkpoints determine that conditions are not favorable for division, the process is halted. When these proteins are normal, cell division is properly regulated and all is well. When these cycle-regulating proteins are unable to perform their jobs, clumps of cells can build up and form tumors. baca juga : kingdom animalia