Gametes and fertilisation

Fertilisation, step by step

Step through what happens when a sperm cell meets an egg cell.

1. 1A sperm cell meets an egg cellThe sperm cell is the male gamete and the egg cell is the female gamete. They meet in a fallopian tube, not in the uterus.+
2. 2The sperm enters the eggOnce the sperm cell gets inside the egg, it loses its tail. The tail has done its job.+
3. 3The two nuclei fuseThe nucleus of the sperm, with its chromosomes, joins with the nucleus already inside the egg. The result is one new nucleus. This fusing is what we call fertilisation.+
4. 4A zygote is formedThe fertilised egg cell is called a zygote. It is the offspring of both parents and the first stage in the life cycle of a new living thing.+
5. 5The zygote divides and moves onThe zygote copies itself and divides into a ball of cells called an embryo. As it divides, the embryo moves down the fallopian tube towards the uterus.+

Does the rule work for a horse?

The halving rule is not just for humans. Commit to an answer before you reveal it.

An adult horse has 64 chromosomes in the nuclei of its body cells. How many chromosomes will there be in one horse sperm cell, in one horse egg cell, and in the foal that results?

What you need to know

• Gametes are sex cells: the sperm cell is the male gamete and the egg cell is the female gamete.
• Each gamete carries half the genetic material. In humans that is 23 chromosomes in each gamete, against 46 in the body cells.
• Fertilisation happens when the nucleus of a sperm fuses with the nucleus of an egg, in a fallopian tube, to form a zygote.
• The zygote divides into an embryo, then a foetus grows in the uterus, then a baby is born. Everything after the zygote is growth.

The big picture

Humans are made when two special cells, a sperm and an egg, fuse. Each of these gametes carries half the genetic material (23 chromosomes), so the zygote they make has the full 46, half from each parent. The zygote then grows through embryo, foetus and baby.

⚠ Watch out

Mixing up what a gamete carries and what the zygote carries. A gamete has 23 single chromosomes, which is half a set. It is the zygote, and the body cells that grow from it, that have 46: two copies of each chromosome, one from each parent.

Memory hook

Halve it, join it, same again: 46 becomes 23 + 23, which becomes 46.

Check yourself

A human zygote has 46 chromosomes. How many came from the egg, and why can't the sperm carry 46 too? (23 from each. A 46-chromosome sperm would give the zygote more than 46.)