Cell cycle
Also known as: cell division cycle, cell cycle and mitosis
The cell cycle is the ordered sequence of events a cell goes through to duplicate its contents and divide into two daughter cells. It consists of interphase — the G1, S, and G2 phases — followed by mitosis and cytokinesis.
Most of a dividing cell's life is spent in interphase. G1 is the growth phase, in which the cell increases in size and synthesizes proteins and organelles. S phase is DNA synthesis, when the genome is replicated so each chromosome consists of two identical sister chromatids. G2 is a second growth and preparation phase in which the cell checks replication fidelity and assembles the machinery for division. Cells that stop dividing exit into G0, a quiescent state that may be temporary or permanent.
M phase follows and comprises mitosis plus cytokinesis. Mitosis proceeds through prophase (chromosomes condense, the nuclear envelope breaks down, spindle forms), metaphase (chromosomes align at the metaphase plate), anaphase (sister chromatids separate to opposite poles), and telophase (nuclear envelopes reform and chromosomes decondense). Cytokinesis then splits the cytoplasm, yielding two genetically identical daughter cells.
Progression is controlled by cyclins and cyclin-dependent kinases (CDKs), whose paired activity rises and falls to drive transitions, and by checkpoints that halt the cycle when something is wrong. The G1/S checkpoint, or restriction point, verifies cell size, nutrients, and DNA integrity before committing to replication; the G2/M checkpoint verifies that replication is complete and undamaged; the spindle checkpoint confirms every chromosome is properly attached before anaphase. The tumor suppressors Rb and p53 enforce these controls, and loss of that enforcement is a central mechanism in cancer.
Tissues differ in cycling behavior. Labile cells such as bone marrow, gut epithelium, and skin divide continuously; stable cells such as hepatocytes sit in G0 but re-enter the cycle after injury; permanent cells such as neurons, cardiac myocytes, and skeletal muscle remain in G0 for life. This distinction underlies chemotherapy design — the USMLE Step 1, MCAT, and PTCE all test how cell cycle–specific antineoplastic agents (such as antimetabolites acting in S phase and vinca alkaloids or taxanes acting in M phase) differ from cell cycle–nonspecific agents, and why rapidly dividing tissues account for the characteristic toxicities.
Key takeaways
- The cell cycle runs G1 → S → G2 (interphase) → M phase (mitosis and cytokinesis).
- DNA is replicated during S phase, producing sister chromatids joined at the centromere.
- Cyclin-CDK complexes drive progression, and checkpoints at G1/S, G2/M, and the spindle stop a faulty cycle.
- Rb and p53 enforce checkpoint control, and their loss contributes to cancer.
- Labile, stable, and permanent cell types differ in whether and when they re-enter the cycle from G0.