A-Level Chemistry: Amino Acids, Proteins and DNA

The molecules of life appear near the end of the A-level Chemistry course, where organic chemistry meets biology and medicine. The questions set on them are chemical rather than biological: you are expected to write the ions an amino acid forms, draw peptides from their amino acids and back again, name the bonds that hold a protein in shape, calculate Rf values and explain, in terms of bonding, how an anticancer drug attacks DNA. This material trains exactly those skills.

It begins with amino acids as molecules that are both acidic and basic. You write the zwitterion of glycine, explain why zwitterions give amino acids high melting points and solubility in water, and give the ions formed in strongly acidic and strongly alkaline solution. You then join amino acids by condensation to form peptide links, recognising that glycine and alanine can combine in two different orders, and reverse the process by acid hydrolysis, which releases the amino acids as their protonated forms.

Next comes protein structure: the primary structure as the sequence of amino acids, the secondary structure of α-helices and β-pleated sheets held by hydrogen bonds, and the tertiary structure maintained by hydrogen bonds, ionic interactions and sulfur–sulfur bonds. Thin-layer chromatography is used to separate the amino acids from a hydrolysed protein, to make them visible with ninhydrin or ultraviolet light and to identify them from their Rf values. Enzymes are introduced as protein catalysts with a stereospecific active site that accepts only one enantiomer, and as targets for drugs that work by blocking that site.

The last part deals with DNA and cisplatin. You describe a nucleotide (phosphate, 2-deoxyribose and one of four bases), the sugar–phosphate backbone of a strand and the hydrogen bonds between complementary base pairs, A with T and C with G, that hold the double helix together. You then explain how the square planar platinum(II) complex cisplatin binds to guanine by ligand replacement and stops DNA replication, why the trans isomer cannot do the same, and why the drug has adverse effects that must be weighed against its benefits.

Four formats are offered. The 13-question quiz uses condensed formulas throughout; the chromatography question shows the developed plate, as on the exam. The flashcards cover the key terms and structures. The written work has eight questions to answer by hand, from the ions of serine to the mode of action of cisplatin, each with a reference answer. The oral exam lets you explain these ideas to an examiner, one question at a time.

The content is based on the A-level chemistry content on amino acids, proteins, enzymes, DNA and anticancer drugs, as set out for example in the AQA A-level specification. It is revision practice written by Zestly, not an exam board resource and not medical advice.

• Write the zwitterion of an amino acid and the ions it forms in acidic and alkaline solution
• Show how peptide links form and how acid hydrolysis breaks a peptide into its amino acids
• Describe primary, secondary and tertiary protein structure and the bonds that maintain them
• Separate and identify amino acids by thin-layer chromatography using Rf values
• Explain the stereospecific action of enzymes and how an inhibitor drug works
• Describe DNA structure and base pairing, and explain how cisplatin prevents DNA replication and why it has adverse effects

Practice material written by Zestly, based on the A-level chemistry content on amino acids, proteins and DNA (for example AQA A-level Chemistry 7405, section 3.3.13, including the action of anticancer drugs).