The cell cycle is the ordered sequence of events through which a cell duplicates its genetic material and usually divides into two daughter cells. It coordinates growth, DNA replication, chromosome segregation, and cell division. In eukaryotes, the conventional division cycle comprises G₁, S, G₂, and M phases, governed by molecular controls that coordinate successive events and respond to conditions inside and outside the cell. The cycle encompasses preparation for division as well as division itself. (ncbi.nlm.nih.gov)
Phases and chromosome duplication
G₁, S, and G₂ together constitute interphase, the interval between successive mitotic phases. Interphase is not an inactive period: cells carry out metabolism, synthesize cellular components, and prepare or duplicate their genetic material. Growth commonly continues across much of the cycle rather than belonging exclusively to one phase. (ncbi.nlm.nih.gov)
- G₁ phase, or first gap phase, follows division. Cells grow and respond to signals that influence whether they proceed toward another division.
- S phase, or synthesis phase, is the period in which nuclear DNA is replicated. Each duplicated chromosome consists of two sister chromatids.
- G₂ phase, or second gap phase, separates DNA synthesis from mitosis. Cells continue preparing for division, while regulatory pathways can delay mitotic entry if replication is incomplete or DNA is damaged.
- M phase includes mitosis, which partitions duplicated chromosomes into daughter nuclei, and usually cytokinesis, which divides the cytoplasm. These processes overlap in time. (ncbi.nlm.nih.gov)
During mitosis, chromosomes become highly condensed, and the mitotic spindle organizes their separation. Sister chromatids segregate to opposite poles before daughter nuclei are established. Replication doubles DNA content but does not itself double the number of chromosome sets: a diploid cell remains diploid during S and G₂, although each chromosome has been duplicated. The usual outcome of a completed mitotic cycle is two cells with the same chromosome complement as the parent. (ncbi.nlm.nih.gov)
Molecular control
Central regulators are cyclin-dependent kinases (CDKs), enzymes whose activities depend on association with regulatory proteins called cyclins. Different cyclin–CDK complexes promote DNA replication or mitotic events. Cyclin abundance changes through regulated synthesis and destruction, while activating or inhibitory phosphorylation and CDK-inhibitory proteins provide additional control. These mechanisms generate successive changes in kinase activity rather than a simple, continuously running timer. (ncbi.nlm.nih.gov)
CDKs modify target proteins by phosphorylation, altering processes such as replication initiation, chromosome condensation, and spindle assembly. Regulated protein degradation also makes important transitions directional. The anaphase-promoting complex/cyclosome promotes destruction of proteins that restrain chromatid separation and helps remove mitotic cyclins. Their degradation lowers mitotic CDK activity, enabling exit from mitosis. Coupling replication initiation to changing CDK activity also helps prevent a second round of DNA replication before division. (ncbi.nlm.nih.gov)
Checkpoints and proliferation signals
Cell-cycle checkpoints are surveillance mechanisms that delay progression when essential processes are incomplete or disrupted. They are regulatory pathways, not additional anatomical stages. DNA-damage responses can act in G₁, during S phase, and in G₂. Such responses provide time for DNA repair and reduce the likelihood of replicating or segregating damaged genetic material. (ncbi.nlm.nih.gov)
The spindle assembly checkpoint restrains anaphase when chromosomes have not achieved appropriate spindle attachment. This links chromatid separation to the machinery responsible for distributing chromosomes. In mammalian cells, the p53 protein provides another important control: after DNA damage, it can increase expression of p21, a CDK inhibitor, thereby delaying cell-cycle progression. Depending on the cellular context, damage responses can also lead to apoptosis. (ncbi.nlm.nih.gov)
Signals controlling proliferation are integrated with this machinery. In many animal cells, passage through the G₁ restriction point reduces dependence on continued growth-factor stimulation for completing the cycle. This commitment mechanism is related to, but distinct from, surveillance of damaged DNA. Nutrient availability and extracellular signals therefore influence whether cells divide, alongside controls monitoring internal events. (ncbi.nlm.nih.gov)
Nondividing states and biological variation
Cells can leave the proliferative cycle and enter G₀. In reversible quiescence, they remain metabolically active and can return to division after suitable stimulation. Other cells withdraw much more permanently as they become specialized; most mature neurons, for example, do not normally divide. G₀ is therefore not a single uniform physiological condition. (ncbi.nlm.nih.gov)
Cycle duration varies substantially among organisms, tissues, and developmental stages. Some early embryonic cycles alternate rapidly between S and M phases with little or no intervening G₁ or G₂, partitioning existing egg cytoplasm into progressively smaller cells. The four-phase scheme describes a common organization, not a universal timetable. (ncbi.nlm.nih.gov)
Research and relation to cancer
Researchers study cycle progression using microscopy, DNA-synthesis labeling, and flow cytometry. DNA-binding fluorescent dyes distinguish populations with unreplicated, intermediate, or doubled DNA content. DNA content alone generally groups G₀ with G₁ and G₂ with M; additional markers are needed to separate those states. Pulse-labeling newly synthesized DNA allows investigators to follow cells through subsequent phases. (ncbi.nlm.nih.gov)
Disrupted growth and division controls contribute to cancer. Changes affecting genes involved in proliferation, growth suppression, or DNA repair can allow inappropriate division and survival. Fundamental discoveries about cycle regulation were recognized by the 2001 Nobel Prize in Physiology or Medicine, awarded jointly to Leland Hartwell, Tim Hunt, and Paul Nurse for identifying key cell-cycle regulators. (cancer.gov)