Sexual and asexual reproduction, meiosis · GCSE Biology

Sexual reproduction

Sexual reproduction involves the joining (fusion) of male and female gametes: sperm and egg cells in animals, pollen and egg cells in flowering plants.
Gametes are made by meiosis. Because genetic information from two parents is mixed, the offspring show variation.

Asexual reproduction involves only one parent and no fusion of gametes.
There is no mixing of genetic information, so the offspring are genetically identical to the parent and each other (clones).
Only mitosis is involved.

Meiosis happens in the reproductive organs (ovaries and testes) and halves the number of chromosomes in gametes.
First the genetic information is copied. Then the cell divides twice to form four gametes, each with a single set of chromosomes.
All the gametes are genetically different from each other.
At fertilisation, gametes join to restore the normal number of chromosomes. The new cell divides by mitosis, and as the embryo develops its cells differentiate.

Advantages of sexual reproduction: it produces variation, so if the environment changes some offspring may survive (natural selection); humans can use it in selective breeding.
Advantages of asexual reproduction: only one parent is needed; it is faster and uses less time and energy; many identical offspring can be produced when conditions are good.
Some organisms do both. Malaria parasites reproduce asexually in the human host and sexually in the mosquito. Many fungi reproduce asexually by spores and sexually to give variation. Many plants make seeds sexually and also reproduce asexually by runners (strawberries) or bulb division (daffodils).

Sexual reproduction: Reproduction involving the fusion of male and female gametes.
Asexual reproduction: Reproduction with one parent and no gametes, giving identical offspring.
Gamete: A sex cell (sperm, egg or pollen nucleus) with half the normal number of chromosomes.
Meiosis: Cell division that makes four genetically different gametes, each with half the chromosome number.
Fertilisation: The fusion of a male and female gamete.
Clone: An organism genetically identical to its parent.
Variation: Differences between individuals of the same species.
Runner: A stem that grows along the ground and forms new plants asexually.
Spore: A cell released by fungi that can grow into a new organism.