Cells in the reproductive organs divide by meiosis to form gametes. First, copies of the genetic information are made. The cell then divides twice, which forms four gametes. Each gamete has a single set of chromosomes, so meiosis halves the number of chromosomes. The gametes are all genetically different from each other.
A human body cell has 46 chromosomes, so a human gamete has 23. When a sperm and an egg join at fertilisation, the new cell has 23 + 23 chromosomes. This restores the full number of chromosomes, which is 46.
The new cell then divides by mitosis, so the number of cells increases. As the embryo develops, the cells differentiate, which means they become specialised for different jobs.
