Preview
IB DP Biology — Unit D: Continuity and Change | SL | D1.3
📋 What You Will Learn
- Identify the types of gene mutations — substitution, insertion, and deletion — and their effects on amino acid sequences
- Explain how mutagens increase the rate of mutation and give examples of chemical, physical, and biological mutagens
- Describe the mechanism of CRISPR-Cas9 gene editing, including the roles of guide RNA and Cas9 endonuclease
- Analyse how frameshift mutations differ from point mutations in their consequences for protein structure and function
- Evaluate the ethical implications and potential applications of gene editing technologies in medicine and agriculture
- Apply knowledge of mutation types to interpret unfamiliar data on genetic disease and protein function
🔍 Guiding Question How do changes in the base sequence of DNA affect the structure and function of proteins, and what are the implications of technologies that allow us to deliberately alter these sequences?
🌍 Why This Matters In 2023, the FDA approved the first CRISPR-based therapy — Casgevy — to treat sickle cell disease, a condition caused by a single nucleotide substitution in the gene encoding haemoglobin. The same molecular logic that causes disease can now be harnessed to cure it.
The Problem the Genome Had to Solve
Every time a cell divides, roughly 3.2 billion base pairs must be copied with extraordinary fidelity. DNA polymerase is remarkably accurate — it makes approximately one error per billion nucleotides added. Yet errors still happen. Environmental agents accelerate them. And occasionally, those errors are not corrected.
A mutation is not inherently catastrophic. Most mutations are silent, neutral, or corrected before they matter. But some alter the amino acid sequence of a protein so profoundly that the protein can no longer fold, bind, or catalyse. Understanding mutations means understanding the bridge between genotype and phenotype — the central concern of the theme of Continuity and Change.
📖 Key Definition — Mutation A mutation is a change in the base sequence of DNA. Mutations may affect a single nucleotide (gene mutation) or involve larger chromosomal rearrangements. In this subtopic, the focus is on gene mutations — changes within a single gene.
Types of Gene Mutation
The genetic code is read in triplets — codons of three bases that each specify one amino acid. This triplet structure means that not all mutations are equal. A change in one base might swap one amino acid for another, or it might do nothing at all. But delete a single base, and the entire reading frame shifts — every codon downstream is misread. The consequences depend entirely on the type of mutation.
Substitution Mutations
A substitution mutation occurs when one nucleotide is replaced by a different nucleotide. Because the reading frame is preserved — the number of bases has not changed — only the codon containing the substitution is affected. This makes substitutions the least disruptive class of gene mutation, though their consequences still vary enormously.
There are three possible outcomes of a substitution. A silent mutation changes a codon but, due to the degeneracy of the genetic code, the same amino acid is still encoded — no change in protein sequence