Asexual and sexual reproduction, gametes

Last time

Before we start: last time, in Antibiotics and antibiotic resistance, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

One parent or two?

A strawberry plant sends out runners, and each grows into a new plant exactly like it. A puppy looks a bit like each parent, but not exactly like either. Two ways to reproduce: asexual and sexual. Today: how they differ, and the gametes that make sexual reproduction work.

Asexual reproduction

Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent. There are no gametes and no fertilisation. Examples: bacteria splitting in two, potato tubers, strawberry runners, and yeast budding.

Asexual: good and bad

Advantages: only one parent is needed, and it is fast, so a population can grow quickly in good conditions. Good features are passed on exactly. Disadvantage: there is no variation, so if conditions change, or a disease arrives, all the offspring may die.

Sexual reproduction

Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote. The offspring are genetically different from each other and from their parents. This variation means some may survive if conditions change. But it is slower, and usually needs two parents to meet.

Haploid and diploid

Body cells are diploid: they have two sets of chromosomes. In humans that is forty-six. Gametes are haploid: they have one set of chromosomes. In humans that is twenty-three. At fertilisation, two haploid nuclei fuse, so the zygote is diploid again: twenty-three plus twenty-three is forty-six.

Sperm and egg

Compare the two human gametes. The sperm is small and moves with a tail. The egg is large and does not move. Each feature helps fertilisation.

Adaptations of gametes

The sperm's flagellum lets it swim to the egg, powered by its many mitochondria. Its acrosome contains enzymes that digest the jelly coat, so the sperm can enter. The egg has a food store for the early embryo. Its jelly coat changes after fertilisation so only one sperm gets in.