Year 1 · Reproduction in plants and animals · Section 6.6

Sexual reproduction

Sex cells

Sexual reproduction involves the fusion of a male sex-cell nucleus with a female sex-cell nucleus. The female sex cell is an egg cell. In flowering plants, pollen carries cells that produce the male sex cells; a pollen grain is not itself a single sex cell.

Fertilisation produces a zygote, or fertilised egg cell, which can develop into a new organism. Two sex cells are involved, but they do not always come from two separate organisms. In flowering plants, fertilisation can follow self-pollination or cross-pollination.

How sex cells form

Meiosis is a special type of division that reduces the chromosome number by half and contributes to genetic variation. DNA is copied once before meiosis, followed by two nuclear divisions. There is no second round of DNA copying between these divisions.

In animals, meiosis is involved directly in producing sex cells. In flowering plants, meiosis produces spores that develop into structures which then produce the sex cells.

In asexual reproduction, such as tulips forming new bulbs, ordinary cell division normally preserves the parent's genetic information. Sexual reproduction brings together different genetic combinations.

Fertilisation

Suppose a plant species has 24 chromosomes in its ordinary body cells. A male sex cell and an egg cell each contain 12. Their nuclei fuse during fertilisation, producing a zygote with 24 chromosomes.

The zygote then develops through ordinary cell division. It receives one set of nuclear chromosomes from each sex cell. Different combinations of parental genetic information create variation among offspring.

Animals

In animals, a sperm cell fuses with an egg cell. Their production involves meiosis, so offspring inherit a new combination of genetic information.

• External fertilisation: Takes place outside the female's body, as in many fish and frogs. Eggs and sperm are released into water.
• Internal fertilisation: Takes place inside the female's body, as in insects, birds and mammals.

Many animals laying eggs on land produce protective coverings, such as shells, that reduce water loss. Eggs developing in water often do not need a hard shell.