Gametes and Fertilisation

Gametes and Fertilisation


  • Gametes are the sex cells produced by organisms for reproduction.
  • There are two types of gametes: sperm (male gamete) and ova or eggs (female gamete).
  • Gametes are produced in the gonads (testes in males and ovaries in females) through a process called meiosis.
  • They are different from other cells as they contain only one set of chromosomes (haploid), unlike body cells which contain two sets (diploid).
  • This means that when sperm and egg unite in fertilisation, the resulting offspring has the correct number of chromosomes (diploid).
  • Each gamete contains different combinations of genes which accounts for genetic variation.


  • Meiosis is a type of cell division that produces four non-identical daughter cells.
  • It involves two divisions, unlike mitosis which involves only one.
  • The first division separates the homologous chromosomes, and the second division separates the sister chromatids.
  • This process ensures that each gamete has one chromosome from each pair.
  • Crossing over of genetic material during meiosis leads to further genetic variation.


  • Fertilisation is the process whereby one sperm and one egg combine to create a zygote.
  • The fertilised egg then begins to divide by mitosis, forming a multicellular embryo.
  • Fertilisation is a random process; which sperm fertilises the egg is purely chance. This introduces further genetic variation.
  • Once the sperm penetrates the outer layers of the egg, the egg undergoes a rapid change that prevents any other sperm from penetrating – this is called the cortical reaction.
  • Fertilisation concludes with the fusion of the nuclei of the sperm and egg, combining the genetic material from both parents.