The cell cycle is the sequence of events a cell goes through to grow and divide into two identical daughter cells. It has four main phases: two growth phases (G1 and G2), a DNA-copying phase (S), and a division phase (M). Most of the cycle is spent preparing for division, not actually dividing.
The Four Phases at a Glance
The first three phases, G1, S, and G2, are collectively called interphase. This is when the cell does the bulk of its work: growing larger, copying its DNA, and assembling the proteins it needs. Interphase accounts for the vast majority of the cell cycle’s total time. The final phase, M phase, is when the cell physically splits in two. It’s dramatic but relatively brief.
Here’s what happens in each phase:
- G1 (first gap phase): The cell grows, produces proteins, and carries out its normal functions. It also runs a series of internal checks to decide whether conditions are right to commit to division.
- S (synthesis phase): The cell copies all of its DNA so each future daughter cell will have a complete set. By the end of S phase, every chromosome has been duplicated.
- G2 (second gap phase): The cell continues growing and synthesizes additional proteins needed for division. It also inspects the newly copied DNA for errors before moving forward.
- M (mitosis and cytokinesis): The cell divides its duplicated chromosomes into two identical sets, then physically splits into two separate cells.
How DNA Gets Copied in S Phase
During S phase, the cell doesn’t just unzip its DNA and copy it in one sweep. Replication starts at many specific spots along each chromosome called origins. At each origin, a molecular machine called the replisome assembles. This complex unwinds the two strands of DNA and builds new complementary strands in both directions from the starting point, creating two branching “replication forks” that move away from each other.
The groundwork for this process is actually laid in G1, when the cell marks (or “licenses”) which origins are allowed to fire. This licensing step is critical because it prevents any stretch of DNA from being copied twice, which would cause dangerous errors. Once S phase begins, signaling proteins activate those licensed origins in a coordinated wave, and replication proceeds until the entire genome has been duplicated.
How a Cell Divides During M Phase
Mitosis itself unfolds in several distinct stages. During prophase, the copied chromosomes condense into tightly packed structures visible under a microscope. Each chromosome at this point consists of two identical sister chromatids joined at a region called the centromere. Meanwhile, a structure called the mitotic spindle begins forming from opposite sides of the cell. The spindle is made of protein fibers called microtubules that will pull the chromosomes apart.
Next, during prometaphase and metaphase, the spindle fibers attach to each chromosome’s centromere, and the chromosomes line up along the middle of the cell. This alignment is essential: each sister chromatid must be connected to fibers from opposite poles of the spindle so they’ll be pulled in opposite directions.
Anaphase is when the actual separation happens. The protein glue holding sister chromatids together is broken down, and motor proteins pull each chromatid toward opposite ends of the cell. In telophase, new nuclear membranes form around each set of chromosomes, and the chromosomes uncoil back to their loosely packed form.
After mitosis, cytokinesis physically divides the cytoplasm. In animal cells, a ring of protein filaments pinches the cell membrane inward like a drawstring, eventually cinching the cell in two. Plant cells handle this differently: because they have rigid cell walls, they build a new wall from the inside out, assembling it from small vesicles that fuse together at the center of the cell and expand outward until the two daughter cells are fully separated.
Checkpoints That Prevent Errors
The cell cycle isn’t a runaway train. Built-in surveillance systems called checkpoints pause the process if something goes wrong. There are three major ones.
The G1 checkpoint (sometimes called the restriction point) is the cell’s first big decision. Here, the cell evaluates whether it has grown enough, whether nutrients are available, and whether its DNA is intact. If conditions aren’t favorable, the cell stalls in G1 or exits the cycle entirely. This checkpoint acts as a gatekeeper: once a cell passes it, the cell is essentially committed to dividing.
The G2/M checkpoint fires just before the cell enters mitosis. Proteins inspect the newly replicated DNA, checking for structural damage or incomplete replication. If DNA replication isn’t finished or errors are detected, the cell halts here to allow repair before attempting to divide. There’s also a monitoring system active during S phase itself that stabilizes the replication machinery if it stalls, preventing half-copied DNA from collapsing into breaks.
The spindle checkpoint operates during mitosis, at the transition from metaphase to anaphase. It ensures every chromosome is properly attached to spindle fibers from both poles before allowing the sister chromatids to separate. Without this check, daughter cells could end up with the wrong number of chromosomes.
What Drives the Cycle Forward
The engine of the cell cycle is a family of proteins called cyclin-dependent kinases, or CDKs. These enzymes are always present in the cell, but they’re inactive on their own. They only switch on when they bind to partner proteins called cyclins, which rise and fall in concentration at specific points in the cycle.
Think of CDKs as locks and cyclins as keys. When the right cyclin accumulates during a particular phase, it binds its CDK partner and activates it. The activated complex then chemically modifies target proteins (by adding phosphate groups to them), flipping molecular switches that push the cell into the next phase. For example, cyclin D pairs with CDK4 and CDK6 during G1 to help the cell commit to division, while cyclin E pairs with CDK2 to trigger entry into S phase.
The system also has brakes. Proteins called CDK inhibitors can block these complexes, keeping the cycle paused when checkpoints detect problems. The balance between cyclins, CDKs, and their inhibitors determines whether a cell moves forward, waits, or stops dividing altogether.
The G0 Resting Phase
Not every cell is actively cycling. Many cells in your body exit the cycle after G1 and enter a resting state called G0. Cells in G0 are alive and metabolically active, but they’re not preparing to divide. They’re carrying out their specialized functions: a liver cell processing toxins, a muscle cell contracting, a nerve cell transmitting signals.
Some cells in G0 can re-enter the cycle if they receive the right growth signals. Liver cells, for instance, are normally quiescent but can start dividing rapidly after injury. Other cells, like mature neurons, are essentially locked in G0 permanently. This is one reason brain and spinal cord injuries are so difficult to recover from. Cells in G0 are also generally resistant to chemotherapy drugs, which typically target actively dividing cells.
What Happens When the Cycle Goes Wrong
Cancer is fundamentally a disease of the cell cycle. When the genes controlling checkpoints or CDK activity mutate, cells can divide uncontrollably. The most commonly mutated gene in human cancers is TP53, which encodes a protein called p53. Under normal conditions, p53 responds to DNA damage during G1 by activating a CDK inhibitor that halts the cycle, giving the cell time to repair the damage before copying it. When p53 is mutated, this brake fails. Damaged DNA gets replicated and passed to daughter cells, and those errors become permanent parts of the genome.
Studies in mice with mutated p53 show increased susceptibility to lung tumors, bone cancers, and lymphomas. In humans, p53 mutations appear in roughly half of all cancers. The spindle checkpoint can also fail: if chromosomes aren’t distributed correctly during mitosis, daughter cells end up with too many or too few chromosomes, a condition called aneuploidy that’s a hallmark of many aggressive tumors.
How Bacteria Divide Differently
Everything described above applies to eukaryotic cells, the type found in animals, plants, and fungi. Bacteria are far simpler. They have a single circular chromosome, no nucleus, and far fewer internal structures. Instead of mitosis, bacteria divide through binary fission: they copy their one chromosome, the two copies move to opposite ends of the cell, and the cell splits down the middle. There’s no spindle apparatus, no condensation of chromosomes, and no elaborate checkpoint system. This simplicity is one reason bacteria can reproduce so quickly, sometimes dividing every 20 minutes under ideal conditions.