Sexual ReproductionCambridge IGCSE Biology: Revision notes

Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote, producing offspring that are genetically different from each other. Fertilisation is the fusion of the nuclei of gametes.

Gametes (sex cells) are haploid — they contain a single set of chromosomes. When two haploid gametes fuse at fertilisation, the resulting zygote is diploid, containing two sets of chromosomes.

Advantages of sexual reproduction: produces genetic variation, which helps a population survive a changing environment or disease, and allows new combinations of desirable traits to be bred into crops. Disadvantages: it is slower, requires two parents (a mate must be found), and can break up combinations of genes that worked well together.

A typical insect-pollinated flower has these structures:

Pollination is the transfer of pollen grains from an anther to a stigma.

• Self-pollination: pollen transferred from the anther of a flower to the stigma of the same flower, or a different flower on the same plant
• Cross-pollination: pollen transferred from the anther of a flower to the stigma of a flower on a different plant of the same species

Insect-pollinated flowers tend to have large, brightly coloured petals, scent, nectar, and pollen grains that are sticky or spiky so they attach to insects. Wind-pollinated flowers have small, dull petals (or none), feathery stigmas that hang outside the flower to catch airborne pollen, anthers loosely attached and hanging outside the flower, and pollen grains that are small, light and smooth, produced in huge numbers.

Cross-pollination increases variation in a population, helping it respond to environmental change, but relies on pollinators or wind being present. Self-pollination is more reliable when pollinators are scarce but produces less variation.

After pollination, if the pollen grain is compatible, it germinates on the stigma and grows a pollen tube down through the style towards an ovule in the ovary. The pollen tube carries the male nucleus to the ovule. Fertilisation occurs when the pollen nucleus fuses with a nucleus in the ovule, forming a zygote that develops into a seed.

Germination of seeds requires three environmental conditions: water, oxygen, and a suitable temperature. Without all three, a seed will remain dormant.

Fertilisation is the fusion of the nuclei of a sperm (male gamete) and an egg cell (female gamete). Sperm and egg cells are adapted for their roles:

• Sperm: a flagellum for swimming, many mitochondria for energy, and enzymes in the acrosome to digest a path into the egg
• Egg cell: energy stores for the developing embryo, and a jelly coat that changes after fertilisation to prevent more than one sperm entering

Compared with sperm, egg cells are far larger, non-motile (do not swim), and produced in much smaller numbers, while sperm are tiny, motile, and produced in vast numbers.

After fertilisation, the zygote divides repeatedly to form an embryo — a ball of cells — which implants into the lining of the uterus.

Some pathogens and toxins (e.g. alcohol, nicotine) can cross the placenta and affect the fetus.

At puberty, testosterone (from the testes) and oestrogen (from the ovaries) bring about the development of secondary sexual characteristics. Oestrogen and progesterone also control the menstrual cycle, which involves cyclical changes in the ovaries (egg maturation and release) and the uterus lining (thickening, then shedding if no pregnancy occurs), regulated by FSH, LH, oestrogen and progesterone.