OCR GCSE Biology A: DNA structure, protein synthesis and enzyme action (J247) – Revision Notes

These notes condense module B1.2 (statements B1.2a to B1.2g) of OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018. For explanations and worked examples, go to the study guide. Test yourself with the practice questions, tick off topics on the printable checklist, find the rest of the course on the OCR GCSE Biology hub, and see Cell level systems revision notes for neighbouring topics. The diagnostics show which parts to start with.

B1.2d and B1.2e are Higher Tier only. Everything else is for both tiers.

Key terms

Term	Meaning
Polymer	Long molecule made of many repeating monomers
Nucleotide	Monomer of DNA: sugar + phosphate + one base
Double helix	Two DNA strands wound around each other
Complementary bases	A with T, G with C
Gene	Section of DNA that codes for one protein
Triplet	Group of three bases that codes for one amino acid
mRNA	Single-stranded copy of a gene; carries the code to the cytoplasm
tRNA	Carries amino acids to the ribosome
Metabolism	All the chemical reactions in a cell or organism
Enzyme	Biological catalyst (a protein)
Active site	The part of the enzyme with a specific shape that the substrate fits into
Denatured	Enzyme’s shape, including the active site, is permanently altered, so it cannot work

DNA in five lines (B1.2a to B1.2c)

• DNA is a polymer made of nucleotides.
• Two strands form a double helix.
• Each nucleotide has the same sugar and phosphate group, plus one of four bases: A, T, G or C.
• Sugar and phosphate form the outside of each strand; bases pair in the middle.
• Pairs: A-T and G-C only.

Method: find percentages of bases

1. Find the partner of the base you are given (same percentage).
2. Add the two known percentages.
3. Subtract from 100.
4. Divide by 2 for the other pair.

Example: 35% thymine gives 35% adenine, 70% in total, 30% left, so 15% each of G and C.

Method: write the complementary strand

Swap every base for its partner: A↔T, G↔C. Example: GGATC pairs with CCTAG.

Protein synthesis (B1.2d) - Higher Tier only

1. DNA unzips around the gene.
2. A copy is made as mRNA in the nucleus (transcription).
3. mRNA moves out of the nucleus into the cytoplasm.
4. The mRNA sequence is translated at a ribosome (translation); tRNA carries the amino acids.
5. Amino acids join in the order set by the mRNA. The chain folds into the protein.

Mnemonic for the stages: copy in the nucleus, read in the cytoplasm.

Method: gene to protein

1. Write the template strand in triplets.
2. Swap each base for its partner to give mRNA triplets (RNA uses U instead of T, so A pairs with U).
3. Look up each triplet in the table the question gives you.
4. Write the amino acids in order.

DNA structure and the protein made (B1.2e) - Higher Tier only

• Sequence of bases in the gene → sequence of triplets → order of amino acids.
• Different amino acid order → different folding → different shape → different function.
• For an enzyme, a changed shape can mean a changed active site, so the substrate no longer fits.
• A base change may alter one amino acid, or none (the new triplet can specify the same amino acid as the old one). A deleted or added base shifts every later triplet.
• Number of bases = number of amino acids × 3 (coding sequence only).

Enzymes (B1.2g)

Idea	Key points
Role	Speed up metabolic reactions; not used up
Active site	Specific shape; substrate binds here
Lock and key	Substrate shape matches the active site, so each enzyme is specific
Denaturation	Shape of active site lost, permanent; caused by high temperature or extreme pH

Effects on rate

Factor	Effect	Reason
Temperature (up to optimum)	Rate increases	More collisions with more energy
Temperature (above optimum)	Rate falls to zero	Denaturation
pH	Rate highest at optimum, falls on either side	Active site shape changes
Substrate concentration	Rises, then levels off	All active sites occupied
Enzyme concentration	Rises in proportion while substrate is plentiful	More active sites

Optimum conditions differ between enzymes. Not every enzyme has an optimum of 37 °C, and not every optimum pH is 7.

Experiments and rates (B1.2f)

• Catalase with hydrogen peroxide: volume of oxygen in a set time.
• Amylase with starch: time for iodine to stop turning blue-black.
• Change one variable; control the others; repeat and take a mean.
• Water bath for temperature; buffer solutions for pH.

Method: calculate a rate

1. Rate = amount of product ÷ time (or amount of substrate used ÷ time).
2. Write the unit as a compound measure: cm³/s, g/min.
3. From a graph, rate = gradient. Use a small interval at the start for the initial rate.
4. If only a time is measured, 1 ÷ time is a measure of rate: a shorter time means a faster reaction.

Percentage change = (new - old) ÷ old × 100.

Worked reminder: 45 cm³ of oxygen in 90 s is 45 ÷ 90 = 0.50 cm³/s. If the rate falls from 0.50 to 0.20 cm³/s, the change is (0.20 - 0.50) ÷ 0.50 × 100 = 60% decrease.

Must-know distinctions

Pair	Difference
Transcription and translation	Transcription copies the gene into mRNA in the nucleus; translation reads the mRNA in the cytoplasm to build the protein
DNA and mRNA	DNA is a double-stranded helix that stays in the nucleus; mRNA is a single-stranded copy that carries the code out
mRNA and tRNA	mRNA carries the code; tRNA carries amino acids
Slowed and denatured	Cold slows an enzyme and the effect can be reversed; heat or extreme pH denatures it permanently
Optimum and limiting factor	The optimum is the best value of a variable; a limiting factor is whatever stops the rate rising further
Substrate and product	Substrate goes in and fits the active site; product comes out

Describing graphs

A good description has three parts: what the line does, a number from the graph to prove it, and a reason.

1. Say what happens, such as “the rate rises then levels off”.
2. Quote data with units, such as “from 0.35 to 0.55 cm³/s between 35 and 45 °C”.
3. Explain using enzyme ideas: collisions, active sites, denaturation.

Second reminder: a pH investigation gives the times for starch to disappear as 120 s at pH 5, 60 s at pH 6 and 40 s at pH 7. Using 1000 ÷ time, the rates are 8.3, 16.7 and 25.0. Doubling the rate from pH 5 to pH 6 shows that pH 5 is further from the optimum. The data alone do not show where the optimum is; testing more values around pH 7 would.

Quick self-test

1. Name the monomer of DNA.
2. Write the strand paired with GGATC.
3. A DNA sample is 35% thymine. State the percentage of cytosine.
4. (Higher Tier only) Template CAT TGC. Write the mRNA.
5. (Higher Tier only) How many bases are needed to code for 50 amino acids?
6. (Higher Tier only) Where does translation take place, and what does tRNA carry?
7. Calculate the rate when 45 cm³ of oxygen forms in 90 s.
8. Starch disappears in 125 s. Calculate 1000 ÷ time as a measure of rate.
9. Explain why no product forms at a very high temperature.
10. Explain why a substrate concentration graph levels off.
11. A rate falls from 0.50 to 0.20 cm³/s. Find the percentage decrease.
12. Give two things to control when testing the effect of pH on amylase.

Answers

1. Nucleotide.
2. CCTAG.
3. 15%.
4. GUA ACG.
6. In the cytoplasm, at a ribosome; tRNA carries amino acids.
7. 0.50 cm³/s.
8. 8.0 (per 1000 s).
9. The enzyme is denatured: its active site has been permanently altered, so the substrate cannot bind.
10. All the active sites are occupied, so extra substrate cannot be processed faster; another factor is limiting.
11. 60% decrease.
12. Any two from: temperature, volume of enzyme, concentration of enzyme, volume and concentration of starch.

Where marks are usually lost

• Writing “protein” or “sugar” when asked what DNA is made of.
• Pairing A with C, or A with G, in a complementary strand.
• Saying enzymes are “killed” or “die” instead of denatured.
• Saying that cold denatures an enzyme.
• Treating the active site as the whole enzyme. It is one region, and its shape matches the substrate.
• Saying a graph “stays the same” without saying why: say the active sites are saturated or the substrate has run out.
• Missing units on a rate, or dividing the wrong way round.
• Not controlling variables in an experiment plan, or changing two at once.
• Mixing up transcription (making mRNA in the nucleus) with translation (making the protein in the cytoplasm).
• Suggesting every enzyme works best at 37 °C or at pH 7.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), for first assessment in 2018, published by OCR: module B1.2.