A-Level Biology May/June 2024 Q6(a): Describe three ways in which the structure of messenger RNA (mRNA) differs from the str…

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  1. Messenger RNA (mRNA) is composed of a single polynucleotide strand, whereas DNA is composed of two polynucleotide strands arranged in a double helix.
  2. The pentose sugar in the nucleotides of mRNA is ribose, while the pentose sugar in the nucleotides of DNA is deoxyribose.
  3. mRNA contains the nitrogenous base uracil (U) in place of thymine. DNA contains the nitrogenous base thymine (T) instead of uracil.

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  How the marks are awarded

  • B1 — The first point correctly identifies that mRNA is single-stranded while DNA is double-stranded (or a double helix).
  • B1 — The second point correctly states that the sugar in mRNA is ribose, whereas the sugar in DNA is deoxyribose.
  • B1 — The third point correctly identifies the difference in nitrogenous bases, stating that mRNA has uracil (U) while DNA has thymine (T).

  Common mistakes

  • Making an incomplete comparison, for example, stating 'mRNA has uracil' without mentioning that DNA has thymine.
  • Being too vague, such as saying 'the sugars are different' without naming ribose and deoxyribose.
  • Confusing structural differences with functional roles, for example, writing 'mRNA leaves the nucleus but DNA does not'.
  • Stating that DNA has base pairing but mRNA does not, which is an oversimplification as tRNA has base-paired regions and mRNA can fold back on itself, although the primary structure is single-stranded.

  Examiner tip: When asked to describe differences, ensure every point you make is a direct comparison that explicitly mentions both of the items in the question.

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