Cambridge O Level Biology · Syllabus 5090 · Development of Organisms and Continuity of Life
Mitosis
What is Mitosis?
Nuclear division giving rise to genetically identical cells in which the chromosome number is maintained. Mitosis provides the cells needed for growth, for the repair of damaged tissues, for the replacement of worn-out cells, and for asexual reproduction.
This definition is part of the Development of Organisms and Continuity of Life chapter in Cambridge O Level Biology.
Mitosis in context
Life is continuous because cells divide, and the two kinds of nuclear division do opposite things to chromosome number. Mitosis maintains the chromosome number and produces genetically identical cells, which is how an organism grows, repairs damaged tissue, replaces dying cells and reproduces asexually. Meiosis is a reduction division: it halves the chromosome number from diploid to haploid and produces genetically different cells, which is how gametes are made. Fertilisation — the fusion of a male and a female nucleus — puts the two haploid sets back together and restores the diploid number. In humans that cycle reads 46 → 23 → 46 in every generation, which is exactly why chromosome number does not double each time a child is born.
Common mistakes with Mitosis
- “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.
- “Mitosis halves the chromosome number.” Why wrong Mitosis maintains it — that is half the definition. Halving is what meiosis does. Say instead Mitosis: 46 → 46. Meiosis: 46 → 23.
- “Meiosis produces genetically identical cells.” Why wrong It contradicts the definition and removes the whole point of meiosis, which is variation. Say instead Meiosis produces genetically different cells; mitosis produces genetically identical ones.
- “The embryo grows by meiosis.” Why wrong If it did, the chromosome number would halve at every division and the embryo would soon have none. Say instead All growth after fertilisation is by mitosis. Meiosis happened earlier, to make the gametes.
- “Asexual reproduction produces variation.” Why wrong The offspring come from one parent by mitosis, so they carry the same genes. Any variation you can see is environmental, not genetic. Say instead Asexual reproduction produces genetically identical offspring; there is very little genetic variation between them.
Examiner tips on Mitosis
- The single most valuable habit in this topic. Before writing any answer that mentions pollination, fertilisation, meiosis or mitosis, say the definition silently to yourself first. Most lost marks in Topic 16 are not gaps in knowledge — they are the right idea attached to the wrong word.
Questions students ask about Mitosis
What is the difference between mitosis and meiosis?
Mitosis is nuclear division giving rise to genetically identical cells in which the chromosome number is maintained; it produces two cells and is used for growth, repair of damaged tissues, replacement of dying cells and asexual reproduction. Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells; it produces four cells and is used to make gametes. In humans, mitosis runs 46 → 46 while meiosis runs 46 → 23.
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.
What exactly is a stem cell?
A stem cell is an unspecialised cell that divides by mitosis to produce daughter cells which can become specialised. Because the division is mitosis, every daughter cell carries the same genes; what differs after differentiation is which of those genes are used. That is why a nerve cell and a red blood cell in the same person contain identical DNA but look nothing alike.