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A1.2.2—Components of a nucleotide

Nucleotides are built from a phosphate group, a pentose sugar, and a nitrogenous base, with DNA and RNA differing in sugar and bases.

Syllabus
First assessment 2025
Objective
A1.2.2
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1B
Typical marks1–3

Common command terms

  • Identify
  • Draw

Recent exam appearances

May 2025Paper1B HL · TZ23(c)(ii)[ 1 ]A1.2.2—Components of a nucleotide
May 2025Paper1B HL · TZ23(c)(i)[ 3 ]A1.2.2—Components of a nucleotide
May 2022Paper2 HL · TZ18(a)[ 3 ]A1.2.2—Components of a nucleotide
May 2022Paper1 HL · TZ16[ 1 ]A1.2.2—Components of a nucleotide
Practice this objective

Coverage 2022–2025 · Updated 14 Jul 2026

Build a Nucleotide from Three Parts

A nucleotide contains three components: a phosphate group, a pentose sugar and a nitrogenous base.

The sugar is the central connector: the base and phosphate are both attached to it. DNA nucleotides contain deoxyribose; RNA nucleotides contain ribose.

For the IB diagram convention, draw the phosphate as a circle, the pentose sugar as a pentagon and the base as a rectangle, with both phosphate and base connected to the sugar.

A DNA nucleotide can be represented as phosphate-circle -> deoxyribose-pentagon -> adenine-rectangle.

A nitrogenous base alone is not a nucleotide; the sugar and phosphate are also required.

Components of a nucleotide

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Draw.

Command terms

Identify / Draw

What earns marks

Build the answer around this relationship: Components of a nucleotide should be described using precise molecular vocabulary.

Representative question

Question 1

[Maximum number: 3]

Draw a labelled diagram to show the structure of a DNA nucleotide.

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

  • Components of a nucleotide 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.