Stages of mitosis for A-Level Biology

Mitosis is the part of the cell cycle in which a single parent cell divides into two genetically identical daughter cells. It has four named stages – prophase, metaphase, anaphase, and telophase – followed by cytokinesis, which splits the cytoplasm and forms two separate cells. Each stage is defined by what the chromosomes are doing, not by the colour of the cell or the shape of the nucleus.

This guide walks through each stage in the order it happens, explains the AQA exam wording, covers why mitosis matters for growth and repair, and lists the mistakes that students lose marks on every year.

Mitosis is one part of the wider cell cycle. The cell cycle has two main phases: Interphase and the M phase (which itself covers mitosis and cytokinesis). Most of the time, a cell is sitting in interphase, growing, copying its DNA, and preparing the components it needs for division.

Interphase is itself split into G1, S, and G2. DNA replication happens in S phase, so by the time the cell enters mitosis, every chromosome already consists of two identical sister chromatids joined at the centromere. Mitosis separates those chromatids into two new nuclei, and cytokinesis then splits the cytoplasm to produce two new cells.

Prophase is the first stage of mitosis. The chromosomes, which were long and thin during interphase, condense and become visible under the light microscope. Each chromosome appears as two identical sister chromatids joined at the centromere.

At the same time, the nuclear envelope begins to break down and the spindle fibres (made of protein microtubules) start to form from the centrosomes at opposite poles of the cell. By the end of prophase, the chromosomes are short, thick, and ready to be moved.

Metaphase is the stage where the chromosomes line up along the equator of the cell. The spindle fibres attach to the centromere of each chromosome, and the chromosomes are pulled into position along a line called the metaphase plate.

The AQA mark-scheme wording for this stage is: Chromosomes line up at the equator of the cell, attached to spindle fibres by their centromeres. Both halves of that sentence carry a mark. Saying "the chromosomes line up" without mentioning the spindle fibres only gets you half the marks.

Anaphase is the shortest stage of mitosis. The centromeres divide and the spindle fibres contract, pulling the sister chromatids apart. Each chromatid is dragged towards opposite poles of the cell, centromere first.

The AQA wording is: Sister chromatids are separated and pulled to opposite poles of the cell by the spindle fibres. Once the chromatids have separated, each one is now classed as an individual chromosome, even though it is only made of one strand of DNA.

Telophase is the reverse of prophase. The chromosomes arrive at the two poles, and new nuclear envelopes form around each set. The chromosomes decondense and become long and thin again, ready to function as templates for transcription. The spindle fibres break down.

At the end of telophase, the cell has two complete nuclei but is still a single cell. The physical splitting of the cytoplasm happens in cytokinesis, which is technically not part of mitosis itself.

Cytokinesis is the division of the cytoplasm that completes the cell cycle. In animal cells, a contractile ring of actin filaments pinches the cell in two, forming a cleavage furrow. In plant cells, a cell plate forms across the middle of the cell and grows outwards to become a new cell wall.

AQA tends to test the difference between plant and animal cytokinesis at least once every couple of years. Make sure you can describe both mechanisms, not just the one you saw drawn in a textbook diagram.

Mitosis is essential for growth, repair, and asexual reproduction. A human zygote becomes a 30-trillion-cell adult through repeated mitotic divisions. Damaged tissues such as skin and gut lining are constantly replaced by mitosis. Organisms like bacteria, yeast, and strawberry runners use mitosis to produce genetically identical offspring.

Mitosis is also tightly regulated. When the control mechanisms break down – for example through mutations in tumour suppressor genes such as p53 – cells can divide uncontrollably. This is the cellular basis of cancer, and it is a common follow-up question in AQA Paper 2.

Examiner reports for AQA A-Level Biology flag the same mistakes year after year. Most of them come down to imprecise language or muddling mitosis with meiosis.