Mitosis and the cell cycle: In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.

The idea

Mitosis is the stage of the cell cycle in which the copied chromosomes are separated and the nucleus divides. The chromosomes, each consisting of two sister chromatids, line up at the equator of the cell. The chromatids are then pulled apart by spindle fibres, so that one complete set of chromosomes moves to each end of the cell. Once the sets are separated, a new nuclear membrane forms around each group and the nucleus divides, producing two genetically identical nuclei. For example, in a human cell with 46 chromosomes, each new nucleus receives 46 chromosomes identical to those of the parent cell. This process ensures genetic continuity and is essential for growth, repair and asexual reproduction.

What earns the marks

• Mitosis is the part of the cell cycle in which the nucleus divides to form two genetically identical nuclei.
• The chromosomes line up and are separated so that one complete set of chromosomes is pulled to each end of the cell.
• Spindle fibres attach to the chromosomes and pull the sister chromatids apart.
• A nuclear membrane forms around each set of chromosomes, so two new nuclei are produced.
• Each daughter nucleus receives the same number and type of chromosomes as the parent cell.
• The separated chromosomes are genetically identical, which maintains genetic continuity.

Common mistakes and how to put them right

• Saying that the cell divides into two cells during mitosis.

  Put it right: Mitosis itself is nuclear division; the cytoplasm usually divides afterwards in cytokinesis.

• Thinking that the chromosomes are pulled to the ends before they have been copied.

  Put it right: DNA replication occurs during interphase, so each chromosome already consists of two sister chromatids before mitosis begins.

• Believing that the daughter nuclei are genetically different.

  Put it right: Mitosis produces two nuclei that are genetically identical to each other and to the parent nucleus.

Exam tips

• Use the phrase 'one set of chromosomes is pulled to each end of the cell' when describing the key event of mitosis.
• Distinguish clearly between mitosis (nuclear division) and cytokinesis (division of the cytoplasm).
• When asked why mitosis is important, link it to growth, repair and asexual reproduction, and to the production of genetically identical cells.