Sexual and asexual reproductionIB MYP Biology: Revision notes

Sexual reproduction involves two parents. Each parent makes special sex cells called gametes (sperm and egg cells in animals, pollen and egg cells in flowering plants).

The male and female gametes join in fertilisation to form a zygote, which divides and grows into a new organism.

The offspring get genes from both parents, so they show genetic variation: they are different from their parents and from each other.

Advantages: variation means some offspring may survive changes in the environment or a new disease. Disadvantages: two parents are needed, it takes more time and energy, and fewer offspring are produced at once.

Asexual reproduction involves one parent, no gametes and no fertilisation. The offspring are genetically identical to the parent (clones).

• Bacteria: binary fission. A bacterium copies its DNA and divides into two identical cells. In good conditions this can happen about every 20 minutes.
• Plants: runners, such as strawberry plants, are stems that grow along the ground and produce new plants.
• Fungi: many fungi, such as moulds, release tiny spores, each of which can grow into a new fungus.

Advantages: fast, only one parent is needed, no mate is required, and good features are kept. Disadvantages: no variation, so a disease or change in conditions can harm all the offspring.

A life cycle is the series of stages an organism goes through.

Mammal: fertilisation of an egg by a sperm, then the embryo develops inside the mother, birth, young mammal, adult that makes gametes.

Insects change form in metamorphosis:

• Incomplete metamorphosis (e.g. grasshopper): egg, nymph, adult. The nymph looks like a small adult without wings and grows by shedding its skin.
• Complete metamorphosis (e.g. butterfly): egg, larva, pupa, adult. The larva (caterpillar) looks very different from the adult. Inside the pupa the body is rebuilt.

Flowering plants reproduce sexually. The male gametes are in pollen grains made in the anther, and the female gametes are in the ovules in the ovary.

1. Pollination: pollen is transferred from an anther to a stigma, by insects or by wind.
2. Fertilisation: a pollen tube grows down to the ovule and a male gamete fuses with the female gamete. The ovule becomes a seed and the ovary becomes a fruit.
3. Seed dispersal: seeds are spread by wind (wings or parachutes), animals (hooked fruits, or fruits that are eaten) or water.

Dispersal reduces competition for light, water and mineral ions between the new plants and their parent.

The seed then germinates to grow into a new plant, completing the life cycle.