Mitosis and Cytokinesis: Nuclear and Cellular Division

Mitosis is the division of a cell’s nucleus into two genetically identical nuclei, in four stages (prophase, metaphase, anaphase and telophase), and cytokinesis is the division of the cytoplasm that completes the formation of two daughter cells. In brief: mitosis produces two diploid cells that are identical to the parent cell. It is used for growth, tissue repair and asexual reproduction. A human cell with 46 chromosomes divides by mitosis into two cells with 46 chromosomes each.

Mitosis is how a fertilized egg becomes a body of trillions of cells, and how a cut heals. This guide describes each stage of mitosis and cytokinesis, explains why mitosis matters, compares animal and plant cells, gives a method for identifying stages under the microscope, works through a chromosome-counting example and corrects common mistakes.

What is mitosis?

Mitosis is a type of nuclear division in eukaryotic cells that follows the S phase of interphase. By then, each chromosome has been copied and consists of two identical sister chromatids joined at a centromere. The aim of mitosis is to distribute one complete copy of each chromosome to each daughter nucleus.

Mitosis is a continuous process, but for convenience it is divided into stages. Its main structure is the mitotic spindle, a set of microtubules that extend from two poles of the cell and move the chromosomes. In animal cells, the spindle forms from centrosomes, each of which contains a pair of centrioles.

What are the stages of mitosis?

Prophase

The chromatin condenses by coiling and becomes visible as chromosomes, each with two sister chromatids. The nucleolus disappears. The centrosomes move toward opposite poles of the cell, and spindle microtubules begin to form between them. Towards the end of prophase, the nuclear envelope breaks up.

Metaphase

The spindle fibers attach to the centromeres of the chromosomes, and the chromosomes line up along the equator (metaphase plate) in the middle of the cell. This is the stage when the chromosomes are most condensed and easiest to count. The spindle checkpoint makes sure that every chromosome is properly attached.

Anaphase

The centromeres split, and the sister chromatids separate. Each chromatid is now a separate chromosome. The spindle fibers shorten and pull the chromosomes toward opposite poles, with the centromeres leading and the arms trailing, so that the chromosomes look V-shaped. The cell also elongates. The number of chromosomes in the cell temporarily doubles, to 92 in a human cell.

Telophase

The chromosomes arrive at the poles and uncoil into chromatin. A new nuclear envelope forms around each set, and the nucleoli reappear. The spindle breaks down. Now there are two nuclei in the cell, each with the same number of chromosomes as the original nucleus.

What is cytokinesis?

Cytokinesis usually begins during late anaphase or telophase, and it divides the cytoplasm and organelles between the two cells.

In animal cells, a ring of actin and myosin filaments forms around the equator under the cell membrane and contracts, pinching the cell in two, a process called cleavage. A visible indentation, the cleavage furrow, forms.
In plant cells, the stiff cell wall prevents furrowing. Instead, vesicles from the Golgi apparatus gather at the middle of the cell and fuse to form a cell plate, which grows outward until it reaches the cell wall. It develops into a new cell wall and membranes between the two cells.

Stages of mitosis compared

Key events in each stage of mitosis
Stage	Chromosomes	Nuclear envelope	Spindle
Prophase	Condense and become visible	Breaks down at the end	Starts to form
Metaphase	Lined up at the equator	Absent	Attached to centromeres
Anaphase	Chromatids pulled to opposite poles	Absent	Fibers shorten
Telophase	At poles, uncoiling	Re-forms around each set	Disassembles

Why is mitosis important?

Growth: a multicellular organism increases in size by adding cells. A human starts from one cell and has tens of trillions in adulthood.
Tissue repair and replacement: cells lost from the skin, the gut lining and blood are replaced. The red bone marrow produces millions of new blood cells every second.
Asexual reproduction: in organisms such as yeast, hydra and many plants, mitosis produces offspring that are genetically identical to the parent.
Genetic stability: because daughter cells are identical, the genetic information is conserved from cell to cell in a body.

Worked example: counting chromosomes and DNA

A human body cell has 46 chromosomes and, at the start of the cell cycle (G1), 6.4 picograms (pg) of DNA in the nucleus. Complete the following table for each stage.

Chromosome number and DNA content during the cell cycle and mitosis
Stage	Chromosomes per cell	Chromatids per cell	DNA per cell (pg)
G1	46	46	6.4
After S phase and in prophase, metaphase	46	92	12.8
Anaphase	92	92	12.8
Each daughter cell after cytokinesis	46	46	6.4

S phase: DNA replication doubles the DNA content to 2 × 6.4 = 12.8 pg. The number of chromosomes stays at 46, but each is now made of two chromatids, giving 92 chromatids.
Anaphase: the sister chromatids separate. Each becomes a chromosome, so the number of chromosomes in the cell is 92. The DNA content is unchanged at 12.8 pg.
After cytokinesis: each daughter cell receives 46 chromosomes and 12.8 ÷ 2 = 6.4 pg of DNA.

The pattern of “double, then halve” shows that mitosis maintains the chromosome number and DNA content of the original cell. Note that the number of chromosomes is counted by the number of centromeres, so the chromosome number rises only in anaphase, when the centromeres split. The value of 6.4 pg is a typical figure for a human diploid cell and is used here for illustration.

Chromosomes per cell in a human cell, as in the worked example. G1 and G2 are the gaps of interphase (DNA is replicated between them, but the chromosome count stays 46), then prophase, metaphase, anaphase and telophase, and End is each daughter cell after cytokinesis. The number doubles to 92 in anaphase when the centromeres split, and returns to 46 in each daughter cell.

How do you identify stages under the microscope?

Onion root tips are commonly used, because they contain many dividing cells. The root tip is softened in warm hydrochloric acid, stained with a dye such as acetic orcein or toluidine blue, and squashed under a cover slip to give a single layer of cells. Then use the following guide.

Interphase: a clear nucleus, with a nucleolus, no visible chromosomes.
Prophase: a nucleus with thread-like condensed chromosomes, no nucleolus.
Metaphase: chromosomes in a line across the center of the cell.
Anaphase: two groups of chromosomes moving apart, V-shaped.
Telophase: two separate groups of chromosomes at the poles of the cell, with a cell plate in plant cells.

How was mitosis discovered?

Walther Flemming observed and drew the stages of mitosis in animal cells in 1882 and called the process mitosis, from the Greek word for thread, because of the thread-like chromosomes. Eduard Strasburger described similar events in plants. Theodor Boveri later showed that normal development requires the correct set of chromosomes, and he suggested in 1914 that abnormal chromosome numbers could cause cancer.

Which mistakes are common?

Mistake 1: Mixing up the order of the stages

A memory aid is PMAT: prophase, metaphase, anaphase, telophase.

Mistake 2: Saying that DNA replicates during mitosis

DNA replication happens in the S phase of interphase, before mitosis begins.

Mistake 3: Believing that mitosis halves the chromosome number

Mitosis maintains the number. Meiosis halves it.

Mistake 4: Confusing mitosis with cytokinesis

Mitosis is the division of the nucleus. Cytokinesis is the division of the cytoplasm.