Describe the structure of a DNA nucleotide and explain how nucleotides are joined together to form a single strand of DNA.

AQA A-Level Biology (7402) — 3.1.5 Nucleic Acids · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A DNA nucleotide consists of a deoxyribose sugar, a phosphate group and one of four nitrogenous bases (adenine, thymine, cytosine or guanine). The 3‑hydroxyl group of the deoxyribose of one nucleotide reacts with the 5‑phosphate of the next nucleotide in a condensation reaction, forming a phosphodiester bond. Repeating this linkage creates a sugar‑phosphate backbone that runs along the length of the DNA strand.

Examiner tips

Use the exact terms – deoxyribose, phosphate, adenine, thymine, cytosine, guanine, phosphodiester bond, sugar‑phosphate backbone.
Show the condensation reaction (3‑OH + 5‑P → phosphodiester + H₂O).
Mention the backbone runs along the strand, bases project inward.
Keep the answer concise – 5 marks, so one sentence per point.

Common mistakes

Confusing ribose with deoxyribose; writing ribose instead of deoxyribose.
Omitting the phosphate group or the bases in the description.
Using the wrong bond type – e.g., saying hydrogen bond instead of phosphodiester.

Mark scheme (5 marks)

A DNA nucleotide is made up of a deoxyribose (sugar) molecule
A DNA nucleotide contains a phosphate group
A DNA nucleotide contains one of four bases (adenine, thymine, cytosine or guanine)
Nucleotides are joined by condensation reactions / covalent bonds form between the sugar of one nucleotide and the phosphate of the next
This forms a sugar-phosphate backbone along the length of the strand

Key terms in this question

nucleotide