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D1.3.9 (HL)—CRISPR-Cas9 gene editing

CRISPR-Cas9 uses guide RNA targeting and Cas9 cutting to alter DNA sequences by deletion, insertion, disruption or replacement in selected genes.

Syllabus
First assessment 2025
Objective
D1.3.9
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks3

Common command terms

  • Explain

Recent exam appearances

May 2025Paper2 ["HL"] · TZ27(c)[ 3 ]D1.3.9 (HL)—CRISPR-Cas9 gene editing
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

CRISPR-Cas9 Targets a Chosen DNA Sequence

HL only

CRISPR-Cas9 gene editing uses a guide RNA to direct the Cas9 enzyme to a complementary DNA target, where Cas9 cuts the DNA so the sequence can be changed.

Changing the guide RNA changes the target sequence. After cutting, gene editing can delete DNA, insert or replace a sequence, or disrupt the gene so that it no longer functions.

Choose a target → design a complementary guide RNA → guide RNA directs Cas9 → Cas9 cuts near the target → the DNA is edited → verify the sequence and biological result.

Successful-use example from an official IB paper: CRISPR-Cas9 repair of a mutated DMD gene restored expression of dystrophin. Depending on the mutation, editing can replace a changed codon, add DNA missing after a deletion, or remove DNA added by an insertion.

Restoring dystrophin expression demonstrates a successful molecular result; it does not by itself prove complete, safe clinical treatment. Delivery, unintended edits and regulation must still be considered.

CRISPR-Cas9 gene editing

HL only

Assessment in practice

3 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: Guide RNA directs Cas9 to a complementary DNA target.

Representative question

Question 1

[Maximum number: 3]

Explain ways in which CRISPR-Cas9 gene editing could be used to change the mutated dystrophin protein produced.

HL Gene Editing Evidence

HL only

Gene knockout makes a specific gene non-functional to investigate phenotype; model organisms such as mice, Drosophila, zebrafish, and Arabidopsis support KO libraries. Guide RNA directs Cas9 to a complementary DNA target sequence; Cas9 cutting enables deletion, replacement, insertion, or gene disruption. Conserved sequences remain similar across species or long evolutionary times; conservation suggests essential function, lower mutation rate, or strong purifying selection.

Concept essentials

  • Guide RNA directs Cas9 to a complementary DNA target.
  • Cas9 cutting creates the opportunity for DNA sequence change.
  • CRISPR-Cas9 can disrupt, delete, insert or replace DNA.
  • Editing a disease gene can alter the protein produced from that gene.