Gametes and Fertilisation

Gametes

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

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

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.