Cell cycle
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The cell cycle, or cell division cycle, is the cycle of events in a eukaryotic cell from one cell division to the next. It consists of interphase, mitosis, and usually cell division. The cell cycle is regulated by cyclins and cyclin-dependent kinases. Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules in the regulation of the cell cycle.
Phases
The phases of the cell cycle are:
- The G0 phase is a period in the cell cycle where cells exist in a quiescent state.
- The G1 phase is the first growth phase.
- S phase, during which the DNA is replicated, where S stands for the Synthesis of DNA.
- G2 phase is the second growth phase, also the preparation phase for the cell.
- M phase or mitosis and cytokinesis, the actual division of the cell into two daughter cells.
A surveillance system, so-called "checkpoints", monitor the cell for DNA damage and failure to perform critical processes. Checkpoints can block progression through the phases of the cell cycle if certain conditions are not met. For instance, there is a checkpoint which monitors DNA replication and keeps cells from proceeding to mitosis before DNA replication is completed. Similarly, the spindle checkpoint blocks the transition from metaphase to anaphase within mitosis if not all chromosomes are attached to the mitotic spindle.
If this system senses a problem, a network of signaling molecules instructs the cell to stop dividing. They can let the cell know whether to repair the damage or initiate programmed cell death, a form of which is called apoptosis. Programmed cell death ensures that the damaged cell is not further propagated. For example, a certain protein, called p53, acts to accept signals provoked by DNA damage. It responds by stimulating the production of inhibitory proteins that then halt the DNA replication process. Without proper p53 function, DNA damage can accumulate unchecked. A direct consequence is that the damaged gene progresses into a cancerous state. Today, defects in p53 are associated with a variety of cancers, including some breast and colon cancers.
Some cells, such as neurons, never divide once they become locked in a G0 phase. However, recent data has shown that neurons undergoing cell death re-enter the cell cycle. Addition of cell cycle inhibitors prevent this type cell death called apoptosis.