Form 4 · Paper 2 essay guide

Cell Division, Paper 2 essay guide

How Cell Division appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Describing the cell cycle and the stages of mitosis: Define the cell cycle as interphase followed by the mitotic phase. → Describe interphase: growth and DNA replication into sister chromatids. → State the four stages of mitosis in order, PMAT. → Describe what happens to the chromosomes in each stage. → Explain how cytokinesis differs between animal and plant cells.
Comparing mitosis and meiosis and explaining the importance of meiosis: State the number of divisions and daughter cells for each process. → Compare the ploidy and genetic make-up of the daughter cells. → State where in the organism each process occurs and its purpose. → Explain how meiosis keeps the chromosome number constant across generations. → Explain how meiosis produces genetic variation.
Explaining chromosomes, the diploid and haploid number, and fertilisation: Define a chromosome and explain that it carries genes made of DNA. → Explain diploid (2n) and haploid (n) using the human numbers 46 and 23. → Explain why body cells are diploid and gametes are haploid. → Describe how fertilisation restores the diploid number in the zygote. → Explain why this keeps the chromosome number of the species constant.
Explaining how loss of control of the cell cycle leads to tumours and cancer: State that the cell cycle is normally controlled so cells divide only when needed. → Explain that uncontrolled division produces a mass of cells called a tumour. → Distinguish a benign tumour from a malignant one. → Explain why a malignant tumour is dangerous, using the idea of invasion and spread. → Note factors that raise the risk of this loss of control.

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Introduction	State what the question asks and define the key term it uses, such as 'mitosis', 'diploid' or 'tumour', in one sentence.
Point 1	Name the structures or stages involved, such as the chromosomes, the stages of division, or the daughter cells, and state what each is.
Point 2	Develop the description or comparison the question asks for, such as the order of the stages or the differences between mitosis and meiosis.
Point 3	Support the answer with the correct numbers or a named example, such as 46 and 23 chromosomes or a malignant tumour, explained fully.
Conclusion	Draw the ideas together, linking the process to its purpose, such as growth, the production of gametes or the constant chromosome number.

Key terms to include

chromosome
gene
DNA
diploid
haploid
homologous pair
sister chromatid
interphase
DNA replication
prophase
metaphase
anaphase
telophase
cytokinesis
mitosis
meiosis
gamete
fertilisation
genetic variation
tumour
malignant

A model essay outline

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.

Frequently asked questions

What is the difference between mitosis and meiosis?

Mitosis produces two genetically identical diploid cells and is used for growth and repair. Meiosis produces four genetically different haploid cells (gametes) and halves the chromosome number, which keeps the species number constant at fertilisation and adds variation. Both processes begin from the same parent cell, but they are used for entirely different biological purposes.

Why must gametes be haploid?

If gametes were diploid, fertilisation would double the chromosome number every generation. Because meiosis makes haploid gametes, the diploid number is restored, not doubled, when the sperm and egg fuse, keeping it constant across generations. This restoration of the diploid number at fertilisation is the whole reason meiosis exists.

How is cancer related to cell division?

Cancer arises when the normal controls on the cell cycle fail and cells divide continuously without stopping. This uncontrolled mitosis forms a tumour, and a malignant tumour can spread to other tissues. Not every tumour is cancerous, only a malignant one that invades and spreads is classified as cancer.