Proteins are natural polymers formed from amino acid monomers. Explain how amino acids join together to form a polypeptide, and describe two structural features of DNA that show it is also a polymer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Amino acids join together by condensation polymerisation, forming a peptide bond between the carboxyl group of one amino acid and the amino group of the next. In this reaction a molecule of water is released. Each amino acid has both an amino and a carboxyl group, allowing it to react with two other monomers and form a chain.
DNA is a polymer made from nucleotide monomers. Each nucleotide contains a phosphate group, a pentose sugar and a nitrogenous base. Two such polymer chains run antiparallel and are held together by hydrogen bonds between complementary bases, giving DNA its double‑helix structure.
DNA is a polymer made from nucleotide monomers. Each nucleotide contains a phosphate group, a pentose sugar and a nitrogenous base. Two such polymer chains run antiparallel and are held together by hydrogen bonds between complementary bases, giving DNA its double‑helix structure.
Examiner tips
- Use the term ‘condensation polymerisation’ and mention ‘peptide bond’ and ‘water released’ for the amino acid part.
- State that DNA is made of nucleotides and that it has two strands forming a double helix.
- Show the two structural features (nucleotide monomers and double‑helix).
Common mistakes
- Confusing dehydration synthesis with addition of water; forgetting to mention water loss.
- Describing DNA as a single chain instead of two antiparallel strands.
- Using vague terms like ‘chain’ without specifying nucleotides or the double‑helix structure.
Mark scheme (5 marks)
- Amino acids join together by condensation polymerisation
- Water is lost/released when amino acids join together
- Amino acids have two functional groups (that allow them to form a chain / react with two other monomers)
- DNA is made from nucleotide monomers (nucleotides are the monomers of DNA)
- DNA consists of two polymer chains in the form of a double helix
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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