A biochemist is studying the structure of DNA. She notes that DNA is a natural polymer made from nucleotide monomers joined together by condensation polymerisation. Explain why DNA is described as a condensation polymer, and compare the type of polymerisation used to make DNA with addition polymerisation.

OCR A-Level Chemistry A (H432) — 6.2 Nitrogen compounds, polymers and synthesis · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

DNA is a naturally occurring polymer found in the nuclei of cells. It is formed when nucleotide monomers join together in a condensation polymerisation reaction. Addition polymerisation is another type of polymerisation used to make many synthetic polymers.

Model answer (5 marks)

DNA is a condensation polymer because each nucleotide monomer has two reactive groups (the 5′‑phosphate and the 3′‑hydroxyl) and, when they join, a small molecule – water – is lost.

In condensation polymerisation the reaction proceeds with the elimination of a small molecule (usually H₂O or CH₃OH). The monomers must therefore possess two functional groups that can react.

By contrast, addition polymerisation involves monomers that contain a carbon–carbon double bond (an alkene). The double bond opens and the monomers link together without any small‑molecule by‑product; no water or other small molecules are released.

Thus DNA is formed by condensation polymerisation, whereas many synthetic polymers (e.g. polyethylene) are produced by addition polymerisation.

Examiner tips

  • Use the term "condensation polymer" and mention water loss; state the two functional groups on nucleotides; contrast with addition polymerisation’s lack of by‑product and use of alkenes; keep answer concise and to the point.

Common mistakes

  • Saying DNA is made by addition polymerisation; confusing the type of bond formed; forgetting to mention the loss of water or the two functional groups on nucleotides.

Mark scheme (5 marks)

  1. In condensation polymerisation, monomers join together and lose small molecules
  2. The monomers in condensation polymerisation have two functional groups
  3. In addition polymerisation, monomers join together without losing any small molecules / no by-products are produced
  4. Addition polymerisation uses monomers that contain a carbon–carbon double bond / are unsaturated (alkenes)
  5. DNA can be described as a condensation polymer because the nucleotide monomers join together releasing small molecules / water, forming long polymer chains

Key terms in this question

condensation polymerisation · addition polymerisation · monomers · polymer

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