The M phase is the stage of the cell cycle where a cell physically divides into two. It takes about 1 hour in a typical human cell, making it the shortest phase in a cycle that lasts roughly 24 hours total. The M phase has two main parts: mitosis, where the duplicated chromosomes are sorted into two identical sets, and cytokinesis, where the cell itself splits in half.
The rest of the cell cycle, called interphase, is spent growing and copying DNA. G1 lasts about 11 hours, S phase (when DNA is copied) about 8 hours, and G2 about 4 hours. The M phase is the payoff for all that preparation.
What Triggers the M Phase
A cell enters the M phase when a specific protein complex becomes active. One protein, called cyclin B1, builds up during the growth phases and partners with an enzyme called CDK1. Their activation at the end of G2 is the key event that flips the switch to mitotic entry. This activation is reinforced by a positive feedback loop, meaning once it starts, it accelerates rapidly.
The cell stays in its mitotic state as long as cyclin B1 levels remain high. Later, when chromosomes are properly sorted, cyclin B1 gets broken down, CDK1 shuts off, and the cell can exit division. This rise-and-fall pattern of cyclin B1 acts like a master timer for the entire M phase.
Prophase: Chromosomes Take Shape
The M phase begins with prophase, when chromosomes condense from loose, spread-out DNA into compact, visible structures. Each chromosome at this point consists of two identical copies (sister chromatids) joined together, produced when the DNA was duplicated earlier in S phase.
At the same time, the cell starts building the mitotic spindle, a structure made of protein fibers that will pull chromosomes apart. Two structures called centrosomes, which duplicated during interphase, migrate to opposite sides of the cell and serve as the spindle’s poles. By the end of prophase, the membrane surrounding the nucleus breaks down, giving the spindle fibers access to the chromosomes.
Prometaphase and Metaphase: Lining Up
Once the nuclear membrane dissolves, spindle fibers reach inward and attach to each chromosome at a specialized docking site called the kinetochore. During prometaphase, chromosomes are pulled back and forth as they get connected to spindle fibers from both poles. This tug-of-war eventually positions every chromosome at the center of the cell, forming what’s called the metaphase plate.
Metaphase is where a critical quality-control step kicks in: the spindle assembly checkpoint. This system detects whether every single chromosome is properly attached to spindle fibers from both sides. If even one chromosome is unattached or incorrectly connected, the checkpoint blocks the cell from moving forward. It does this by preventing activation of a protein complex (the APC/C) that would otherwise trigger the next step. Only when every chromosome passes inspection does the checkpoint release, allowing the cell to proceed into anaphase.
Anaphase: Chromosomes Separate
Once the checkpoint is satisfied, an enzyme cuts the molecular glue holding sister chromatids together. The two copies of each chromosome are pulled toward opposite poles of the cell by the spindle fibers, with motor proteins helping drive the movement. This is the moment the genetic material is actually divided. Each pole ends up with a complete, identical set of chromosomes.
Because every chromosome is transported independently by the spindle, the cell needs to coordinate their movement so that all chromosomes on each side end up enclosed in a single new nucleus. This coordination depends on signals between the spindle machinery and the nuclear membrane components that will soon reassemble.
Telophase: Rebuilding the Nucleus
In telophase, the separated chromosome clusters recruit membrane material and nuclear proteins to rebuild a nuclear envelope around each set. This process works by reversing the chemical changes that dismantled the original nucleus at the start of mitosis. Enzymes called phosphatases strip away the molecular tags that had caused the nuclear membrane to break apart.
Sheets of membrane from the cell’s internal network wrap around the chromosome masses. As they do, nuclear pores (the channels that control what enters and exits the nucleus) are rapidly rebuilt within the new membranes. A protein called BAF keeps the membranes pressed against the surface of the chromosome cluster, preventing them from slipping between individual chromosomes. Meanwhile, remaining spindle fibers between the two nuclei are severed by specialized cutting proteins, clearing the way for the membrane to fully seal. By the end of telophase, two complete nuclei exist within a single cell.
Cytokinesis: Splitting the Cell
Cytokinesis overlaps with the final stages of mitosis and physically divides the cell into two daughter cells. In animal cells, this happens through a contractile ring, a belt of protein fibers that forms just beneath the cell membrane at the cell’s equator.
The ring is built from two main proteins: actin filaments and myosin motors. Myosin grabs onto actin filaments and pulls on them, generating a force of about 4 piconewtons per motor cluster. Through a self-organizing process, randomly oriented actin filaments get captured by myosin, rotated into alignment, and bundled together into a tight ribbon roughly 0.1 micrometers wide. Three reinforcing mechanisms drive this: myosin clusters zip along actin filaments to align them, tension from myosin pulling on multiple filaments brings them into parallel, and the bundled filaments in turn recruit more myosin into a dense ribbon.
As the ring tightens, it pinches the cell membrane inward, creating a visible groove called the cleavage furrow. The furrow deepens until the cell is split into two separate daughter cells, each with its own nucleus and roughly equal share of the cell’s contents.
What Happens When the M Phase Goes Wrong
Errors during chromosome separation lead to daughter cells with the wrong number of chromosomes, a condition called aneuploidy. In developing embryos, aneuploidy is a major cause of miscarriages. In adult tissues, it is a hallmark of cancer.
Cancer cells frequently make chromosome segregation errors during division, a trait known as chromosomal instability. This instability gives tumors a kind of evolutionary flexibility. Cells with different chromosome combinations arise constantly, and those that happen to grow faster or resist treatment get selected for. Specific patterns of gained or lost chromosomes show up repeatedly across tumors and are associated with increased proliferation, the ability to spread to other organs, and resistance to therapy.
How the M Phase Differs in Meiosis
The M phase described above applies to mitosis, the division of ordinary body cells. In meiosis, the type of division that produces sperm and egg cells, the M phase happens twice and follows different rules.
In mitosis, homologous chromosomes (your maternal and paternal copies) behave independently. Each chromosome’s two sister chromatids attach to spindle fibers from opposite poles, so the chromatids get pulled apart. In the first meiotic division, however, homologous chromosomes pair up and physically connect through crossover points called chiasmata. The sister chromatids of each chromosome attach to spindle fibers from the same pole, oriented side by side rather than back to back. This means whole chromosomes, not individual chromatids, get separated in meiosis I, cutting the chromosome number in half.
A second division, meiosis II, then separates the sister chromatids in a process that looks much more like mitosis. The end result is four cells, each with half the original chromosome count, ready to function as reproductive cells.