Explain how the base sequence of DNA acts as a store of genetic information.

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)

DNA contains four nitrogenous bases – adenine, thymine, guanine and cytosine – which can be arranged in many different orders. The order of these bases along a polynucleotide strand encodes the instructions for making proteins, because the sequence of bases determines the sequence of amino acids in a polypeptide.

The very large number of base pairs in a DNA molecule means that there are an enormous number of possible sequences, giving DNA a huge information‑carrying capacity.

Because base pairing is complementary (A with T, G with C), the sequence can be copied accurately during DNA replication, so the genetic information is stable and heritable and is passed on to daughter cells and offspring.

Examiner tips

  • Use the exact terms – bases, sequence, complementary base pairing, replication
  • Show the link between base sequence and amino‑acid sequence
  • Mention the large number of base pairs for information capacity
  • Explain how base pairing ensures faithful copying

Common mistakes

  • Confusing the role of RNA with DNA, or omitting the complementary base‑pairing point
  • Using vague phrases like "DNA stores information" without explaining how the sequence encodes proteins
  • Failing to mention the large number of base pairs and the resulting information capacity

Mark scheme (4 marks)

  1. DNA contains four different nitrogenous bases (adenine, thymine, guanine, cytosine), allowing variation in sequence.
  2. The sequence of bases along a polynucleotide strand encodes instructions / specifies the sequence of amino acids in a polypeptide.
  3. The large number of base pairs in a DNA molecule allows an enormous variety of different sequences, giving a very large information-carrying capacity.
  4. The information is stable and heritable because complementary base pairing allows the sequence to be faithfully copied and passed to daughter cells / offspring.

Key terms in this question

base sequence

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