Sexual Reproduction (Cambridge (CIE) IGCSE Biology): Revision Note

Gametes & zygotes

Sexual reproduction

• Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other

• Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring

Gametes

• A gamete is a sex cell (in animals: sperm and ovum; in plants pollen nucleus and ovum)

• Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells - we say they have a haploid nucleus

• This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells

• In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomes

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

• This contains the full 46 chromosomes, half of which came from the father and half from the mother - we say the zygote has a diploid nucleus

Haploid & diploid cells: Extended

• The nuclei of gametes are haploid

  • A haploid nucleus contain half the number of chromosomes of a normal body cell

  • In humans, a haploid cell has 23 chromosomes

• The nucleus of a zygote is diploid

  • A diploid nucleus contains the same number of chromosomes as a normal body cell

  • In humans, this is 23 pairs of chromosomes

  • The zygote continues to stay diploid as it grows into a foetus and embryo during pregnancy

Advantages & disadvantages of sexual reproduction: Extended

• Advantages of sexual reproduction:

  • increases genetic variation within a population

  • this variation gives the population a better chance of adapting to a changing environment

  • a population with more variation is less likely to be wiped out entirely by a single disease, since some individuals may be resistant

• Disadvantages of sexual reproduction:

  • finding a mate costs time and energy

  • individuals that are isolated from others may struggle to find a mate at all

• Many crop plants reproduce sexually:

  • This is an advantage as it means variation is increased and a genetic variant may be produced which is better able to cope with weather changes, or produces significantly higher yield

  • The disadvantage is that the variation may lead to offspring that are less successful than the parent plant at growing well or producing a good harvest