Reproduction and meiosis

Meiosis leads to non-identical cells being formed, while mitosis leads to identical cells being formed. Everything in this section follows from that one difference.

Sexual reproduction involves the joining (fusion) of male and female gametes: sperm and egg cells in animals, pollen and egg cells in flowering plants. There is mixing of genetic information, which leads to variety in the offspring. The formation of gametes involves meiosis.

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

Some organisms reproduce by both methods, depending on the circumstances, and the specification names three:

• Malarial parasites reproduce asexually in the human host, but sexually in the mosquito. • Many fungi reproduce asexually by spores, but also reproduce sexually to give variation. • Many plants produce seeds sexually, but also reproduce asexually — by runners, such as strawberry plants, or by bulb division, such as daffodils.

Knowledge of reproduction in organisms is restricted to those mentioned. You are, however, expected to explain the advantages and disadvantages for any organism if you are given the information — so learn the reasoning, not the list.

The reasoning is a trade-off between speed and insurance. Asexual reproduction is fast and cheap, and it works while conditions stay the same. But every offspring is identical, so a change in the environment — or a disease — affects all of them equally. Sexual reproduction is slower and needs a mate, but the variation it produces means some offspring may survive a change that kills the rest.

The malaria example carries a piece of history worth knowing. Understanding that the parasite reproduces sexually in the mosquito is what identified the mosquito as the vector rather than as a coincidence — and that historical development in our understanding of the cause is exactly what made prevention possible, by targeting mosquito breeding sites and using nets rather than treating the illness alone.

Cells in reproductive organs divide by meiosis to form gametes. When a cell divides to form gametes:

• copies of the genetic information are made • the cell divides twice to form four gametes, each with a single set of chromosomes • all gametes are genetically different from each other.

Gametes join at fertilisation to restore the normal number of chromosomes. The new cell then divides by mitosis, the number of cells increases, and as the embryo develops the cells differentiate.

Knowledge of the stages of meiosis is not required — only what it achieves.

The halving is the point. A human body cell has 46 chromosomes, in 23 pairs. Meiosis produces gametes with 23 single chromosomes each, so when a sperm and an egg fuse the new cell has 46 again. Without the halving, the number would double every generation.