Cell
Cycle and its Regulation
In
response to internal demands and outside conditions, there are
several possible ways that cells can commit. They can state as
they are, commit suicide, differentiate to adapt to the demands,
or divide to multiply themselves. The process by which one cell
divides into two is fundamental to the growth of an embryonic
cell or uncontrolled proliferation of a cancer cell.
Cell cycle refers to the division of a single eukaryotic
cell into two daughter cells via four serial biologically defined
phases: first gap (G1), DNA synthesis (S), second gap (G2) and
mitosis (M). Completion of specific sequence of events in each
phase is required before the cell can proceed from one phase to
another. When the cell is cycling, it must decide whether to commit
itself to DNA synthesis or to exit cell cycle, and enter into
a state of quiescence. Cell division is tightly regulated at discrete
checkpoints in cell cycle. The cell stops at these checkpoints
before proceeding to the following phases. A checkpoint at G1
phase, or a restriction (R) point, is particularly important in
mammalian cells. Additional checkpoints have also been identified
in G2 and M phases, and more are being discovered.
Cyclins,
when combine with their respective kinases (known as cyclin-dependent
kinases CDKs), drive cells through different checkpoints by phosphorylating
different protein sets. Cyclins work by oscillating their levels
at specific points in cell cycle. For instance, G1 cyclins attain
their peak amounts in G1 phase, and mitotic cyclins reach their
highest levels in G2/M phase. Activities of cyclin-CDK complexes
can be further regulated by activating and inhibiting kinases,
activating phosphatase, and CDK inhibitors.
