A1.2.3—Sugar-phosphate bonding
Nucleotides join by condensation reactions to form a continuous sugar-phosphate backbone, leaving bases exposed to carry sequence information along each strand.
- Syllabus
- First assessment 2025
- Objective
- A1.2.3
- Level
- SL
Exam analysis
Chance of appearing6%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper1
Typical marks1
Common command terms
- Identify
- Outline
Scoring notes
- Common mistake
- Mixing up covalent sugar-phosphate bonds with hydrogen bonds between complementary bases.
Recent exam appearances
May 2023Paper1 SL · TZ210[ 1 ]A1.2.3—Sugar-phosphate bonding
November 2021Paper1 SL · TZ010[ 1 ]A1.2.3—Sugar-phosphate bonding
May 2021Paper1 SL · TZ29[ 1 ]A1.2.3—Sugar-phosphate bonding
May 2018Paper1 SL · TZ111[ 1 ]A1.2.3—Sugar-phosphate bonding
November 2015Paper1 SL · TZ08[ 1 ]A1.2.3—Sugar-phosphate bonding
Practice this objective
Coverage 2010–2023 · Updated 14 Jul 2026
Link Nucleotides through the Sugar–Phosphate Backbone
Nucleotides join when a phosphate links the sugar of one nucleotide to the sugar of the next, forming a repeating backbone.
The covalent sugar–phosphate links make the strand continuous and give it a direction. Bases project from the backbone, so their order can vary without breaking the structural chain.
When reading a strand, separate:
- the stable sugar–phosphate backbone
- the variable base sequence
- the direction from one end to the other
Joining many nucleotides produces a strand with an alternating sugar–phosphate backbone and a base attached to each sugar.
The bases do not form the backbone; confusing the code with the support structure reverses the roles.
Sugar-phosphate bonding
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Outline.
Command terms
Identify / Outline
What earns marks
Build the answer around this relationship: Sugar-phosphate bonding 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
Outline the bonding between DNA 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
- Sugar-phosphate bonding 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.