Reproduction · Process
Gametes and fertilisation
Two cells fuse and become one. Each brought half a set of instructions; neither brought anything like half the material.
Start here
An egg cell has several thousand times the volume of a sperm cell.
An egg is around 0.1 mm across — just visible without a microscope, the largest cell in the human body. A sperm’s head is about 0.005 mm. They contribute exactly the same number of chromosomes: twenty-three each.
Equal instructions, wildly unequal size. Where has all the extra gone?
Into everything the new organism needs for its first few days: cytoplasm, mitochondria, a food store, and all the machinery for building a body. The sperm brings chromosomes and a means of arriving. Equal genetic contribution, radically unequal material contribution — and both of those facts matter.
Two specialised cells · six features
Every difference has a reason
0 of 6 opened
These are the specialised cells from Specialised cells, doing the clearest job in the body. Tap a row to see why each cell has what it has.
Why: The egg is large because of what it carries, not because of what it does. A sperm only has to travel, and travelling is easier the smaller you are.
Why: The egg is carried along the oviduct by cilia and by muscle in the wall. Only one of the two gametes needs its own propulsion, and it is the cheap one.
Why: The sperm’s mitochondria release energy for swimming and are destroyed after fusion. Every mitochondrion in your body came from the egg.
Why: The fertilised egg has to survive several days of dividing before it implants and can be supplied. Everything it spends in that time was loaded before fertilisation.
Why: The one row where the two are identical. Half a set is only useful if it is going to be added to another half — which is why a gamete cannot live independently.
Why: A direct consequence of cost. Something cheap to make can be made in vast numbers and mostly wasted; something expensive cannot.
Relative diameter, to scale
Egg cell
0.1 mm across
Sperm cell head
0.005 mm across
Those bars are diameters. Because volume goes as the cube of the diameter, a twentyfold difference in width is roughly an eight-thousandfold difference in volume — and the egg’s cytoplasm is where nearly all of that sits.
The process, in order
Five steps, and only one of them is fertilisation.
Release · Ovary oviduct
One mature egg cell leaves an ovary and enters the oviduct, where cilia and muscle begin moving it along. It remains able to be fertilised for roughly a day.
Transfer and travel · Vagina uterus oviduct
Semen is transferred into the vagina. Sperm swim through the cervix and uterus into the oviduct — a journey of about 15 cm for a cell 0.005 mm long. Hundreds of millions set out; a few hundred arrive.
Fertilisation · Oviduct — and nowhere else
One sperm passes through the egg’s outer layer and its nucleus fuses with the egg’s nucleus. 23 plus 23 makes 46. The outer layer then changes so no other sperm can enter. This step, and only this step, is fertilisation.
Dividing · Travelling down the oviduct
The single fertilised cell divides into two, then four, then eight, over about five days, while still travelling. It is a ball of cells before it has arrived anywhere.
Implantation · Uterus lining
The ball of cells embeds itself in the thickened lining of the uterus, where it can be supplied. This is a separate event from fertilisation, several days later and in a different organ — and it is where pregnancy begins.
Key fact
Fertilisation is the moment the nucleus of a sperm fuses with the nucleus of an egg, forming a single cell with a full set of chromosomes. It happens in the oviduct, and everything before and after it is a different step with a different name.
Think again
“Fertilisation is when the sperm reaches the egg.”
Arriving is not fertilising. Many sperm reach the egg and touch its surface; fertilisation is the single event in which one sperm’s nucleus fuses with the egg’s nucleus, and it happens once. The distinction is not fussiness — the moment of fusion is what triggers the egg to change its outer layer so that no second sperm can enter, and it is what produces the full set of forty-six chromosomes. Use *reaches* and you have described a crowd; use *fuses* and you have described the event. The same care applies to the words either side of it: implantation is a different step, days later and in a different organ, and pregnancy begins there rather than at fertilisation.
“Identical twins happen when two eggs are fertilised.”
That is the recipe for non-identical twins, and it is worth having both straight because the two arise at completely different moments. Non-identical twins: two eggs released, two sperm, two separate fertilisations — two ordinary siblings who happen to share a pregnancy, and they can be different sexes. Identical twins: one egg, one sperm, one fertilisation — and then, within the first fortnight, the ball of cells splits in two and each half continues as a whole embryo. Same chromosomes in both, always the same sex. Notice what that means: identical twins are not caused by anything about the gametes at all. The cause happens after fertilisation is over.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Define it precisely
Which of these is fertilisation?
Rung 2 · The one that catches people
A sperm and an egg each carry 23 chromosomes, but an egg has thousands of times more material. What is the extra material for?
Rung 3 · Explain the two designs
Explain why a sperm cell is small with a tail and many mitochondria, while an egg cell is large and immobile with a food store. Say what each is specialised for, and name the one feature they share.
Rung 4 · Take it somewhere new
A student says identical twins must come from two eggs, because there are two babies. Say whether they are right, explain what actually causes each kind of twin, and give one observation that would tell the two kinds apart.
Key note
A gamete is a sex cell with half a set of chromosomes — 23 in humans. Sperm are small, mobile and numerous; egg cells are large, immobile and few, and carry the food store and cytoplasm. Fertilisation is the fusion of the two nuclei, in the oviduct, producing a single cell with 46 chromosomes. It divides as it travels and implants in the uterus lining several days later.
Going further
Every mitochondrion in your body came from your mother. The egg supplies all the cytoplasm and everything in it; the sperm’s few mitochondria power its journey and are destroyed after fusion. Mitochondria carry a small loop of their own DNA, so that DNA passes down an unbroken line of mothers — and because it is not shuffled with anything each generation, it changes only by slow mutation. That makes it a clock. Comparing mitochondrial DNA across living humans traces every one of us back to a common maternal ancestor in Africa, and it is how the deep structure of human migration was worked out before anyone could sequence a whole genome. A cell’s leftover packing material turned out to be the archive.
Before this lesson
- This follows on from Human reproductive systems.
Connects to
At GCSE this becomes
- Meiosis, sexual and asexual reproduction compared, and inherited variation.
Where to next
Ask Mr Badmus AI
Want to check the difference between reaching and fusing?
If any of this is about you or someone you know, talk to someone you trust — a parent or carer, a teacher, your school nurse, a pharmacist or your GP. Out of school hours: Childline — 0800 1111, free and confidential.
Cell sizes are typical values: egg about 0.1 mm diameter, sperm head about 0.005 mm. This is the biology of the cells and the process; relationships and consent are covered in your RSE and PSHE lessons.
Lesson content © MrBadmusAI.