cell cycle

cell cycle, the ordered sequence of events that occur in a cell in preparation for cell division. The cell cycle is a four-stage process in which the cell increases in size (gap 1, or G1, stage), copies its DNA (synthesis, or S, stage), prepares to divide (gap 2, or G2, stage), and divides (mitosis, or M, stage). The stages G1, S, and G2 make up interphase, which accounts for the span between cell divisions. On the basis of the stimulatory and inhibitory messages a cell receives, it “decides” whether it should enter the cell cycle and divide.

The proteins that play a role in stimulating cell division can be classified into four groups—growth factors, growth factor receptors, signal transducers, and nuclear regulatory proteins (transcription factors). For a stimulatory signal to reach the nucleus and “turn on” cell division, four main steps must occur. First, a growth factor must bind to its receptor on the cell membrane. Second, the receptor must become temporarily activated by this binding event. Third, this activation must stimulate a signal to be transmitted, or transduced, from the receptor at the cell surface to the nucleus within the cell. Finally, transcription factors within the nucleus must initiate the transcription of genes involved in cell proliferation. (Transcription is the process by which DNA is converted into RNA. Proteins are then made according to the RNA blueprint, and therefore transcription is crucial as an initial step in protein production.)

Cells use special proteins and checkpoint signaling systems to ensure that the cell cycle progresses properly. Checkpoints at the end of G1 and at the beginning of G2 are designed to assess DNA for damage before and after S phase. Likewise, a checkpoint during mitosis ensures that the cell’s spindle fibers are properly aligned in metaphase before the chromosomes are separated in anaphase. If DNA damage or abnormalities in spindle formation are detected at these checkpoints, the cell is forced to undergo programmed cell death, or apoptosis. However, the cell cycle and its checkpoint systems can be sabotaged by defective proteins or genes that cause malignant transformation of the cell, which can lead to cancer. For example, mutations in a protein called p53, which normally detects abnormalities in DNA at the G1 checkpoint, can enable cancer-causing mutations to bypass this checkpoint and allow the cell to escape apoptosis.

Discover how ordinary objects work, and expand your understanding of the familiar.

SIGN UP

" height="20" alt="HEW Logo">

Thank you for signing up!"

Back to homepage

By clicking "Sign Up", you consent to the Britannica Privacy Policy and Terms of Use.

More From Britannica

cancer: Proto-oncogenes and the cell cycle

The idea that a cell’s life operated on a cycle marked by distinct phases was introduced in 1953 by Canadian-born English radiobiologist Alma Howard and physicist and cell biologist Stephen R. Pelc. Molecules that serve as the major regulators of the cell cycle were later described by Leland Hartwell, R. Timothy Hunt, and Paul Nurse, who shared the 2001 Nobel Prize for Physiology or Medicine for their discoveries.