Cambridge O Level Biology · Syllabus 5090 · Development of Organisms and Continuity of Life
Zygote
What is Zygote?
The single diploid cell formed when a sperm nucleus fuses with an egg nucleus at fertilisation. In humans a zygote contains 46 chromosomes, 23 from each gamete, and it begins to divide by mitosis to form an embryo.
This definition is part of the Development of Organisms and Continuity of Life chapter in Cambridge O Level Biology.
Common mistakes with Zygote
- “Every cell in the body has a different genetic code.” Why wrong All the body cells of one organism come from one zygote by mitosis, so they carry the same genes. Cells differ because different genes are used in different cells. Say instead Body cells are genetically identical; they differ in which genes are active.
- “Fertilisation produces a haploid zygote.” Why wrong Two haploid nuclei fusing gives two sets of chromosomes, not one. Say instead Fertilisation produces a diploid zygote: 23 + 23 = 46 in a human.
Questions students ask about Zygote
What is the difference between an embryo and a fetus?
At O Level the distinction is one of stage rather than a sharp definition. The zygote is the single diploid cell formed at fertilisation. As it divides by mitosis it becomes a ball of cells, the embryo, which implants in the uterus lining. Later in pregnancy, once the main organs have formed and are recognisable, the developing individual is referred to as a fetus.