The cell cycle consists of five stages that take a single cell from its initial growth through to the production of two daughter cells: G1 (first gap), S (synthesis), G2 (second gap), mitosis, and cytokinesis. The first three stages make up a long preparatory stretch called interphase, during which the cell grows and copies its DNA. Mitosis then divides the duplicated chromosomes into two identical sets, and cytokinesis physically splits the cell in half. What makes this process remarkable is not just its reliability but the elaborate system of molecular controls that keep it on track, and how breakdowns in those controls connect to diseases like cancer.
G1, the First Gap Phase
After a cell is born from its parent’s division, it enters G1, the longest and most variable phase. During G1, the cell grows in size, produces proteins, and carries out whatever specialized job it has in the body. A liver cell metabolizes toxins; a skin cell strengthens the barrier. The cell is also receiving signals from its environment: growth factors, nutrients, and contact with neighboring cells all influence whether it will commit to dividing again or pause.
The key decision point in G1 is sometimes called the restriction point. Once a cell passes this threshold, it no longer needs external growth signals to keep moving forward; it is committed to completing the cycle. A second nutrient-sensing checkpoint later in G1, regulated by the mTOR signaling pathway, ensures that the cell has enough raw materials to support the enormous task of duplicating its genome.1PubMed Central. Regulation of G1 Cell Cycle Progression: Distinguishing the Restriction Point from a Nutrient-Sensing Cell Growth Checkpoint(s) These two checkpoints are distinct: the first responds to growth factor signals, while the second monitors nutritional sufficiency. Together, they prevent a cell from entering DNA replication before it is ready.
S Phase and DNA Replication
S phase is when the cell copies every one of its chromosomes. In a human cell, that means duplicating roughly six billion base pairs of DNA with extraordinary accuracy. By the end of S phase, each chromosome exists as a pair of identical sister chromatids joined at a structure called the centromere. They will stay linked until mitosis pulls them apart.
Copying DNA is not just about the DNA itself. The cell also needs to produce massive quantities of histone proteins, the spool-like structures around which DNA is wound. For a long time, the standard view was that histone production only ramped up once DNA replication was already underway. More recent work using live-cell imaging in human cells revealed a more nuanced picture: histone production actually begins in G1, shortly after the cell commits to dividing, building a small initial pool before DNA synthesis starts. The bulk of histone manufacturing then ramps up dramatically once S phase is in full swing.2PubMed Central. Replication-dependent histone biosynthesis is coupled to cell-cycle commitment This staggered approach makes biological sense: having some histones ready before replication begins prevents newly copied DNA from sitting exposed.
G2, the Second Gap Phase
After S phase finishes, the cell enters G2, a shorter gap phase focused on preparation for division. The cell continues to grow, synthesizes proteins needed for mitosis, and performs a critical quality check on the freshly duplicated DNA. Any replication errors that slipped through during S phase get repaired here. The cell also begins organizing the structural machinery it will need to physically separate its chromosomes, including components of the mitotic spindle.
G2 is relatively brief compared to G1, but it is not optional. Cells that skip or rush through G2 risk entering mitosis with damaged DNA, which can lead to mutations in both daughter cells. The checkpoint at the G2/M boundary is one of the most important quality control steps in the entire cycle.
Mitosis Divides the Nucleus
Mitosis is the dramatic phase where the cell’s duplicated chromosomes are separated into two identical sets. It unfolds in a rapid, highly choreographed sequence of substages. The chromosomes condense into compact, visible structures. The mitotic spindle, a scaffold of microtubule filaments, assembles and attaches to each chromosome pair. The sister chromatids are then pulled to opposite ends of the cell. Finally, a new nuclear envelope forms around each set of chromosomes, producing two genetically identical nuclei inside what is still, briefly, a single cell.
The whole process typically takes an hour or less, a remarkably short window given the precision required. Every chromosome must be correctly attached to the spindle before the cell proceeds. A surveillance system called the spindle assembly checkpoint monitors this attachment and delays chromosome separation until every connection is secure. When this checkpoint fails, cells can end up with the wrong number of chromosomes, a condition called aneuploidy that is a hallmark of many cancers.3PubMed Central. Mitotic and DNA Damage Response Proteins: Maintaining the Genome Stability and Working for the Common Good
Cytokinesis Splits the Cell
Cytokinesis overlaps with the tail end of mitosis and finishes the job by physically dividing the cytoplasm, organelles, and cell membrane into two separate daughter cells. In animal cells, this works through a contractile ring made of actin and myosin filaments that tightens around the cell’s midsection like a drawstring, pinching it in two.4PubMed Central. Cytokinesis in eukaryotes
Plant cells face a different challenge because they have rigid cell walls. Instead of pinching inward, plant cells build a new wall from the inside out, using vesicles that fuse together at the center of the dividing cell. For years, this difference made animal and plant cytokinesis look like fundamentally separate processes. More recent research has blurred that line, revealing that vesicle trafficking from endosomes plays a surprisingly important role in animal cytokinesis too, and that the terminal phase of animal cell division, guided by midbody microtubules, resembles the plant process more than anyone expected.5Developmental Biology. Cytokinesis in plant and animal cells: Endosomes ‘shut the door’ Once cytokinesis is complete, each daughter cell enters G1, and the cycle can begin again.
G0, the Exit from Cycling
Not every cell keeps dividing. Many cells exit the cycle after mitosis and enter a state called G0. Neurons in your brain, for example, are essentially permanent residents of G0. Mature muscle fibers and fully differentiated red blood cells are similarly out of the game. For these cells, the cycle is not paused so much as voluntarily abandoned in favor of specialization.
G0 is not a single uniform state, though. Some cells in G0 are quiescent, meaning they have stopped dividing but retain the ability to re-enter the cycle if the right signals arrive. A liver cell, for instance, can sit in quiescence for years and then spring back into action after tissue damage. Other cells in G0 have entered senescence, a permanent exit from which there is no return under normal conditions.6PubMed. Cell dormancy plasticity: quiescence deepens into senescence through a dimmer switch Researchers distinguish between the two partly by looking at protein synthesis activity: senescent cells are metabolically busy despite being unable to divide, while quiescent cells tend to be more metabolically subdued.7PubMed Central. Different Stages of Quiescence, Senescence, and Cell Stress Identified by Molecular Algorithm Based on the Expression of Ki67, RPS6, and Beta-Galactosidase Activity
This distinction matters for medicine. Cancer cells are, by definition, cells that refuse to enter G0 when they should. And some cancer treatments work by forcing cancer cells into senescence, so understanding what keeps a cell locked in G0, and what lets it escape, is an active area of research.
Checkpoints Keep the Cycle Honest
Threaded throughout the cycle are molecular checkpoints, essentially surveillance systems that halt progress if something is wrong. The three major ones are the G1/S checkpoint (which blocks DNA replication if the cell is not ready or the DNA is damaged), the G2/M checkpoint (which prevents entry into mitosis if replication errors remain), and the spindle assembly checkpoint during mitosis (which prevents chromosome separation until all chromosomes are properly attached to the spindle).
These checkpoints are not independent of each other. The DNA damage response and the spindle assembly checkpoint share proteins and collaborate to maintain genome stability across the entire cycle.8PubMed Central. A role for the spindle assembly checkpoint in the DNA damage response When DNA damage occurs even during mitosis, the cell can activate pathways that degrade key enzymes and effectively shunt the cell back to an interphase-like state to allow repair.9PubMed Central. DNA damage during the spindle-assembly checkpoint degrades CDC25A, inhibits cyclin-CDC2 complexes, and reverses cells to interphase This kind of redundancy is critical. Losing a single checkpoint layer does not usually doom a cell, because the remaining layers can compensate. It is when multiple checkpoint mechanisms fail simultaneously that genomic instability spirals out of control.
Cyclins and CDKs Drive the Transitions
The engine powering movement through the cell cycle is a family of enzymes called cyclin-dependent kinases, or CDKs. These enzymes are always present in the cell, but they only become active when they bind to a partner protein called a cyclin. Different cyclins rise and fall in concentration at specific points in the cycle, creating what researchers describe as “cyclin waves” that switch different CDKs on and off in sequence.10Signal Transduction and Targeted Therapy. Cyclin-dependent protein kinases and cell cycle regulation in biology and disease Ten cyclins grouped into four major types (A, B, D, and E) partner with cell cycle CDKs to orchestrate the transitions between phases.
In G1, D-type cyclins accumulate in response to growth factor signals and activate CDK4 and CDK6. As the cell approaches the G1/S boundary, E-type cyclins pair with CDK2 to push the cell into DNA replication. Once in S phase, A-type cyclins take over with CDK2 to manage replication, and later A- and B-type cyclins activate CDK1 to trigger entry into mitosis.11PubMed. Cyclin-dependent protein kinases: key regulators of the eukaryotic cell cycle After mitosis, B-type cyclins are rapidly destroyed, resetting the system for the next round.
This sequential activation of CDK-cyclin pairs is tightly regulated by additional brakes. The tumor suppressor protein RB, in its active form, binds to transcription factors of the E2F family and blocks the expression of genes needed for S phase entry. When cyclin D-CDK4/6 and cyclin E-CDK2 phosphorylate RB, E2F is released and switches on those genes.12PubMed Central. Cell cycle regulation: p53-p21-RB signaling If DNA damage is detected, the tumor suppressor p53 activates production of a CDK inhibitor called p21, which blocks CDK activity and prevents RB from being phosphorylated, keeping the brakes firmly on.13Cancer Cell. Mechanisms of Disease: The Retinoblastoma Protein and p53 This p53-p21-RB pathway is one of the most frequently disrupted systems in cancer.
When the Cell Cycle Goes Wrong
Cancer is fundamentally a disease of uncontrolled cell division. Nearly every cancer involves mutations that either activate growth-promoting signals (hitting the gas pedal) or disable checkpoint mechanisms (cutting the brakes). The p53 gene is mutated in roughly half of all human cancers. RB pathway disruptions are found across many tumor types as well.14Cancer Research Communications. Functional Profiling of p53 and RB Cell Cycle Regulatory Proficiency Suggests Mechanism-Driven Molecular Stratification in Endometrial Carcinoma
Understanding these molecular details has led directly to targeted therapies. Drugs that inhibit CDK4 and CDK6, the kinases that phosphorylate RB during G1, have been used with notable success in treating hormone receptor-positive breast cancers and are being tested against many other tumor types.15PubMed Central. CDK4 and CDK6 kinases: From basic science to cancer therapy By blocking these kinases, the drugs essentially reinforce the G1 brake, preventing cancer cells from progressing to S phase and replicating their DNA. Other therapeutic strategies aim to exploit checkpoint deficiencies in cancer cells: if a tumor has lost one checkpoint, targeting a compensatory checkpoint can selectively kill the cancer cells while leaving normal cells, which still have both checkpoints intact, relatively unharmed.16PubMed Central. Defective cell cycle checkpoints as targets for anti-cancer therapies
Not All Cells Follow the Standard Script
The five-stage cycle described above is the textbook version, and most actively dividing cells follow it. But some cell types take deliberate shortcuts. In a process called endoreplication, cells replicate their DNA without ever entering mitosis, producing cells with multiple copies of the genome. This is not an error; it is a feature. Certain cells in plants use endoreplication routinely during normal development and as a response to stress, such as pathogen attack or DNA damage.17PubMed. Molecular control and function of endoreplication in development and physiology
In mammals, the strategy also shows up. Megakaryocytes, the bone marrow cells that produce blood platelets, become polyploid (containing many copies of the genome) before fragmenting into the platelets circulating in your blood. Trophoblast giant cells in the placenta use a similar approach, growing large to form a protective barrier between maternal and embryonic tissues.18PubMed Central. Fundamental differences in endoreplication in mammals and Drosophila revealed by analysis of endocycling and endomitotic cells Polyploidy also contributes to disease: genome doubling is observed in some cancers, and understanding how cells bypass mitosis has implications for understanding how tumors evolve.19PubMed Central. Endoreplication and polyploidy: insights into development and disease
How Scientists Watch Cells Cycle in Real Time
For most of the history of cell biology, studying the cycle meant killing cells at different time points and inferring what happened in between. That changed dramatically in 2008 with the development of FUCCI (fluorescence ubiquitination cell cycle indicator), a technology that color-codes each phase of the cycle using fluorescent proteins. Cells in G1 glow one color, cells in S/G2/M glow another, and transitions are visible as color shifts in real time under a microscope.20PubMed Central. FUCCI Real-Time Cell-Cycle Imaging as a Guide for Designing Improved Cancer Therapy: A Review of Innovative Strategies to Target Quiescent Chemo-Resistant Cancer Cells
FUCCI has been especially valuable for cancer research because it makes quiescent cells visible in a tumor. Chemotherapy tends to kill actively dividing cells, but quiescent cancer cells that have temporarily slipped into G0 can survive treatment and drive relapse. Being able to see which cells are cycling and which are resting in real time is guiding the design of combination therapies that target both populations. Refinements to the original FUCCI system now allow researchers to simultaneously track other cellular structures, like the primary cilium, alongside cell cycle phase, opening up questions about how the cycle coordinates with other aspects of cell behavior.21PubMed. Live Imaging and Analysis of Cilia and Cell Cycle Dynamics with the Arl13bCerulean-Fucci2a Biosensor and Fucci Tools
An Ancient and Deeply Conserved System
The cyclin-CDK system is not a recent evolutionary invention. Comparative studies across plants, animals, and fungi have found that the core regulatory proteins governing the cell cycle, including RB, E2F transcription factors, and cyclins of the A and D types, were already present in the last common ancestor of all eukaryotes.22PubMed Central. Evolution of networks and sequences in eukaryotic cell cycle control Yeast appear to have lost some of these regulators over time rather than never having them, which makes yeasts somewhat misleading as a model for the “primitive” cell cycle.
Reconstructions of early eukaryotic cell cycle control suggest that CDKs were actually among the last major kinases added to the regulatory network. Earlier systems may have relied on other kinases, including checkpoint kinases like Chk1, to coordinate DNA replication and division.23Current Biology. Evolution of Eukaryotic Cell Cycle Regulation: Stepwise Addition of Regulatory Kinases and Late Advent of the CDKs Mathematical models of a primordial cell cycle propose that the earliest version may have been controlled by just two opposing forces: a cyclin-CDK complex pushing the cell toward division and the anaphase promoting complex destroying cyclin to reset the system. This simple toggle between a G1-like state and an S/M-like state, driven by cell growth, could represent the ancestral core that subsequent evolution decorated with the elaborate checkpoint and inhibitor layers seen in modern cells.24PubMed Central. Model scenarios for evolution of the eukaryotic cell cycle