OCR A Level Biology A: Gene mutations and control of gene expression (H420) – Revision Notes

These are condensed notes for OCR A Level Biology A (H420), written to the OCR specification Version 4.1 (April 2026), for first assessment in 2025. They cover module 6.1.1 Cellular control, learning outcomes (a) to (d). The full explanations and worked examples are in the study guide. When you are ready, try the practice set. Course pages: the OCR A Level Biology hub and the printable checklist. The free 10-minute diagnostics show which topics to prioritise. Codon background: the genetic code, transcription and translation.

Specification map

Outcome	One-line summary
6.1.1(a)	Substitution, insertion, deletion; effects beneficial, neutral or harmful
6.1.1(b)	Control at transcription (lac operon, transcription factors), after transcription (primary mRNA editing, intron removal) and after translation (cAMP activation)
6.1.1(c)	Homeobox sequences conserved in plants, animals and fungi; Hox genes and body plans
6.1.1(d)	Mitosis and apoptosis shape body form; cell cycle and apoptosis genes respond to internal and external stimuli

6.1.1(a) Mutations

Definition. A gene mutation is a permanent alteration to the order of nucleotides in a gene.

Mutation	Frame kept?	Typical effect
Substitution	Yes	Silent, missense (one amino acid changed) or nonsense (stop codon)
Insertion of 1 or 2 nucleotides	No	Frameshift: all later amino acids can change
Deletion of 1 or 2 nucleotides	No	Frameshift
Insertion or deletion of 3, 6, 9… nucleotides	Yes	Amino acids added or lost at that site only

Method: predicting the effect

1. Write the mRNA as triplets from the start codon.
2. Make the change, then re-split into triplets from the same start.
3. Translate both and compare.
4. Say which kind of change it is, then say what it does to the protein’s shape and function.

Must-know distinctions

• Silent versus missense. The code is degenerate, so an altered codon can still specify the same amino acid.
• Missense versus nonsense. Missense swaps one amino acid. Nonsense makes a stop codon and a shorter polypeptide.
• Frameshift versus in-frame. Only the number of nucleotides added or lost (multiple of three or not) decides this.
• Beneficial, neutral, harmful. The same allele can be harmful in one environment and beneficial in another (sickle cell allele and malaria).

Small worked reminder

mRNA: AUG CAA UUC GGC UGA gives Met-Gln-Phe-Gly, then stop.

• CAA becomes CAG: both code for Gln, so silent.
• CAA becomes UAA: stop codon, so Met only (nonsense).
• UUC becomes UAC: Phe becomes Tyr (missense).
• First U of UUC deleted: AUG CAA UCG GCU GA, so Met-Gln-Ser-Ala (frameshift, and the stop is lost).
• GGAUCC (six nucleotides) inserted after CAA: Met-Gln-Gly-Ser-Phe-Gly, then stop (in frame; two amino acids added).

6.1.1(b) Control of gene expression

Transcriptional level

Feature of the lac operon	Role
lacI (regulatory gene)	Codes for the repressor protein
Promoter	Binding site for RNA polymerase
Operator	Binding site for the repressor, next to the structural genes
lacZ	Beta-galactosidase: hydrolyses lactose
lacY	Permease: lactose uptake

Lactose absent	Lactose present
Repressor binds the operator	Lactose binds the repressor and changes its shape
RNA polymerase blocked	Repressor leaves the operator
No transcription of lacZ and lacY	RNA polymerase transcribes lacZ and lacY

Reading mutant data. Constitutive (always on) means a fault that stops the repressor working or binding: a faulty repressor gene, or an operator that the repressor cannot bind. A faulty promoter means always off, because RNA polymerase cannot bind.

Eukaryotic transcription factors. Proteins that bind to specific DNA sequences near genes. Activators help RNA polymerase to bind and start transcription. Repressors prevent it. A cell type is defined partly by which factors are active. You need the general role only, not named factors.

Post-transcriptional level

Sequence: transcription gives primary mRNA (exons and introns), introns are removed and exons are joined, giving mature mRNA which leaves the nucleus. Capping and a poly-A tail are also part of editing. Different exon combinations can give different polypeptides from one gene.

Number to remember	Meaning
Nucleotides per codon	3
Amino acids	Coding nucleotides divided by 3, minus one for the stop codon

Worked reminder. A primary mRNA of 1950 nucleotides contains introns totalling 1320. Exons: 1950 − 1320 = 630 nucleotides. Introns are 1320 / 1950 × 100 = 67.7%. Codons: 630 / 3 = 210, so 209 amino acids if one is the stop codon.

Post-translational level

Hormone binds a surface receptor, so the cell makes cAMP. cAMP binds an inactive protein (such as protein kinase A) and changes its shape, so it becomes active. The active enzyme can phosphorylate other proteins. This acts on proteins that already exist, so it needs no new transcription or translation.

6.1.1(c) Body plans

• Homeobox: a conserved sequence of about 180 base pairs, coding for a DNA-binding domain of about 60 amino acids (180 / 3 = 60). Found in plants, animals and fungi. The proteins act as transcription factors.
• Hox genes: a subset of homeobox genes in animals. They sit in clusters. Their order along the chromosome matches the head-to-tail order in which they act.
• Mutation: a change in a Hox gene can change what forms in a body region (legs in place of antennae in a fly).
• Why conserved: a change in a gene that controls the whole layout is likely to be harmful, so natural selection removes it.

Homeobox gene	Hox gene
Any gene containing the sequence	Subset of homeobox genes
Plants, animals and fungi	Animals
Regulates gene expression	Controls body plan along the head-to-tail axis

6.1.1(d) Mitosis and apoptosis

Process	Role in development	Key points
Mitosis	Increases cell number from the zygote; growth; replacement	Daughter cells genetically identical
Apoptosis	Programmed cell death that removes cells that are not needed or are damaged; shapes the body (separates digits, loses a tadpole tail)	Caspase enzymes; cell breaks into membrane-bound fragments; phagocytes engulf them; no inflammation

Balance. Body form depends on cell division and cell death being balanced in the right place at the right time.

Control. The genes that regulate the cell cycle and apoptosis respond to internal stimuli (such as DNA damage) and external stimuli (such as growth factors and death signals from other cells).

Worked reminder. In C. elegans, 1090 somatic cells are made and 131 undergo apoptosis: 959 remain, and 131 / 1090 × 100 = 12.0% die. Mitotic index = cells in mitosis / total cells × 100. If 18 of 240 cells are dividing, index = 7.5%.

Quick self-test

1. State the three types of gene mutation named in the specification.
2. A codon CAA changes to CAG, and both code for Gln. Name this kind of change and say what happens to the polypeptide.
3. Six nucleotides are deleted from the middle of a coding sequence. Does a frameshift occur? Explain.
4. A single nucleotide is inserted near the start of a gene. Why is the effect usually greater than a substitution at the same place?
5. In the lac operon, what does the lacI gene code for?
6. Explain why lactose in the medium leads to beta-galactosidase being made.
7. A strain makes beta-galactosidase when no lactose is present. Suggest two possible causes.
8. State what happens to introns during mRNA processing.
9. Explain how cAMP can activate an enzyme.
10. A homeobox is 180 base pairs. How many amino acids are in the domain it codes for?
11. Give two roles of apoptosis in development.
12. A tissue sample shows 18 cells in mitosis out of 240. Calculate the mitotic index.

Answers

1. Substitution, insertion, deletion.
2. A silent substitution. The same amino acid is coded for, so the polypeptide is unchanged.
3. No. Six is a multiple of three, so the reading frame is kept and two amino acids are lost.
4. It shifts the frame, so every codon after the insertion is read differently and the polypeptide can differ from there on, or end early.
5. The repressor protein.
6. Lactose binds the repressor and changes its shape, so the repressor cannot bind the operator. RNA polymerase can transcribe lacZ, so the enzyme is made.
7. A faulty repressor (from a mutation in lacI) or an operator that the repressor cannot bind.
8. They are removed from the primary mRNA and the exons are joined.
9. cAMP binds to the inactive enzyme and changes its shape, so it becomes active.
10. 180 / 3 = 60 amino acids.
11. Any two of: separating the digits; removing a tadpole’s tail; removing cells that are damaged or not needed.
12. 18 / 240 × 100 = 7.5%.

Where marks are usually lost

• Saying “the gene changes shape” or “the DNA is the enzyme”. Name the molecule: the repressor, the polypeptide, the active site.
• Writing that lactose “turns off” the repressor gene. Say that lactose binds the repressor protein and changes its shape.
• Missing the word frameshift, or forgetting that a deletion of three nucleotides keeps the frame.
• Stating a mutation is harmful without saying how the protein’s structure or function changes.
• Confusing the operator with the regulatory gene, or the promoter with the operator.
• Saying that introns are removed from DNA instead of from the primary mRNA.
• Describing apoptosis as cells being damaged, instead of programmed, controlled death.
• Giving a homeobox sequence and a Hox gene as the same thing.
• Not giving units or working in a percentage question: write the formula, then the answer.

Official syllabus

Source: OCR, A Level Biology A (H420), specification Version 4.1 (April 2026), first assessment 2025, module 6.1.1 Cellular control, learning outcomes (a) to (d).