How Mitosis Divides a Cell in Two: Phases, Checkpoints and Results

Every time a cell divides to make two new cells, it must first make a complete copy of its genetic material and then split that copy so each daughter receives exactly one full set. In most body cells, this process is mitosis. It drives growth in young organisms, replaces worn-out cells in adults, and in many single-celled eukaryotes, such as yeast, it is the main method of reproduction.

This article walks through the stages of mitosis, the control systems that keep it accurate, and how it compares with meiosis, the other type of division.

Where mitosis fits in the cell cycle

Mitosis is one part of the cell cycle, the sequence of events between one division and the next. Most of that cycle is interphase, the period of growth and preparation. During the S phase of interphase the cell copies its DNA, so each chromosome now consists of two identical sister chromatids joined at a region called the centromere.

Mitosis itself is a shorter phase that physically separates those copies. It is usually followed by cytokinesis, which divides the cytoplasm and completes the split into two cells.

The five phases of mitosis

Mitosis is conventionally described as five phases. In reality the process is continuous, and the boundaries between phases are drawn for convenience.

Phase	Main event	What you would see under a microscope
Prophase	Chromatin condenses; spindle forms	Chromosomes become visible as X-shaped pairs
Prometaphase	Nuclear envelope breaks down; spindle attaches	Chromosomes move and jostle
Metaphase	Chromosomes line up at the middle	A neat row across the cell equator
Anaphase	Sister chromatids separate	Two sets move toward opposite ends
Telophase	Nuclei reform; chromosomes loosen	Two nuclei appear inside one cell

Prophase

During prophase the chromatin, which is the loose form of DNA wrapped around proteins called histones, condenses into compact chromosomes. Condensation makes the chromosomes short and sturdy enough to move without tangling. At the same time the spindle begins to assemble from microtubules that grow out from two spindle poles on opposite sides of the cell.

Prometaphase

In prometaphase the nuclear envelope breaks apart, releasing the chromosomes into the cytoplasm. Microtubules from the spindle poles reach the chromosomes and attach to their kinetochores, protein structures built on each centromere. Each sister chromatid ends up tethered to the opposite pole, which is the arrangement needed for an even split.

Metaphase

During metaphase the chromosomes align along the metaphase plate, an imaginary line halfway between the two poles. Tension from microtubules pulling in both directions holds each chromosome in position. This alignment is the last stop before the chromosomes are separated.

Anaphase

Anaphase begins when the sister chromatids separate. Each chromatid is now considered its own chromosome. Microtubules shorten, pulling each chromosome toward the pole it faces. The poles also move farther apart, stretching the cell.

Telophase

In telophase the chromosomes arrive at the poles and begin to decondense. A new nuclear envelope forms around each set. By the end of telophase there are two distinct nuclei inside one cell, each holding a complete copy of the genome.

Cytokinesis: splitting the cytoplasm

Cytokinesis usually overlaps with late anaphase and telophase, and its method depends on the cell type. In animal cells, a ring of actin and myosin filaments tightens beneath the membrane and pinches it into a cleavage furrow, which eventually separates the two halves. In plant cells, the rigid cell wall prevents pinching. Instead, vesicles fuse in the middle to build a new cell plate that grows outward and becomes the new wall between the daughters.

How the cell keeps mitosis on schedule

Mitosis is driven by proteins called cyclin-dependent kinases, or CDKs. These enzymes are active only when they are bound to cyclins, whose levels rise and fall through the cycle. Cyclin-CDK complexes switch on the events of mitosis, while another system called the anaphase-promoting complex triggers the release of sister chromatids in anaphase.

A key safety mechanism is the spindle assembly checkpoint. Before anaphase can begin, the cell checks that every kinetochore is correctly attached to spindle microtubules. If even one chromosome is unattached, a signal holds the cell in metaphase. This delay gives the cell time to fix attachments, and it helps prevent one daughter from receiving too many or too few chromosomes.

Errors that slip past these checks can cause aneuploidy, an abnormal chromosome number. Aneuploidy is common in many cancers, and it is also a major cause of miscarriage in humans.

Chromosome numbers in practice

Human body cells are normally diploid, meaning they carry two copies of each chromosome, for a total of 46 chromosomes arranged in 23 pairs. A single mitotic division starts with a cell that has 46 duplicated chromosomes, or 92 chromatids, and ends with two cells that each have 46 single chromosomes. The chromosome number therefore stays the same across the division. Only the amount of DNA per cell is halved by splitting.

Because each daughter receives identical copies, mitosis produces genetically identical cells, barring rare copying errors. This is why skin cells, blood-forming cells and liver cells all share the same genetic information even though they look and behave very differently.

Mitosis compared with meiosis

Mitosis is often contrasted with meiosis, the division that makes sperm and egg cells. The differences are significant.

Feature	Mitosis	Meiosis
Number of divisions	One	Two
Daughter cells	Two	Four
Chromosome number	Diploid to diploid	Diploid to haploid
Genetic variation	None intended	Crossing over and independent assortment
Main purpose	Growth, repair, asexual reproduction	Producing gametes for sexual reproduction

Mitosis keeps the genome stable, which is essential for tissues that must function reliably. Meiosis deliberately shuffles genetic material, which creates the variation that populations need in order to adapt over time.

Why mitosis matters

Mitosis underlies much of biology. A fertilized egg becomes a body through repeated rounds of mitosis. Wounds heal because surrounding cells divide to replace lost tissue. Many single-celled organisms, including yeast and bacteria, multiply by related processes, although bacteria use a simpler system called binary fission.

Errors in the control of mitosis are central to cancer. Cancer cells often ignore the signals that should stop division, and some cancer treatments target the spindle or the cell-cycle controls to halt their growth.

Key takeaways

Mitosis is the division that copies a cell's chromosomes and distributes one complete set to each daughter cell. Its five phases move the chromosomes from condensation to separation to new nuclei, and cytokinesis finishes the job by splitting the cytoplasm.

The spindle assembly checkpoint and cyclin-CDK controls keep the process accurate. Because the chromosome number is conserved, mitosis produces identical cells for growth and repair, while meiosis produces the varied gametes needed for sexual reproduction.