Meiosis

• Gametes are produced by meiosis
• Gametes only have one copy of each chromosome, so that when gamete fusion takes place, you get the right amount of chromosomes again
• To make gametes which only have half the original number of chromosomes, cells divide by meiosis. This process involves two cell divisions. In humans, it only happens in the reproductive organs (the ovaries and testes)
• Meiosis produces cells with half the normal number of chromosomes
• Before the cell starts to divide, it duplicated its genetic information, forming two armed chromosomes - one arm of each chromosome is an exact copy of the other arm. After replication, the chromosomes arrange themselves into pairs

  
• In the first division in meiosis the chromosome pairs line up in the centre of the cell

  
• After the pairs line up in the middle of the cell, the pairs are then pulled apart so each new cell only has one copy of each chromosome. Some os the father's chromosomes and some of the mother's chromosomes go into each new cell

  
• In the second division, the chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart

  
• You get four gametes, each with only a single set of chromosomes in it. Each of the gametes is genetically different from the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random

  
• The cell produced by gamete fusion replicates itself
• After 2 gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself
• Mitosis repeats many times to produce lots of new cells in an embryo. As the embryo develops, these cells then start to differentiate into different types of specialised cell that make up a whole organism