Explain how the structure of RNA differs from that of DNA, and how these structural differences relate to the distinct functions of each molecule.

IB DP Biology Standard Level (2023 syllabus) — A1.2 Nucleic acids · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

RNA contains a ribose sugar, whereas DNA contains a deoxyribose sugar.
RNA contains uracil as a base instead of thymine, which is found in DNA.
DNA is usually double‑stranded, while RNA is usually single‑stranded.
The double‑stranded, stable structure of DNA is suited to long‑term storage of genetic information, whereas the single‑stranded nature of RNA allows it to be transcribed, translated and act as a short‑lived intermediate in gene expression.

Examiner tips

  • Use the exact terms ‘ribose’, ‘deoxyribose’, ‘uracil’, ‘thymine’, ‘double‑stranded’, ‘single‑stranded’.
  • Show the link between structure and function – storage vs. intermediate.
  • Keep the answer concise – four short points matching the marks.

Common mistakes

  • Confusing ribose with deoxyribose or vice versa.
  • Using ‘single‑stranded’ for DNA or ‘double‑stranded’ for RNA.
  • Omitting the functional link between structure and role.

Mark scheme (4 marks)

  1. RNA contains ribose sugar whereas DNA contains deoxyribose sugar
  2. RNA contains uracil as a base instead of thymine (found in DNA)
  3. DNA is (usually) double-stranded whereas RNA is (usually) single-stranded
  4. The double-stranded stable structure of DNA suits long-term storage of genetic information, whereas the single-stranded nature of RNA allows it to be transcribed / translated / act as a short-lived intermediate in gene expression

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