Before mitosis: DNA replication

Before the cell divides, each chromosome is copied (DNA replication) during interphase. Each chromosome now consists of two identical copies called chromatids, joined at the centromere. Draw a cell with duplicated chromosomes: each shown as an X shape (two chromatids joined at the centre).

Key stages of mitosis

Show four simplified stages: (1) Chromosomes condense and become visible. The nuclear membrane breaks down. (2) Chromosomes line up along the middle (equator) of the cell. Spindle fibres attach to the centromeres. (3) Chromatids separate: the centromere splits and spindle fibres pull one chromatid from each chromosome to opposite poles of the cell. (4) Nuclear membranes form around each set of chromosomes, and the cytoplasm divides (cytokinesis), producing two daughter cells.

The outcome

Mitosis produces two daughter cells that are genetically identical to each other and to the parent cell. Each daughter cell has the same diploid number of chromosomes as the parent. In humans, that is 46 chromosomes (23 pairs). Mitosis is used for growth, repair of damaged tissues and asexual reproduction in some organisms.

How to draw mitosis clearly

Use a consistent number of chromosomes throughout (e.g. 4 chromosomes in the parent cell, 4 in each daughter cell). Show the X shapes clearly in the early stages, then single-line chromosomes after separation. Label the centromere, spindle fibres and cell membrane at each stage. Use arrows between stages to show the sequence.