Fertilisation

New living things are made by sexual reproduction. This needs two special cells called gametes (say it "GAM-eets"). A gamete is simply a sex cell:

• the male gamete, and
• the female gamete.

Fertilisation is the fusion of a male gamete with a female gamete. "Fusion" just means the two cells join together and combine their contents into one. You can think of it like two halves clicking together to make a whole.

When the male gamete and female gamete fuse, they form a single new cell. That new cell has a special name — the zygote — which we look at in the next section.

Exam tip. The marking word the examiner is looking for is fusion (or "join together"). Saying the gametes "meet" or "touch" is not enough — they must fuse to combine into one cell.

The moment fertilisation is complete, the single cell formed is called a zygote. A zygote is just one cell — but it is the very first cell of a brand-new organism.

That one cell does not stay single for long. The zygote divides by cell division (the type of cell division is mitosis). One cell becomes two, two become four, four become eight, and so on. This builds a ball of cells.

As the ball of cells keeps dividing and the cells begin to do different jobs, it develops into an embryo — the early stage of a developing animal or plant before it is fully formed.

So the whole chain of events in spec 3.2 is:

gamete + gamete → fertilisation → zygote → cell division → embryo

Here is the sequence drawn out:

Exam tip. Learn the order of the words: zygote first (one cell), then cell division, then embryo. A common slip is to call the very first cell an "embryo" — the first cell is the zygote.

Inside the nucleus of a cell are thread-like structures called chromosomes, which carry the genetic information. The number of chromosomes matters here.

• A gamete is haploid. Haploid means it has one set of chromosomes — that is half the normal number for the organism.
• A zygote is diploid. Diploid means it has two sets of chromosomes — the full (normal) number.

Why does this work out so neatly? Each gamete brings one set of chromosomes. When the male gamete (one set) fuses with the female gamete (one set), the two sets add together:

haploid gamete (1 set) + haploid gamete (1 set) → diploid zygote (2 sets)

So half + half = full. This is the whole reason gametes must be haploid: if they were already diploid, fertilisation would double the chromosome number every generation, which would be a problem. Being haploid keeps the chromosome number constant from one generation to the next.

A simple way to remember the words:

• Haploid → half (both start with "ha-") → gametes.
• Diploid → double / di- (two) → zygote and the body cells that grow from it.

Exam tip. The exact marking points are: gametes are haploid (one set of chromosomes) and the zygote is diploid (two sets of chromosomes). State the number of sets — not just the words haploid/diploid — to be safe.

Spec 3.2 is really about getting four keywords in the right order and knowing the chromosome numbers. Here they all are in one place:

Term	What it is	Chromosome number
Gamete	A sex cell (male gamete or female gamete)	Haploid — one set (half)
Fertilisation	The fusion of a male gamete and a female gamete	—
Zygote	The single cell formed when the gametes fuse	Diploid — two sets (full)
Embryo	The ball of cells the zygote develops into after cell division	Diploid — two sets (full)

The story in one line:

Two haploid gametes fuse during fertilisation to make a diploid zygote, which divides by cell division to become an embryo.

If you can write that sentence from memory and add "haploid = one set, diploid = two sets", you have the full mark scheme for spec 3.2.

Exam tip. Questions often give you the words jumbled up and ask you to put them in order, or to fill blanks in a flow diagram. Practise writing the chain gamete → fertilisation → zygote → cell division → embryo until it is automatic.