Fertilisation

In the syllabus and the exam

Fertilisation sits at the centre of the reproduction and growth topic, marking the point where a haploid sperm and a haploid ovum combine to form a single diploid cell. Candidates are expected to state precisely where this happens in the human body (the oviduct, also called the fallopian tube), and to explain that fertilisation restores the diploid chromosome number that meiosis had earlier halved during gamete formation. Paper 2 questions often ask candidates to sequence events from ovulation through fertilisation to implantation, or to explain why only one sperm normally fertilises an ovum, which is commonly linked to a rapid change in the ovum's membrane that blocks further sperm from entering. A frequent error is using fertilisation and pollination interchangeably, which are entirely separate processes belonging to human/animal reproduction and plant reproduction respectively; another is confusing the zygote, which is the immediate product of fertilisation, with the embryo, which forms only after the zygote has undergone several rounds of mitotic division. Some candidates also incorrectly state that fertilisation occurs in the uterus rather than the oviduct, costing marks on an otherwise well-answered question. Candidates are also expected to know that fertilisation in humans normally results in a single zygote, and that the rare cases producing non-identical twins involve two separate ova being fertilised by two separate sperm in the same cycle, rather than one zygote splitting. This is also why doctors sometimes advise on the fertile window within a cycle, since fertilisation can only succeed within a limited number of days around ovulation, after which the released ovum is no longer viable.

Example

A candidate describing conception writes: 'Fertilisation takes place in the oviduct, where a sperm fuses with the ovum's nucleus to form a diploid zygote, restoring the full chromosome number of the species.'

Related terms

ZygoteDiploidHaploid

Source:SRC-DSKP-EN