Mammalian Gametes (Edexcel International A Level Biology): Revision Note

Gamete Specialisation

Gametes

• Gametes are the sex cells of an organism

  • For example, the sperm and egg (ovum) cells in humans

• Gametes fuse during fertilisation to form a zygote

  • Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)

• These sex cells are formed during meiosis and only have one copy of each chromosome, so they are haploid cells

  • For humans, that means the sperm and egg cells contain 23 single chromosomes in their nucleus

Sexual reproduction involves the fusing of two gametes to form a zygote that contains DNA from both parents

Mammalian gametes are specialised for their functions

• Mammalian gametes have adaptations to increase the chances of fertilisation and successful development of an embryo

• Sperm cells:

  • Have a flagellum (tail) that allows them to swim towards the egg cell

  • Contain many mitochondria that provide energy for movement of the flagellum (swimming)

  • An acrosome that contains digestive enzymes to break down the protective glycoprotein layer (a jelly-like coating known as the zona pellucida) surrounding the egg cell - sperm cells must penetrate this layer in order to fertilise the egg

• Egg cells:

  • Are much larger than sperm cells as most of their internal space contains food to nourish a growing embryo

  • Have follicle cells that form a protective coating

  • Have a jelly-like glycoprotein layer, known as the zona pellucida, that forms an impenetrable barrier after fertilisation by a sperm cell has occurred, to prevent other sperm nuclei from entering the egg

• These features are summarised in the diagrams and tables below 

Structure of a mammalian sperm cell

Structure of a mammalian egg cell

Adaptations of Mammalian Gametes Table