Explain how differences in the base composition of RNA compared with DNA relate to the distinct functions of each nucleic acid.

IB DP Biology Higher 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 uracil instead of thymine, whereas DNA contains thymine (a methylated base). The lack of a methyl group on uracil means RNA synthesis costs less energy and resources, which is advantageous because RNA molecules are short‑lived and produced in large quantities. The methyl group on thymine allows DNA repair enzymes to distinguish thymine from uracil, so that spontaneous deamination of cytosine to uracil can be detected and corrected, maintaining the fidelity of the genetic code. RNA also contains ribose with a 2′‑OH group, whereas DNA contains deoxyribose. The 2′‑OH makes RNA more reactive and less chemically stable, which suits its temporary role in transcription and translation but means DNA, lacking the 2′‑OH, is better suited to long‑term storage of genetic information.

Examiner tips

  • Mention both uracil vs thymine and ribose vs deoxyribose; link each difference to a functional advantage; keep answer concise to fit 4 marks.

Common mistakes

  • Confusing uracil with thymine; forgetting the 2′‑OH difference; not linking base composition to function.

Mark scheme (4 marks)

  1. RNA contains uracil instead of thymine, whereas DNA contains thymine (a methylated base).
  2. Uracil in RNA is not methylated and therefore costs less energy/fewer resources to synthesise, which is advantageous because RNA molecules are short-lived/produced in large quantities and continually broken down and remade.
  3. The methyl group on thymine allows DNA repair enzymes to distinguish thymine from uracil, so that spontaneous deamination of cytosine to uracil can be detected and corrected, maintaining the fidelity of the genetic code.
  4. RNA also contains ribose (with a 2′-OH group) rather than deoxyribose, which makes RNA more reactive/less chemically stable than DNA; this suits RNA's temporary role in transcription/translation but means DNA, lacking the 2′-OH, is better suited to long-term storage of genetic information.

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