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What is DNA actually made of, and how does its shape let it store the instructions for life?

Describe the structure of DNA as a double helix of nucleotides with complementary base pairing

A focused answer to the O-Level outcome on DNA structure. Nucleotides, the double helix, the four bases, complementary base pairing, and why the structure suits storing information.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
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What this dot point is asking

This outcome asks you to describe the structure of DNA: the small units it is built from, the double-helix shape, and the base-pairing rule that holds the two strands together. The deeper point is that this structure is exactly what lets DNA store information and be copied faithfully.

The answer

The building block: the nucleotide

DNA is a polymer built from repeating units called nucleotides. Each nucleotide has three parts:

A sugar (deoxyribose).
A phosphate group.
One of four nitrogen-containing bases: adenine (A), thymine (T), cytosine (C) or guanine (G).

The double helix

Two strands of nucleotides twist around each other to form a double helix, often pictured as a twisted ladder.

The sugar and phosphate groups alternate to form the two sides, called the sugar-phosphate backbone.
The bases point inwards and form the rungs of the ladder.

Complementary base pairing

The two strands are joined by hydrogen bonds between the bases, and the bases pair in a fixed way:

Adenine (A) pairs with thymine (T).
Cytosine (C) pairs with guanine (G).

This is complementary base pairing. Because of it, if you know the sequence of one strand you can always work out the other.

Why the structure matters

The fixed pairing rule means the two strands carry the same information in complementary form. This is what allows DNA to be copied accurately, since each strand can act as a template for a new partner. The order of the bases along a strand is the code that stores genetic information.

Working out a complementary strand

A strand of DNA reads $5'$ - A A T G C C A G - $3'$ . Write the complementary strand and check the base-pairing balance.

Step 1: Apply the pairing rule to each base

Pair each base in turn: A goes to T, A goes to T, T goes to A, G goes to C, C goes to G, C goes to G, A goes to T, G goes to C.

Step 2: Write the complementary sequence

Reading the pairs in order gives T T A C G G T C.

Step 3: Count the bases

The original strand has $3$ adenines, $1$ thymine, $1$ guanine and $3$ cytosines. The complementary strand has $1$ adenine, $3$ thymines, $3$ guanines and $1$ cytosine.

Step 4: Check the balance

Across the two strands, the number of A equals the number of T (three A and one A make four; one T and three T make four), and the number of C equals the number of G. This balance is a useful check that the pairing was done correctly.

Examples in context

Example 1. Designing a DNA probe. To detect a specific gene, scientists make a short single strand of DNA whose bases are complementary to part of that gene. It binds only where the sequence matches, using exactly the A-with-T, C-with-G rule. The probe's whole function depends on base pairing.

Example 2. Reading a sequence both ways. Because the strands are complementary, a sequencing machine that reads one strand effectively gives the other for free. This redundancy, built into the structure, is also why DNA can be checked and repaired.

Try this

Q1. State the three parts of a DNA nucleotide. [3 marks]

Cue. A sugar (deoxyribose), a phosphate group, and a nitrogen-containing base.

Q2. Name the base that pairs with (a) adenine and (b) cytosine. [2 marks]

Cue. (a) Thymine. (b) Guanine.

Q3. Write the complementary strand for the sequence G G T A C. [2 marks]

Cue. Apply A with T and C with G: C C A T G.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksDescribe the structure of a DNA molecule, including the parts of a nucleotide and the rule of complementary base pairing.

Show worked answer →

Examiners want the building blocks, the overall shape, and the base-pairing rule.

DNA is made of two strands twisted into a double helix. Each strand is a chain of units called nucleotides. A nucleotide has three parts: a sugar (deoxyribose), a phosphate group, and one of four nitrogen-containing bases.

The two strands are held together by hydrogen bonds between the bases, following complementary base pairing: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). The sugar and phosphate groups form the backbone of each strand.

What markers reward: the double-helix shape, the nucleotide made of sugar, phosphate and a base, the four bases, the pairing rule A with T and C with G, and the hydrogen bonds holding the strands together.

Original3 marksOne strand of a section of DNA has the base sequence A T G C A. Write the base sequence of the complementary strand and explain how you worked it out.