Meiosis (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Meiosis

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

• The number of chromosomes must be halved when the gametes are formed

  • Otherwise, there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)

• When a cell divides by meiosis:

  • copies of the genetic information are made

  • the cell divides twice

  • four gametes are formed

  Each gamete:

  • has a single set of chromosomes

  • is genetically different from the others

Comparing meiosis and mitosis

Fertilisation

• Gametes join at fertilisation to restore the normal number of chromosomes (23 pairs 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 to perform particular functions and form all the body tissues of the offspring

  • The process is known as cell differentiation