Form 4 · Worked answers

Cell Division, worked answers

Fully worked answers for Cell Division, original structured and essay questions with mark-scheme keywords highlighted.

Book a Trial Class

One-hour paid trial · Same-day reply

How this topic is examined

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.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

A human body cell contains 46 chromosomes. (a) State the chromosome number of a human gamete. (b) Explain the terms diploid and haploid using these numbers.

[4]

Show answer

(a) A human gamete contains 23 chromosomes. (b) Diploid means a cell has its chromosomes in homologous pairs, one of each pair from each parent, so a human body cell is diploid with 23 pairs, which is 46 chromosomes. Haploid means a cell has only one chromosome of each pair, so a human gamete is haploid with 23 single chromosomes. The gamete therefore has half the number found in a body cell.

diploidhaploidhomologous pair2346

Describe what happens during interphase of the cell cycle.

[3]

Show answer

During interphase the cell grows larger and carries out its normal activities. It replicates its DNA so that each chromosome is copied into two identical sister chromatids joined at a centromere. Interphase is the longest stage of the cell cycle, and no visible division of the nucleus takes place during it; the cell is preparing for the division that follows.

interphasegrowsDNA replicationsister chromatidslongest

Name the stage of mitosis in which (a) the chromosomes line up at the equator of the cell and (b) the sister chromatids are pulled to opposite poles. Describe what is happening to the chromosomes in each.

[4]

Show answer

(a) The chromosomes line up at the equator during metaphase, where each is attached to spindle fibres. (b) The sister chromatids are pulled to opposite poles during anaphase, so that each pole of the cell receives one complete set of chromosomes. In metaphase the duplicated chromosomes are arranged in a single plane across the middle of the cell, while in anaphase the centromeres separate and the chromatids move apart.

metaphaseequatoranaphasesister chromatidspoles

Explain why gametes must be haploid.

[4]

Show answer

A gamete must be haploid so that, when two gametes fuse at fertilisation, the diploid number is restored rather than doubled. If gametes were diploid, fertilisation would combine two diploid sets and the chromosome number would double every generation. Because meiosis halves the number to produce haploid gametes, the zygote returns to the normal diploid number, keeping the chromosome number of the species constant across generations.

haploidfertilisationdiploiddoubledconstant

Compare mitosis and meiosis by stating, for each, the number of daughter cells produced, the chromosome number of those cells, and whether they are genetically identical or different.

[6]

Show answer

Mitosis produces two daughter cells, each diploid with the same chromosome number as the parent cell, and both are genetically identical to the parent and to each other. Meiosis produces four daughter cells, each haploid with half the chromosome number of the parent cell, and each is genetically different from the parent and from the others. Mitosis is used for growth and repair, while meiosis produces gametes; naming the number, the ploidy and the genetic outcome for each division is what a comparison question credits.

twofourdiploidhaploididenticaldifferent

Using the idea of the cell cycle, explain how a tumour forms and why a malignant tumour is dangerous.

[5]

Show answer

The cell cycle is normally controlled so that cells divide only when new cells are needed. When this control fails, cells divide continuously and without stopping, and the extra cells build up into an abnormal mass called a tumour. A malignant tumour is dangerous because it can invade the surrounding tissue and spread through the blood or lymph to other parts of the body, forming secondary growths that disrupt the organs they reach. The harm therefore comes from cells dividing when they should not.

control failscontinuouslytumourmalignantinvadespread

State two differences between a benign tumour and a malignant tumour.

[3]

Show answer

A benign tumour stays in one place and does not spread to other parts of the body, whereas a malignant tumour can spread through the blood or lymph to form secondary growths elsewhere. A benign tumour is usually enclosed and grows slowly, while a malignant tumour invades the surrounding tissue. It is the malignant tumour that is classified as cancer.

benigndoes not spreadmalignantinvadecancer

Phrasing that earns marks

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.

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.