Mitosis has five main phases: prophase, prometaphase, metaphase, anaphase, and telophase. These are followed by cytokinesis, which physically splits the cell in two. The entire process takes less than an hour in a typical human cell, representing only about 5% of the total cell division cycle. The rest of that time is spent in interphase, where the cell grows and copies its DNA in preparation.
Why Cells Need Mitosis
Mitosis produces two genetically identical daughter cells from one parent cell. This is how your body grows, replaces worn-out tissue, and repairs damage. Some tissues rely on mitosis far more than others. Your skin, the lining of your gut, salivary glands, and blood-forming cells in your bone marrow are constantly dividing to replace cells that die or slough off. These are called labile cell types, and they cycle through mitosis on a near-continuous basis throughout your life.
Prophase: Chromosomes Condense
Prophase is the first and longest phase of mitosis. The loosely organized DNA inside the nucleus compacts into tightly coiled chromosomes, each made of two identical copies (sister chromatids) joined at a structure called the centromere. This condensation is critical because trying to move loose, tangled DNA strands to opposite ends of a cell would be like trying to sort two tangled fishing lines by pulling them apart.
While the chromosomes are condensing, the cell begins building the spindle, a scaffold of protein fibers called microtubules. In animal cells, two centrosomes (small organizing structures that were duplicated earlier in the cell cycle) migrate toward opposite sides of the cell, trailing microtubules behind them like the ribs of an opening umbrella. The nuclear envelope, the membrane surrounding the nucleus, is still intact at this point.
Prometaphase: The Nuclear Envelope Breaks Down
Prometaphase begins when the nuclear envelope disintegrates. This is one of the most dramatic moments in cell division. Specific enzymes break apart the structural scaffold (the lamina) that holds the nuclear membrane together, causing it to fragment into small pieces. With this barrier gone, spindle microtubules can now reach the chromosomes directly.
Each chromosome has a specialized protein complex called a kinetochore on each side of its centromere. These kinetochores act like docking stations. Because the two kinetochores on a chromosome face opposite directions, they naturally grab microtubules coming from opposite poles of the spindle. Motor proteins at each kinetochore then begin pulling the chromosome back and forth, jostling it toward the center of the cell. This tug-of-war is what sets up the next phase.
Metaphase: Chromosomes Line Up at the Center
Metaphase is defined by a single, visually striking event: all the chromosomes align along the middle of the cell, forming what’s called the metaphase plate. This lineup isn’t just for show. Research has demonstrated that the equatorial position of the metaphase plate is essential for symmetric cell division. If chromosomes aren’t centered before the cell proceeds, the two daughter cells can end up with unequal amounts of material.
The cell has a built-in quality control system for this called the spindle assembly checkpoint. It works by detecting whether every single kinetochore is properly attached to spindle fibers from both poles. Even one unattached kinetochore triggers checkpoint proteins to block the cell from moving forward. This gives the cell enough time to correct any attachment errors and reposition chromosomes to the center. Only when every chromosome passes inspection does the cell get the green light for the next phase.
Anaphase: Chromosomes Pull Apart
Anaphase is the shortest phase but the most visually obvious. Once the spindle assembly checkpoint is satisfied, the “glue” holding sister chromatids together dissolves, and the two copies of each chromosome are pulled to opposite poles of the cell. This happens through two overlapping mechanisms.
In the first step (sometimes called anaphase A), the microtubules attached to kinetochores shorten by breaking down at their ends, reeling each chromatid toward its respective pole. In the second step (anaphase B), the spindle itself elongates. Microtubules between the two poles slide past each other, pushing the poles further apart. In some organisms these two steps happen one after the other; in others they overlap. The result is the same: one complete set of chromosomes at each end of the cell.
Telophase: Nuclei Re-Form
Telophase essentially reverses what happened in prophase and prometaphase. A new nuclear envelope assembles around each cluster of chromosomes, sealed together by specialized membrane-repair machinery that also disassembles the remaining spindle microtubules passing through the envelope. Nuclear pores, the channels that control what enters and exits the nucleus, are rebuilt and inserted into the new membranes. The chromosomes begin to relax and decondense back into their loosely organized form, and the internal scaffolding of each new nucleus (the lamina) reassembles. By the end of telophase, the cell contains two distinct, fully enclosed nuclei.
Cytokinesis: The Cell Splits in Two
Cytokinesis overlaps with telophase but is technically a separate process. It divides the cytoplasm, organelles, and other cellular contents between the two new cells. How this happens depends on the type of cell.
- Animal cells use a contractile ring made of actin and myosin proteins (the same proteins that power muscle contraction). This ring tightens around the cell’s equator like a drawstring, pinching the membrane inward to form a cleavage furrow that deepens until the cell is cleaved in two.
- Plant cells can’t pinch inward because they have a rigid cell wall. Instead, they build a new wall from the inside out. Remnants of the spindle form a structure called the phragmoplast, which guides vesicles carrying wall material to the center of the cell. These vesicles fuse to form a cell plate that expands outward until it reaches the existing cell wall, creating a complete partition.
How Long Each Phase Takes
In a commonly studied human breast cell line, the entire cell cycle averages about 21 hours. Cells spend roughly 4 hours in the first growth phase, 9 hours copying DNA, and 5 to 6 hours in the second growth phase. Mitosis itself averages just 48 minutes, with a standard deviation of only about 6 minutes, meaning the cell keeps this process remarkably consistent in length even when earlier phases vary.
Within that roughly 48-minute window, prophase and prometaphase together consume the most time, while anaphase is the quickest, often lasting only a few minutes. The tight timing of mitosis appears to be insulated from variations in the rest of the cell cycle, meaning even if a cell takes longer than usual to grow or copy its DNA, the actual division process stays on a reliable internal clock.