AQA A-Level Chemistry: Amino acids, proteins and DNA (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.3.13 Amino acids, proteins and DNA of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.3.13.1 to 3.3.13.5. The whole section is A-level only; organic chemistry is on Paper 2, and Paper 3 can test any content.

Learn the ideas first in the study guide and the revision notes. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics help you decide what to revise first.

Data. Mr values: glycine 75.0, alanine 89.0, valine 117.0, cysteine 121.1, water 18.0. Valine is H₂N–CH(CH(CH₃)₂)–COOH, alanine is H₂N–CH(CH₃)–COOH, cysteine is H₂N–CH(CH₂SH)–COOH and glycine is H₂N–CH₂–COOH.

Questions

1. Draw the structure of valine as a zwitterion and state what is meant by the term zwitterion. [2]

2. Lysine is H₂N–(CH₂)₄–CH(NH₂)–COOH. Draw the structure of the ion present when lysine is dissolved in an excess of hydrochloric acid. [2]

3. Aspartic acid is HOOC–CH₂–CH(NH₂)–COOH. Draw the structure of the ion present when aspartic acid is dissolved in an excess of aqueous sodium hydroxide. [2]

4. Glycine and valine react to form dipeptides.

(a) Draw the structures of the two dipeptides that contain one glycine and one valine. [2] (b) Calculate the relative molecular mass of one of these dipeptides. [1] (c) State the type of reaction that forms a peptide link. [1]

5. A tripeptide has the structure H₂N–CH(CH₃)–CO–NH–CH(CH₂SH)–CO–NH–CH₂–COOH.

(a) Give a reagent and the conditions used to hydrolyse the tripeptide. [1] (b) Draw the structures of the three species present in the acidic hydrolysis mixture. [3] (c) Calculate the relative molecular mass of the tripeptide. [1] (d) State how many different tripeptides can be made if each of these three amino acids is used once. [1]

6. An amino acid mixture is separated by thin-layer chromatography. The baseline is 1.0 cm above the bottom edge of the plate. After the run, the solvent front is 10.0 cm above the bottom edge. Two spots are found at 3.7 cm and 6.4 cm above the bottom edge. Standards on the same plate gave Rf values of glycine 0.30, alanine 0.45 and valine 0.60.

(a) Amino acids are colourless. Describe one way to locate them on the plate. [2] (b) Calculate the Rf value of each spot. [2] (c) Identify the two amino acids in the mixture. [1] (d) Explain why the baseline is drawn in pencil and not in ink. [1]

7. This question is about protein structure.

(a) State what is meant by the primary structure of a protein. [1] (b) Explain how the α-helix structure of a protein is held in shape. [3] (c) A protein chain contains two cysteine units far apart in its sequence. Explain how these can help to maintain the tertiary structure of the protein. [2]

8. This question is about enzymes and drug action.

(a) A drug molecule has one chiral centre. Explain why only one enantiomer of the drug can bind to the active site of an enzyme. [3] (b) Describe how a drug can act as an enzyme inhibitor, and state how computers help in designing such drugs. [2]

9. This question is about DNA.

(a) Describe the structure of a nucleotide. [2] (b) Describe how nucleotides are linked to form a single strand of DNA. [2] (c) A section of one strand has the base sequence GCCATGCG. Give the base sequence of the complementary strand and calculate the number of hydrogen bonds between the two strands in this section. [2] (d) A sample of double-stranded DNA contains 22% adenine. Deduce the percentage of guanine. [1]

10. Cisplatin, cis-[Pt(NH₃)₂Cl₂], is used as an anticancer drug.

(a) Explain how cisplatin prevents DNA replication in cancer cells. [3] (b) Explain why cisplatin can have adverse effects. [2] (c) State why society must assess drugs such as cisplatin before they are used. [1]

11. A dipeptide Y has Mr = 188.0. Y is hydrolysed and the products are separated by thin-layer chromatography. The solvent front is 8.0 cm from the baseline, and two spots are at 2.4 cm and 5.6 cm from the baseline. Standards on the same plate gave Rf values of alanine 0.30, cysteine 0.55, glycine 0.40 and valine 0.70.

(a) Calculate the Rf value of each spot. [2] (b) Identify the two amino acids. [1] (c) Show that your answer to (b) is consistent with the Mr of Y. [1] (d) Draw the two possible structures of Y. [2] (e) Explain why the chromatogram alone cannot tell you which structure Y has. [1] (f) Draw the structure of the ion formed by the amino acid with the higher Rf value in alkaline solution. [1]

Answers

1. H₃N⁺–CH(CH(CH₃)₂)–COO⁻ [1]. A zwitterion is an ion with both a positive and a negative charge but no overall charge [1]. [2] Examiner insight: A zwitterion drawn without the + and − charges shown is unlikely to earn the structure mark.

2. Both amine groups are protonated: H₃N⁺–(CH₂)₄–CH(NH₃⁺)–COOH [1]. The –COOH group is unchanged, and the ion has an overall charge of +2 [1]. [2] Examiner insight: The side-chain –NH₂ is basic too; protonating only the α-amine group gives the wrong ion.

3. Both acid groups lose H⁺: ⁻OOC–CH₂–CH(NH₂)–COO⁻ [1]. The –NH₂ group is unchanged, and the ion has an overall charge of −2 [1]. [2] Examiner insight: In alkali, an –NH₃⁺ group is not possible; leave every amine as –NH₂.

4. (a) H₂N–CH₂–CO–NH–CH(CH(CH₃)₂)–COOH [1] and H₂N–CH(CH(CH₃)₂)–CO–NH–CH₂–COOH [1]. (b) 75.0 + 117.0 − 18.0 = 174.0 [1]. (c) Condensation [1]. [4] Examiner insight: Each dipeptide needs a full –CO–NH– link; a link drawn as –CO–O– or –C–N– with the C=O missing is a different compound.

5. (a) Heat under reflux with concentrated hydrochloric acid [1]. (b) H₃N⁺–CH(CH₃)–COOH [1]; H₃N⁺–CH(CH₂SH)–COOH [1]; H₃N⁺–CH₂–COOH [1]. (c) 89.0 + 121.1 + 75.0 − (2 × 18.0) = 249.1 [1]. (d) 3 × 2 × 1 = 6 [1]. [6] Examiner insight: “Present in the acidic mixture” means protonated amine groups; neutral amino acids answer a different question.

6. (a) Spray the plate with ninhydrin and warm it [1]; the amino acids show as purple spots [1]. (Alternative: use a plate that fluoresces and shine ultraviolet light on it; the spots appear dark.) (b) Solvent distance = 10.0 − 1.0 = 9.0 cm. Spot 1: (3.7 − 1.0) ÷ 9.0 = 0.30 [1]. Spot 2: (6.4 − 1.0) ÷ 9.0 = 0.60 [1]. (c) Glycine and valine [1]. (d) Ink would dissolve in the solvent and move up the plate; pencil does not [1]. [6] Examiner insight: Distances must be measured from the baseline; using the bottom edge gives 0.37 and 0.64 and the wrong identification.

7. (a) The sequence of amino acids in the protein chain [1]. (b) Hydrogen bonds hold the coil in shape [1]. They form between the C=O of one peptide link and the N–H of another [1], on different turns of the helix, where a lone pair on the δ− O attracts the δ+ H [1]. (c) The –SH groups of the two cysteine side chains form a covalent S–S bond [1]. This joins two distant parts of the chain and fixes the folded 3D shape [1]. [6] Examiner insight: “Held by hydrogen bonds” scores little unless you say which groups take part.

8. (a) The active site is stereospecific: its binding groups are in fixed 3D positions [1]. One enantiomer has the groups at its chiral centre lined up with the binding sites [1]. The other has them arranged differently in space, so it cannot make the same interactions [1]. (b) The drug binds to the active site and blocks it, so the substrate cannot bind [1]. Computers model the shape of the active site and test how well possible drug molecules fit [1]. [5] Examiner insight: “The other enantiomer is the wrong shape” is too vague; tie the answer to the 3D arrangement of groups and the binding sites.

9. (a) A phosphate ion bonded to 2-deoxyribose [1], which is in turn bonded to one of the bases adenine, cytosine, guanine or thymine [1]. (b) Covalent bonds form between the phosphate group of one nucleotide and the 2-deoxyribose of the next [1], giving a sugar–phosphate–sugar–phosphate chain with the bases attached to the sugars [1]. (c) CGGTACGC [1]. There are six G–C pairs and two A–T pairs: (6 × 3) + (2 × 2) = 22 [1]. (d) T = 22%, so G + C = 100 − 44 = 56% and G = 28% [1]. [7] Examiner insight: Linking the base to the phosphate, instead of to the sugar, is a common slip in (a).

10. (a) Cisplatin takes part in a ligand replacement reaction: a Cl⁻ ligand is replaced [1]. A bond forms between platinum and a nitrogen atom on guanine in DNA [1]. With both cis Cl⁻ ligands replaced, Pt binds to two guanines and distorts the DNA, so the strands cannot be copied and the cell cannot divide [1]. (b) It also binds to DNA in healthy cells and stops them dividing [1], and it reaches healthy tissues, causing side effects such as kidney damage or nausea [1]. (c) Its benefits in treating cancer must be balanced against its adverse effects [1]. [6] Examiner insight: Name the atom and the base: “Pt bonds to DNA” without “nitrogen on guanine” is incomplete.

11. (a) 2.4 ÷ 8.0 = 0.30 [1]; 5.6 ÷ 8.0 = 0.70 [1]. (b) Alanine and valine [1]. (c) 89.0 + 117.0 − 18.0 = 188.0, which matches the Mr of Y [1]. (d) H₂N–CH(CH₃)–CO–NH–CH(CH(CH₃)₂)–COOH [1] and H₂N–CH(CH(CH₃)₂)–CO–NH–CH(CH₃)–COOH [1]. (e) Both structures give the same two amino acids on hydrolysis, so the order is not shown [1]. (f) Valine: H₂N–CH(CH(CH₃)₂)–COO⁻ [1]. [8] Examiner insight: In a “show that”, write the full sum with the water subtracted; quoting 188.0 alone shows no working.

Where marks are usually lost

• Ignoring the extra –NH₂ in lysine or the extra –COOH in aspartic acid.
• Forgetting to subtract water once per peptide link when calculating Mr.
• Measuring TLC distances from the bottom edge rather than from the baseline.
• Saying “hydrogen bonds” in protein structure without naming C=O and N–H.
• Treating S–S bonds as intermolecular forces rather than covalent bonds.
• Writing the complementary strand by reversing the sequence instead of pairing A–T and G–C.
• Giving vague adverse effects for cisplatin without saying it also affects healthy cells.

Next steps

• Amino acids, proteins and DNA revision notes
• Amino acids, proteins and DNA study guide
• AQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.13 Amino acids, proteins and DNA (3.3.13.1 to 3.3.13.5), A-level only.