Fertilisation

Explained

Two gametes, one new cell

Gametes are sex cells. Each carries half the genetic information of an ordinary body cell, which is what haploid means, and fertilisation is the fusion of a male gamete nucleus with a female gamete nucleus.

The result is a single new cell called a zygote, with a full set of chromosomes restored. That cell then divides by mitosis to become an embryo.

Naming the gametes correctly

In animals the male gamete is the sperm and the female gamete is the egg cell, also called the ovum.

In flowering plants the female gamete is the egg cell inside the ovule. The male gamete is inside the pollen grain, and this is where care is needed: pollen is not itself the gamete. A pollen grain is a structure that contains the male gamete, in the same way that an ovule is a structure containing the female one.

Questions ask for the gamete, not the structure carrying it, and the two are marked differently.

Fusion of nuclei

The word fusion is doing precise work. It is the two nuclei that join, bringing the two sets of chromosomes into one cell.

Half the chromosomes come from each parent, which is why offspring resemble both and why sexual reproduction produces variation. It is also why gametes must be haploid: if they carried a full set, the chromosome number would double every generation.

Where it happens

In humans, fertilisation takes place in the oviduct, not in the uterus. The fertilised egg then travels to the uterus and implants there as an embryo, which is a separate later event.

In flowering plants, fertilisation takes place inside the ovule, after the pollen tube has grown down through the style and the male nucleus has travelled along it. Pollination, which is the transfer of pollen to the stigma, comes earlier and is not the same thing.

The sequence, in order

Pollination or mating, then the gametes meet, then fertilisation, then the zygote, then mitosis, then the embryo.

Each of those words means something different, and questions frequently supply one and ask for the next. Zygote is a single cell; embryo is what it becomes after dividing. Writing that fertilisation produces an embryo skips a step that the mark scheme treats as its own.

External and internal fertilisation

Many fish and amphibians release gametes into water, where fertilisation happens outside the body. It needs no mate contact but wastes vast numbers of gametes, so those animals produce enormous quantities.

Mammals, birds and reptiles fertilise internally, which protects the gametes and greatly increases the chance of success, so far fewer are needed.

If asked why an external fertiliser produces so many eggs, the answer is the low probability that any individual gamete meets another, not a difference in how fertilisation itself works.