Explain how the structure of a DNA nucleotide is related to its role in forming the polynucleotide strand.

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)

A DNA nucleotide consists of a deoxyribose sugar, a phosphate group and a nitrogenous base.

The phosphate group of one nucleotide forms a covalent phosphodiester bond with the 3′‑hydroxyl of the deoxyribose of the next nucleotide, linking the nucleotides into a chain.

This linkage creates a sugar–phosphate backbone that gives the polynucleotide strand its structural integrity and directionality (5′ to 3′).

The nitrogenous bases project from the backbone; their specific sequence stores genetic information because the order of bases encodes the instructions for the organism.

Examiner tips

Use the exact terms – deoxyribose, phosphate, nitrogenous base, phosphodiester bond, sugar–phosphate backbone, 5′→3′ directionality.
Show the chain‑forming bond explicitly – phosphate of one to 3′‑OH of the next.
Explain that the base sequence is the information‑carrying part.
Keep the answer concise – 4 points, 1 sentence per point.

Common mistakes

Confusing ribose with deoxyribose; writing ribose instead of deoxyribose.
Omitting the 5′→3′ directionality of the backbone.
Failing to mention that the base sequence stores genetic information.

Mark scheme (4 marks)

A DNA nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
Covalent bonds (phosphodiester bonds) form between the phosphate group of one nucleotide and the deoxyribose sugar of the next, linking nucleotides together.
This produces a sugar–phosphate backbone that gives the polynucleotide strand its structural integrity/directionality (5' to 3').
The sequence of bases projecting from the backbone stores genetic information, because the specific order of bases encodes instructions (is not restricted by any structural rule).

Key terms in this question

nucleotide · polynucleotide