The three main phases of the cell cycle are interphase, mitosis, and cytokinesis. Interphase is by far the longest, occupying roughly 90% of the total cycle, while mitosis and cytokinesis together make up the remaining fraction. Each phase has a distinct job: interphase prepares the cell by growing and copying its DNA, mitosis divides the nucleus, and cytokinesis splits the entire cell into two daughter cells.
Interphase: Growth and DNA Replication
Interphase is the workhorse stage of the cell cycle. It looks quiet under a microscope, which is why early biologists called it the “resting phase,” but at the molecular level it is anything but restful. This is when the cell grows larger, duplicates all of its DNA, and manufactures the proteins it will need for division. Interphase itself breaks down into three sub-phases that always occur in the same order: G1, S, and G2.
G1 (Gap 1) begins right after a cell finishes dividing. The cell is metabolically active, producing proteins and organelles, and steadily increasing in size. It does not copy its DNA yet. G1 is typically the longest and most variable sub-phase. Cells that receive signals to stop dividing exit the cycle here and enter a resting state called G0. Many cells in your body, including most nerve cells and mature muscle cells, spend their entire lifespan in G0. This state is often reversible: when conditions change, such as the return of growth signals, a G0 cell can re-enter G1 and resume the cycle.
S phase (Synthesis) is when every chromosome is faithfully copied so that the cell ends up with two complete sets of DNA. This is a high-stakes process. Errors made during DNA replication can become permanent mutations, so the cell runs constant proofreading and repair machinery throughout S phase.
G2 (Gap 2) follows DNA synthesis. The cell continues to grow and produces the specific proteins it will need for mitosis, such as the structural components of the spindle that will later pull chromosomes apart. G2 also serves as a final quality-control window where the cell checks that DNA replication finished correctly before committing to division.
Mitosis: Dividing the Nucleus
Once the cell has doubled its DNA and grown large enough, it enters mitosis, the phase where the copied chromosomes are physically separated into two identical sets. Mitosis is fast, often lasting only one to two hours in a rapidly dividing human cell, and it unfolds in a tightly choreographed sequence of stages.
During prophase, the loosely organized DNA condenses into compact, visible chromosomes. Each chromosome appears as two identical sister chromatids joined at a central point called the centromere. A structure called the mitotic spindle, made of protein filaments, begins forming as the cell’s two centrosomes migrate toward opposite ends of the cell.
In prometaphase, the nuclear envelope breaks apart, allowing the spindle fibers to reach the chromosomes. The fibers attach to each chromosome at specialized protein structures on the centromere. This connection is what allows the spindle to move chromosomes around.
Metaphase is the alignment step. All chromosomes line up along the midpoint of the cell, sometimes called the metaphase plate. The cell pauses here until every single chromosome is properly attached to spindle fibers from both poles. This is a critical checkpoint: if even one chromosome is not correctly connected and under tension, the cell will not proceed.
Anaphase is the shortest stage. The sister chromatids are pulled apart, and the separated chromosomes move quickly toward opposite poles. By the end of anaphase, each side of the cell holds a full, identical set of chromosomes.
In telophase, new nuclear envelopes re-form around each set of chromosomes, the DNA begins to unwind back into its less condensed form, and the spindle disassembles. At this point, one cell contains two complete nuclei.
Cytokinesis: Splitting the Cell in Two
Cytokinesis overlaps with the tail end of mitosis and finishes the job by dividing the cytoplasm, organelles, and cell membrane into two separate daughter cells. The mechanism differs depending on the type of cell.
In animal cells, a band of protein filaments (mainly actin and myosin, the same proteins responsible for muscle contraction) assembles just beneath the cell surface and tightens like a drawstring. This creates a visible indentation called a cleavage furrow that deepens and pinches inward until the cell is squeezed into two.
Plant cells cannot pinch inward because they are surrounded by a rigid cell wall. Instead, they build a new wall from the inside out. Small vesicles carrying wall-building materials travel along leftover spindle fibers to the center of the cell, where they fuse together to form a structure called the cell plate. The cell plate expands outward until it meets the existing cell wall on all sides, completing the partition.
Checkpoints That Keep the Cycle on Track
The cell cycle is not an automatic conveyor belt. Built-in checkpoints act as molecular gatekeepers, pausing the cycle if something is wrong and only allowing progression when specific conditions are met.
The G1 checkpoint is the major decision point. Here the cell evaluates whether it has grown large enough, whether its DNA is intact, and whether it is receiving external growth signals. If DNA damage is detected, a protein called p53 activates a cascade that halts the cycle so repairs can be completed before the cell copies flawed DNA. When p53 is mutated and stops working, damaged cells can slip through this gate unchecked, which is one of the most common pathways to cancer.
The G2 checkpoint verifies that DNA replication during S phase was completed correctly and that the cell has reached sufficient size. If unreplicated or damaged DNA is found, the proteins that would normally trigger entry into mitosis are kept inactive until the problem is resolved.
The metaphase checkpoint (also called the spindle checkpoint) prevents the cell from pulling chromosomes apart until every chromosome is properly attached to spindle fibers and under tension from both poles. This ensures that each daughter cell receives exactly one copy of every chromosome. Failure at this checkpoint can produce cells with too many or too few chromosomes, a condition linked to miscarriage, developmental disorders, and cancer.
What Drives the Cycle Forward
Progression through each phase is controlled by pairs of proteins: cyclins and their partner enzymes called cyclin-dependent kinases (CDKs). Different cyclin-CDK combinations act at different transitions, like a relay of molecular switches. In G1, D-type cyclins activate CDK4 and CDK6, which begin releasing the brakes on a key growth-suppressor protein called RB. At the G1-to-S transition, another cyclin (cyclin E) paired with CDK2 finishes inactivating RB, giving the cell the green light to start copying DNA. During S phase, cyclin A partners with CDK2 to drive DNA replication forward. As the cell approaches mitosis, cyclin A and then cyclin B pair with CDK1, triggering entry into and completion of mitotic division.
This layered system of cyclins ensures that each phase only begins after the previous one is truly finished. Cancer therapies that target CDK4 and CDK6 exploit this logic by blocking the very first “go” signal at the G1 gate, preventing tumor cells from entering the cycle at all.
Cells That Never Divide and Cells That Never Stop
Not every cell in your body cycles at the same speed. Cells lining the gut and the skin’s outer layer are among the fastest dividers, replacing themselves every few days to keep up with constant wear. Blood-forming stem cells in the bone marrow also cycle frequently. At the other extreme, mature neurons and heart muscle cells exit the cycle into G0 and rarely, if ever, divide again. Liver cells sit in a middle ground: they normally rest in G0 but can re-enter the cycle rapidly after injury, which is why the liver has a remarkable ability to regenerate.
The total length of the cell cycle varies widely. A typical rapidly dividing human cell completes the full cycle in about 24 hours, with the bulk of that time spent in interphase. Embryonic cells during early development can cycle in as little as 8 to 10 hours because G1 is dramatically shortened. The variation almost always comes from differences in G1 length, since S phase, G2, and mitosis tend to stay relatively constant across cell types.