Zygote

In the syllabus and the exam

Zygote is examined as the single cell immediately produced by fertilisation, and candidates must be able to place it correctly in the sequence of early human development: fertilisation forms the zygote, the zygote divides by mitosis to form an embryo, and the embryo implants in the uterus wall before developing into a fetus. A key fact tested in Paper 1 and Paper 2 alike is that the zygote is diploid, containing chromosomes from both parents, which distinguishes it clearly from the haploid gametes that produced it. Structured questions sometimes ask candidates to explain why the zygote must divide by mitosis rather than meiosis at this stage, with the expected answer being that mitosis produces genetically identical cells needed for growth, whereas meiosis would halve the chromosome number again. A common error is describing the zygote as already having body parts or organs; at this stage it is simply a single cell, and organs only begin to form later as the embryo develops. Candidates also sometimes reverse the sequence, writing that the zygote is formed after implantation rather than before it. Candidates should also be aware that the zygote's early mitotic divisions occur without an overall increase in size, so a ball of many small cells forms before implantation, and this detail is occasionally tested when candidates are asked to compare the size of the zygote with the size of the resulting embryo. This distinction is also useful when candidates are asked to explain why identical twins share exactly the same genetic material, since both arise from the mitotic division of a single original zygote rather than from two separately fertilised ova. This is also why fertility treatments that combine a sperm and an ovum outside the body must culture the resulting zygote for a short period, allowing initial mitotic divisions to occur, before it can be transferred into the uterus for implantation to proceed.

Example

Tracing early development, a candidate writes: 'The zygote formed after fertilisation divides repeatedly by mitosis as it travels down the oviduct, becoming a ball of cells before it implants in the uterine wall.'

Related terms

FertilisationDiploidMitosis

Source:SRC-DSKP-EN