Animation of a fertilised human egg cell undergoing its first cell division. Fertilisation occurs when a male sperm cell fuses with a female egg cell, each contributing half a genome to the new organism. After fertilisation, the combined genome begins to divide by mitosis. A spindle structure (brown) forms, with the chromosomes (pink) in a line in the centre. The chromosomes will separate into identical halves and be pulled to each side by the spindle, and the cell will then divide so that two identical cells are produced.