Meiosis (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Meiosis

• Cells in reproductive organs divide by meiosis to form gametes (sex cells)

• During meiosis the number of chromosomes is halved

  • Otherwise, there would be double the number of chromosomes after they join at fertilisation

  • For example, a human body cell has 46 chromosomes (23 pairs); each gamete has a single set of 23 chromosomes – one from each pair, not 23 pairs

• The cells produced during meiosis are genetically different from each other and the parent cells

• The process of meiosis involves:

  • copying the original DNA

  • two rounds of cell division

  • the production of four daughter cells from each parent cell

The importance of meiosis

• Meiosis is important for life cycles because:

  • it produced gametes; this enables sexual reproduction

  • it halves the chromosome number, ensuring that fertilisation restores the normal number of chromosomes

  • it generates genetic variation in offspring

    • Each gamete receives one chromosome from each pair, chosen at random, so meiosis forms new combinations of maternal and paternal chromosomes every time a gamete is made

    • Therefore when gametes fuse randomly at fertilisation, every offspring inherits a different combination of alleles

Fertilisation

• Gametes join at fertilisation to restore the normal number of chromosomes

• When the male and female gametes fuse, they become a zygote (fertilised egg cell)

• This contains the full number of chromosomes, half of which came from the male gamete and half from the female gamete

• The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo

• Cell division continues and eventually many of the new cells produced become specialised (the cells differentiate) to perform particular functions and form all the body tissues of the offspring

• The process of cells becoming specialised is known as cell differentiation