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A1.2.9—Diversity of DNA base sequences

DNA stores vast amounts of information because base sequences can vary in length and order across genes, genomes, and organisms.

Syllabus
First assessment 2025
Objective
A1.2.9
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks4

Common command terms

  • Explain

Recent exam appearances

November 2025Paper2 HL · TZ17(a)[ 4 ]A1.2.9—Diversity of DNA base sequences
Practice this objective

Coverage 2025–2025 · Updated 14 Jul 2026

DNA sequence diversity and genetic code

Sequence diversity and codon conservation.

DNA can vary in both length and base sequence. If a sequence has n positions and each position can contain one of four bases, there are 4ⁿ possible sequences. Even relatively short sequences can therefore store a very large number of different instructions.

Genome size and gene number vary widely between organisms, but neither is a simple measure of organismal complexity. The key idea is that variation in sequence length and order gives DNA an enormous capacity for information storage.

The genetic code is highly conserved across life: the 64 codons have nearly the same meanings in different organisms. Shared codon meanings, together with conserved genes involved in transcription, translation and ribosomes, support the inference that living organisms share a common ancestor.

“Nearly” matters because there are minor exceptions. A synonymous mutation can change a base or codon without changing the amino acid specified, so a change in DNA sequence does not always change the polypeptide sequence.

Diversity of DNA base sequences

Assessment in practice

1–4 marks
How it is assessed

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

Command terms

Explain

What earns marks

Build the answer around this relationship: Diversity of DNA base sequences should be described using precise molecular vocabulary.

Representative question

Question 1

[Maximum number: 4]

Explain the diversity of possible base sequences in nucleic acids.

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

  • Diversity of DNA base sequences 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.