OxfordAQA International AS and A-level Biology: Cell division, mutation and cancer (9610) – Revision Notes

These notes condense sections 3.2.10 and 3.2.11 of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1 (International AS from May/June 2017, International A-level from May/June 2018), together with Required practical 4. Both sections sit in Unit 2, which the specification lists under International AS and A-level. For full explanations read the study guide, then check recall with the practice questions. Course pages: hub, checklist and free diagnostics.

3.2.10.1 Cell cycle at a glance

Phase	What happens
G1	Cell grows; protein content rises; more organelles (mitochondria, ribosomes)
S	DNA replicated
G2	Rapid growth and protein synthesis
Mitosis	Nucleus divides; two daughter cells with identical DNA

G1, S and G2 together make up interphase, the period between mitotic divisions.

Worked reminder: time in each phase. 360 cells are classified from a culture with an 18 hour cycle: 162 in G1, 99 in S, 63 in G2, 36 in mitosis. A phase’s share of the cells matches its share of the cycle time.

• S: 99 / 360 = 27.5%, so 0.275 × 18 = 4.95 hours (about 5.0 hours)
• Mitosis: 36 / 360 = 10.0%, so 1.8 hours
• G1 is 8.1 hours and G2 is 3.15 hours; all four sum to 18.0 hours

3.2.10.2 Mitosis: one-line summaries

Stage	One line to remember
Prophase	Chromosomes condense and become visible; each is two chromatids joined at a centromere
Metaphase	Chromosomes line up at the equator, centromeres attached to spindle fibres
Anaphase	Centromeres divide; fibres shorten and pull chromatids to opposite poles
Telophase	Chromosomes at poles; nuclear envelopes reform; cell divides

Method: recognise a stage from a photograph.

1. Are chromosomes visible as separate structures? If not, it is interphase.
2. If visible but scattered, it is prophase.
3. If in a single line across the middle, it is metaphase.
4. If in two groups moving apart, it is anaphase.
5. If in two groups at the ends with nuclei forming, it is telophase.

Counting reminder. A cell with 2n = 14: 14 chromosomes in G1; 14 chromosomes (28 chromatids) after S phase; 28 chromosomes moving in anaphase; 14 in each daughter. If a G1 nucleus has 6.0 pg DNA, it has 12.0 pg after S phase and 6.0 pg in each daughter cell.

3.2.10.3 Binary fission and conjugation

Feature	Binary fission	Mitosis
Cell type	Prokaryote	Eukaryote
DNA copied	Circular DNA and plasmids	Linear chromosomes
Spindle	None	Yes
Daughter cells	Two; one copy of circular DNA each, plasmid number may vary	Two with identical DNA

Conjugation: DNA (a plasmid copy) passes from one bacterium to another, which may be a different species. This is horizontal gene transmission. Vertical transmission is from parent to daughter cell.

Worked reminder. 15 cells divide every 40 minutes for 4 hours. Divisions = 240 / 40 = 6. Cells = 15 × 2⁶ = 15 × 64 = 960.

3.2.11.1 Gene mutations

• Gene mutation: any alteration to the order of bases in DNA.
• Mutations arise spontaneously during DNA replication.
• Mutagens are physical or chemical agents that increase the frequency of mutations.
• Examples of mutation: base substitution and base deletion.
• Degenerate code: more than one triplet can code for the same amino acid, so some substitutions do not change the amino acid sequence.

Method: predict the effect on the amino acid sequence.

1. Write the original mRNA codons (if given DNA, build the complementary mRNA).
2. Apply the change to the correct base.
3. Regroup in threes from the start codon.
4. Read each codon from the code table supplied.
5. Compare with the original sequence and state what has changed.

Worked reminder. Original mRNA: AUG UCC AAG GGU CAU UGA, giving Met - Ser - Lys - Gly - His - stop.

Change	New codons	Result
Substitution, UCC to UCA	AUG UCA AAG GGU CAU UGA	Ser still; no change to sequence
Substitution, AAG to AAU	AUG UCC AAU GGU CAU UGA	Lys replaced by Asn; one change
Deletion of the fourth base (U)	AUG CCA AGG GUC AUU GA	Met - Pro - Arg - Val - Ile; everything after Met altered

A deletion changes the reading frame, so it generally has a larger effect than a substitution. Substitutions can have a large effect too, for example when they create a stop codon.

3.2.11.2 Cancers

	Benign	Malignant
Growth	Slower	Faster
Edges	Often encapsulated	Invasive, irregular
Spread	Stays in place	Spreads, forming secondary tumours
Cells	Similar to normal	Often abnormal in shape and nucleus

Genes controlling division

Gene type	Normal role	After mutation
Proto-oncogene	Stimulates cell division	Becomes an oncogene: division stimulated too much
Tumour suppressor gene	Slows cell division	Inactivated: slowing lost, rate of division increases

Method: evaluate risk-factor data.

1. Quote figures from the table, including units (cases per 100 000 per year).
2. Compare as a ratio or difference between groups.
3. Describe the trend (rises with exposure, or no pattern).
4. Say whether it shows correlation only, and what else could explain it.
5. Comment on sample size, matching of groups and any known mechanism.

Worked reminder. Illustrative figures for oral cancer, cases per 100 000 per year, by daily alcohol intake: none 3.1; 1 to 2 units 4.0; 3 to 4 units 7.9; 5 or more units 16.5. Ratio for 5 or more units against none: 16.5 / 3.1 = 5.3, so over five times. The increase is steady, which suggests a link. It does not prove cause, because heavy drinkers may differ in other ways, such as smoking.

Drugs and the cell cycle.

Drug action	Cell cycle link
Blocks DNA replication	Acts in S phase; cell cannot complete the cycle
Prevents spindle formation	Acts in mitosis; chromatids cannot be separated

Side effects arise because healthy cells that divide often, such as hair follicle and gut lining cells, are also affected.

Required practical 4: root tip squash

Step	Reason
Cut the tip and warm it in hydrochloric acid	Softens tissue so cells separate
Add stain (such as acetic orcein)	Makes chromosomes visible
Squash under a coverslip	One thin layer of cells
Count cells in each stage	Gives data for the mitotic index

Mitotic index: cells in mitosis divided by all cells counted (× 100 for a percentage).

Worked reminder. Untreated tip: 48 cells in mitosis out of 380. Index = 48 / 380 = 12.6%. Treated tip: 19 out of 360 = 5.3%. The treated tip has a lower index, so fewer cells were dividing. A reliable comparison needs several fields of view for each tip, with the same total counted.

Model answer shapes

Explain how spindle fibres separate chromatids. Spindle fibres are attached to the centromeres. At anaphase the centromeres divide and the fibres shorten. This pulls one chromatid of each pair to each pole, so each daughter nucleus receives one copy of every chromosome.

Explain how a mutation in a tumour suppressor gene can lead to a tumour. The mutation inactivates the gene, so it no longer slows cell division. The rate of division increases and cells divide in an uncontrolled way, forming a mass of cells.

Explain why a mutagen increases cancer risk. A mutagen raises the frequency of mutation. A mutation in a proto-oncogene or a tumour suppressor gene is then more likely, and that can cause the rate of division to rise.

Suggest why a root tip, not a mature part of the root, is used. The cells just behind the tip are dividing, so more cells are in mitosis and a count is possible.

Must-know distinctions

• Chromatid or chromosome: after S phase the chromosome has two chromatids; after anaphase each chromatid is a chromosome.
• Proto-oncogene or oncogene: the first is the normal gene, the second the mutated form.
• Deletion or substitution: deletion shifts the frame; substitution changes one codon.
• Horizontal or vertical: horizontal moves genes between cells; vertical passes them to daughters.

Quick self-test

1. Name the phase in which DNA is replicated.
2. State what increases in number in G1 besides protein.
3. A cell has 2n = 10. How many chromatids are present at metaphase?
4. Which structure do spindle fibres attach to?
5. In which stage do chromatids separate?
6. State what happens to plasmid number in binary fission.
7. Name the process that passes a plasmid between bacteria.
8. A substitution changes CUU to CUC (both Leu). Give the effect on the protein.
9. Give the mitotic index as a percentage: 24 cells in mitosis out of 200.
10. Name the mutated form of a proto-oncogene.

Answers

1. S phase.
2. Organelles, such as mitochondria and ribosomes.
3. 20 chromatids (10 chromosomes, each with two).
4. The centromere.
5. Anaphase.
6. It may vary between the two daughter cells.
7. Conjugation.
8. No change, because the code is degenerate.
9. 24 / 200 × 100 = 12.0%.
10. An oncogene.

Where marks are usually lost

• Writing that DNA replicates during prophase instead of S phase.
• Using “chromosome” and “chromatid” as if they were the same thing.
• Describing anaphase without mentioning the shortening spindle fibres and the centromere.
• Giving only “the code is degenerate” without linking it to the same amino acid being coded.
• Not stating that a deletion alters the reading frame of all later codons.
• Calling every tumour cancerous, instead of separating benign from malignant.
• Making a claim of cause from risk-factor data that show correlation only.
• Dividing by the dividing cells only when calculating the mitotic index.

Official syllabus

Source: OxfordAQA, International AS and A-level Biology (9610), Version 5.1, International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, sections 3.2.10, 3.2.11 and Required practical 4.