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The cell cycle is regulated by cyclins and cyclin-dependent kinases.

1 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
• M phase or mitosis and cytokinesisCytokinesis refers to the division of a eukaryotic cell. Cytokinesis usually occurs after a cell has undergone nuclear division as part of mitosis or meiosis. Except for some special cases, the amount of cytoplasm in each daughter cell is the same. In ani, 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 checkpointThe spindle checkpoint blocks entry into anaphase until all chromosomes are properly attached to the mitotic spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles. Only this patter blocks the transition from metaphasechromosomes align in the middle of the cell. Metaphase is a stage of mitosis in the eukaryotic cell cycle in which condensed chromosomes align in the middle of the cell (biology) before being separated into each of the two daughter cells. One of the cell to anaphaseAnaphase is the stage of meiosis or mitosis when chromosomes separate and move to opposite poles of the cell (opposite ends of the nuclear spindle). Centromeres are broken and chromatids rip apart. Mitosis. 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.