Mitosis and the cell cycle

The DNA also replicates, forming two exact copies of each chromosome. These copies initially remain attached to each other. The genetic material has now doubled, ready to be shared between the two new cells.

Stage 2: mitosis divides the nucleus

During mitosis, the chromosome copies separate. One complete set of chromosomes is pulled to each end of the cell, and the nucleus divides. Each new nucleus therefore receives the same genetic information.

Copying first and separating the copies afterwards is what allows both daughter cells to receive a full set of chromosomes. For example, if a human body cell starts with 46 chromosomes, each daughter cell receives 46 chromosomes—not 23 and not 92.

Stage 3: the cell splits

Finally, the cytoplasm and cell membrane divide, producing two separate daughter cells. Each has its own nucleus containing a complete set of chromosomes. The daughter cells are genetically identical to each other and to the original cell.

Mitosis is therefore one stage within the cell cycle, rather than a name for everything that happens from growth to the final splitting of the cell.

Why mitosis matters for growth and development

Following fertilisation, the single cell formed when a sperm joins an egg divides by mitosis. The new cells divide again, increasing cell number as an embryo develops. Continued cell division supports growth, while cells also become specialised for different functions.

If every cell divides in each round, the number of cells follows the sequence 1 → 2 → 4 → 8. This shows how repeated cell division can build a multicellular organism from one cell. Growth involves increasing cell number, not just making existing cells larger.

Mitosis also supplies new cells for repair and replacement. For example, replacing damaged cells during wound healing requires new cells with the same genetic instructions as the cells they replace.

Recognising mitosis in a biological context

Look for situations in which the number of cells is increasing: an embryo developing, a plant root tip growing or tissue being repaired. In each case, mitosis produces genetically identical cells to add to or replace existing tissue.

In microscope images, look at the arrangement of the chromosomes rather than just the size of the cell. Two groups of chromosomes at opposite ends of one cell are evidence that chromosome copies have separated during mitosis. Two new nuclei within a cell indicate that nuclear division has occurred, even if the cytoplasm has not yet finished dividing.

An intact nucleus on its own is not evidence of mitosis: the cell may be growing and preparing to divide. Use the visible features to describe what is happening, rather than assuming that every cell in growing tissue is dividing at that moment.

What stays the same?

Each daughter cell receives a complete set of genetic information and the same chromosome number as the original cell. A body cell with 46 chromosomes produces two cells with 46 chromosomes each.

Why and where?

• Growth and development: repeated division increases cell number, including during embryo development and plant root growth.
• Repair and replacement: new cells replace damaged or worn-out cells.
• Image evidence: separated chromosome groups at opposite ends of a cell indicate mitosis.