Cell Division
Cells divide to grow, repair damage and reproduce, and this chapter is built around two very different types of division. Mitosis produces identical body cells for growth and repair, while meiosis produces the gametes needed for sexual reproduction.
Both processes start from the same cell cycle, but they lead to cells with completely different chromosome numbers and completely different genetic content, which is exactly why examiners test the two processes side by side rather than in isolation. Understanding this distinction clearly from the start makes the whole chapter far easier to master.
The chapter opens with chromosomes themselves, thread-like structures made of DNA that carry the genes controlling an organism's characteristics. Human body cells are diploid, containing 23 pairs of chromosomes, one member of each pair inherited from each parent.
Understanding diploid and haploid numbers before studying mitosis and meiosis makes it far easier to explain why one division keeps the chromosome number constant while the other halves it, and why exam questions often ask you to state a chromosome number before and after a named division.
A separate content standard covers the cell cycle in detail: a long interphase in which the cell grows and its DNA replicates, followed by the much shorter mitotic phase, when the nucleus divides through prophase, metaphase, anaphase and telophase before the cytoplasm splits in cytokinesis. Meiosis follows a similar overall pattern but involves two successive divisions, which is what produces four cells instead of two.
Sketching the cell cycle as a labelled circular diagram helps you remember the order and relative length of each stage.
The final content standard links cell division to human health, focusing on what happens when the controls on the cell cycle fail. Cells that divide continuously without stopping form a tumour, and a malignant tumour can invade nearby tissue and spread to other parts of the body.
This section is examined by asking you to explain, in terms of cell division, why uncontrolled growth is dangerous, not by asking for medical treatment details. This reinforces the wider idea that cell biology explains the reasons behind a health condition, not just memorised medical facts.
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Content standards in this chapter
- 6.1 Cell Division
- 6.2 The Cell Cycle and Mitosis
- 6.3 Meiosis
- 6.4 Issues of Cell Division on Human Health
Key concepts
- Chromosomes
- Chromosomes are thread-like structures made of DNA found in the nucleus, and each carries many genes that control an organism's characteristics. Human body cells are diploid (2n), containing 23 pairs of chromosomes, 46 in total, with one chromosome of each pair inherited from the mother and one from the father. Knowing this number is often needed to answer questions about gamete or daughter-cell chromosome counts.
- The cell cycle
- The cell cycle has two main stages: interphase, during which the cell grows, carries out its normal functions, and replicates its DNA so each chromosome becomes two identical sister chromatids; and the mitotic phase, when the nucleus divides followed by cytokinesis, which splits the cytoplasm into two separate daughter cells. Interphase lasts considerably longer than the mitotic phase, reflecting the time needed for cell growth and careful DNA replication.
- Mitosis
- Mitosis is nuclear division that produces two daughter cells genetically identical to the parent cell and to each other, both diploid. It proceeds through four stages in order, prophase, metaphase, anaphase and telophase, and is used for growth, for replacing worn-out cells, and for repairing damaged tissue. Recognising these four stages from a photograph or diagram is one of the most frequently tested skills in this chapter.
- Meiosis
- Meiosis is a special type of nuclear division that occurs only in reproductive organs, involving two successive divisions that produce four daughter cells, each haploid (n) and genetically different from one another. It halves the chromosome number so that fusion of two gametes at fertilisation restores the full diploid number.
- Importance of meiosis
- Meiosis keeps the chromosome number constant from one generation to the next; without it, fertilisation would double the chromosome number every generation. It also introduces genetic variation among offspring through the independent separation of chromosome pairs, which is one reason siblings are not genetically identical. Without this variation, populations would have far less raw material for natural selection to act on over time.
- Uncontrolled cell division and cancer
- The cell cycle is normally controlled so that cells divide only when needed. When this control fails, cells divide continuously and without stopping, forming a mass of cells called a tumour. A malignant tumour can invade surrounding tissue and spread to other parts of the body, which is what makes it cancerous. Understanding the stages of the cell cycle helps pinpoint exactly where this normal growth control can fail.
- Diploid and haploid numbers
- A diploid cell (2n) contains chromosomes in homologous pairs, one from each parent, and is the normal condition of body cells produced by mitosis. A haploid cell (n) contains only one chromosome of each pair and is found only in gametes produced by meiosis. Recognising which number a described cell has is often the first step in answering a genetics or cell-division question correctly.
- Stages of mitosis
- In prophase the chromosomes condense and become visible, each already duplicated into two sister chromatids; in metaphase they line up at the equator of the cell; in anaphase the sister chromatids are pulled to opposite poles; and in telophase two new nuclei form before the cytoplasm divides.
- Chromosome behaviour in meiosis
- In the first meiotic division, homologous chromosome pairs separate, halving the chromosome number; in the second division, which resembles mitosis, the sister chromatids of each chromosome separate. The result is four haploid cells, each with a single, unpaired set of chromosomes. This two-step separation is why meiosis, unlike mitosis, cannot be described by a single simple set of four stages.
- Growth, repair and reproduction
- Mitosis and meiosis serve different purposes in the same organism: mitosis increases cell number for growth and replaces damaged cells during repair, while meiosis produces sex cells for reproduction. Confusing the two processes is one of the most common sources of lost marks in this chapter.
How this chapter is examined
SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.
Common exam angles
- Comparing mitosis and meiosis in a table covering the number of daughter cells, chromosome number and purpose.
- Identifying a stage of mitosis from a diagram and describing what is happening to the chromosomes at that stage.
- Explaining why gametes must be haploid and what would happen if they were diploid instead.
- Explaining, using the cell cycle, why uncontrolled cell division leads to the formation of a tumour.
- Calculating or stating the chromosome number of a cell after mitosis or after each stage of meiosis.
- Sequencing the phases of the cell cycle, from interphase through to cytokinesis, in the correct order.
Common mistakes
What students write: Saying mitosis produces gametes.
What earns the mark: Mitosis produces identical body cells for growth and repair; meiosis produces gametes, and confusing the two on an exam script usually costs at least one full mark.
What students write: Writing that meiosis makes two cells.
What earns the mark: Meiosis makes four haploid cells; mitosis makes two diploid cells, remembering the numbers four-and-haploid together, and two-and-diploid together, avoids this mix-up.
What students write: Confusing diploid and haploid.
What earns the mark: Diploid (2n) cells have chromosomes in pairs; haploid (n) gametes have one of each pair. This single-letter difference, 2n versus n, is exactly what an exam question is testing.
What students write: Getting the order of mitosis stages wrong.
What earns the mark: The order is prophase, metaphase, anaphase, telophase (PMAT); a simple way to remember it is the mnemonic 'Please Make Another Tea'.
What students write: Saying meiosis happens in every cell of the body.
What earns the mark: Meiosis occurs only in specialised reproductive organs, the testes and ovaries in humans, not in ordinary body cells, which divide only by mitosis. This matters because structured questions often ask you to name the organ where meiosis takes place.
What students write: Describing a tumour and cancer as exactly the same thing.
What earns the mark: A tumour is any abnormal mass formed by uncontrolled cell division; it only becomes cancer if the tumour is malignant and able to invade and spread to other tissues. This distinction matters for questions that separate ordinary cell growth from disease.
What students write: Thinking chromosome number doubles after mitosis.
What earns the mark: Mitosis produces two cells with the same diploid chromosome number as the parent cell; the number does not double or halve. Chromosome number changes only during meiosis, never during mitosis.
What students write: Forgetting that DNA replication happens before mitosis, not during it.
What earns the mark: DNA replicates during interphase, before the mitotic phase begins, so each chromosome entering mitosis already consists of two identical sister chromatids. The correct order is replication during interphase, followed by division during the mitotic phase.
Study this chapter
Frequently asked questions
What is the difference between mitosis and meiosis?
Why must gametes be haploid?
How is cancer related to cell division?
What happens during interphase?
Why does meiosis produce four different cells instead of two identical ones?
Source:SRC-DSKP-EN, SRC-FORMAT
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