Mitosis, the Cell Cycle and Cancer

During interphase, the cell grows and its DNA is copied exactly. Each chromosome now consists of two identical copies called sister chromatids, joined together. These copies will later be separated so that each daughter cell receives one complete set of genetic information.

An X-shaped chromosome represents a copied chromosome: the two halves are sister chromatids, not the two chromosomes of a pair. Copying the DNA prepares the cell for division; it does not yet produce two cells.

The four stages of mitosis

Prophase — chromosomes become visible. The DNA condenses into compact chromosomes, each containing two sister chromatids. The nuclear membrane breaks down, allowing the chromosomes to be moved within the cell.

Metaphase — chromosomes line up in the middle. The copied chromosomes line up along the equator, or centre, of the cell. Spindle fibres attach to them. This arrangement allows the two copies of each chromosome to be separated towards opposite ends.

Anaphase — the copies separate. Spindle fibres pull the sister chromatids apart towards opposite poles of the cell. Once separated, each chromatid is a chromosome in its own right. Each pole receives one copy of every chromosome, so both ends receive identical sets.

Telophase — two nuclei form. New nuclear membranes form around the chromosomes at each pole. There are now two nuclei within the cell, each containing the same genetic information.

Cytokinesis completes cell division

During cytokinesis, the cytoplasm divides and the two daughter cells become separate. Each has its own cell membrane and nucleus.

The overall result is two genetically identical diploid daughter cells, with identical sets of chromosomes to those in the parent cell before its DNA was copied. The key connection is that exact copying during interphase, followed by equal separation during mitosis, preserves the genetic information.

Why mitosis matters

Growth requires an increase in cell number. Repeated mitotic divisions produce more body cells as an organism develops and grows.

Repair and replacement require new cells to take the place of damaged or dead cells. For example, cells lining the intestines divide to replace cells shed as food moves through the digestive system. The replacement cells receive the same genetic information as the cells that produced them.

Asexual reproduction involves one parent. Mitosis produces genetically identical cells, allowing offspring to inherit the same genetic information as that parent. Unlike growth or repair, the outcome here is a new organism rather than simply more cells within an existing organism.

Cancer: when control of division is lost

Normally, the cell cycle is controlled so that cells divide when needed and stop when enough cells have been produced. Genes help regulate this process.

Changes in a cell’s DNA, called mutations, can disrupt this control. Affected cells may continue dividing when they should stop. This uncontrolled cell division can produce a tumour, a mass of cells.

Cancer results from changes in cells that lead to uncontrolled division. The problem is therefore not mitosis itself—normal mitosis is essential—but the loss of control over when division occurs.

• Interphase: cell grows; DNA is copied to form sister chromatids.
• Prophase: chromosomes condense; nuclear membrane breaks down.
• Metaphase: chromosomes line up at the cell’s equator.
• Anaphase: spindle fibres pull sister chromatids to opposite poles.
• Telophase: new nuclear membranes form.
• Cytokinesis: cytoplasm divides, producing separate cells.

Mitosis is nuclear division: prophase to telophase.

Outcome and importance

One diploid parent cell produces two genetically identical diploid daughter cells, each with the same sets of chromosomes as the parent. In humans: 46 chromosomes in each daughter cell.

Mitosis enables growth, repair and replacement, and asexual reproduction.

Cancer

Changes in DNA can disrupt cell-cycle control → uncontrolled cell division → possible tumour formation.