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A1.2.6—DNA as a double helix

DNA forms a double helix from two antiparallel polynucleotide strands held together by hydrogen bonds between complementary bases in the molecule.

Syllabus
First assessment 2025
Objective
A1.2.6
Level
HL

Exam analysis

Chance of appearing12%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1–2

Common command terms

  • Describe
  • Explain
  • Deduce
  • Draw
  • Sketch

Scoring notes

Common mistake
Mixing up covalent sugar-phosphate bonds with hydrogen bonds between complementary bases.

Recent exam appearances

November 2025Paper1A HL · TZ31[ 1 ]A1.2.6—DNA as a double helix
November 2024Paper1 HL · TZ08[ 1 ]A1.2.6—DNA as a double helix
May 2024Paper1 HL · TZ19[ 1 ]A1.2.6—DNA as a double helix
May 2023Paper1 HL · TZ125[ 1 ]A1.2.6—DNA as a double helix
May 2023Paper1 HL · TZ222[ 1 ]A1.2.6—DNA as a double helix
Practice this objective

Coverage 2013–2025 · Updated 14 Jul 2026

The DNA Double Helix Stores Two Matching Copies

DNA consists of two antiparallel polynucleotide strands whose complementary bases pair inside a double helix.

Hydrogen bonding between paired bases holds the strands together, while the sugar–phosphate backbones face outward. Because each base has a partner, either strand can guide construction of the other during copying. This arrangement combines stability with a built-in template.

The helix depends on three linked features:

  • complementary base pairing
  • antiparallel strand direction
  • an external sugar–phosphate backbone

If one strand reads A–G–C, the opposite strand reads T–C–G at the matching positions.

The two strands are not identical copies in sequence; they are complementary and run in opposite directions.

DNA as a double helix

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Describe / Explain / Deduce.

Command terms

Describe / Explain / Deduce / Draw / Sketch

What earns marks

Build the answer around this relationship: DNA as a double helix should be described using precise molecular vocabulary.

Watch for

Mixing up covalent sugar-phosphate bonds with hydrogen bonds between complementary bases.

Representative question

Question 1

[Maximum number: 6]

Draw a simple labeled diagram to show the structure of a double stranded DNA molecule, comprising four nucleotides.

Retrieve The Core Rules

The core chain is: nucleotides have three parts; condensation builds the sugar-phosphate backbone; base order stores information; complementary pairing lets DNA copy and express that information.

  • Nucleotide parts: phosphate, pentose sugar, nitrogenous base.
  • Polymer rule: condensation forms a sugar-phosphate backbone.
  • Code rule: base order stores information and triplet codons specify amino acids.
  • Pairing rule: A-T or A-U, and C-G, enables replication and gene expression.
  • DNA/RNA contrast: DNA uses deoxyribose/T and is usually double-stranded; RNA uses ribose/U and is usually single-stranded.

Concept essentials

  • DNA as a double helix should be described using precise molecular vocabulary.
  • Structure and information must be kept separate: backbones provide continuity while bases carry sequence meaning.
  • Exam answers should match the command term and avoid adding unsupported extra claims.