Mutation and gene editing — IB Diploma Biology SL
IB Biology D1.3: substitutions, insertions and deletions, causes of mutation, and germ and somatic cells.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Gene mutations are structural changes to genes at the molecular level: substitutions, insertions or deletions of bases. A base substitution produces a single-nucleotide polymorphism (SNP), which may or may not change one amino acid because the code is degenerate. Insertions and deletions often cause a frameshift, changing every later codon, so the polypeptide is likely to stop functioning.
Gene mutations are caused by mutagens and by errors in DNA replication or repair. Chemical mutagens include benzo[a]pyrene in tobacco smoke; mutagenic radiation includes ultraviolet light, X-rays and gamma rays. Mutation is random: it can occur anywhere in the genome, though some bases mutate more often, and no natural mechanism deliberately changes a base to change a trait.
A mutation in a germ cell can be passed to offspring and is then in every cell of the new organism. A mutation in a somatic cell affects only that cell and its descendants and is not inherited, though it may lead to cancer. Gene mutation is the original source of all genetic variation, essential for evolution by natural selection.
Exam questions
5 exam-style questions (15 marks), each with its mark scheme.
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10 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of molecules (continuity and change) on one page, so you can see where this subtopic fits.
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Quick check questions
What is a single-nucleotide polymorphism (SNP)?
Show answer
A position in the genome where a single base differs between individuals, resulting from a base substitution mutation.
Why does deleting one base usually have a larger effect than substituting one base?
Show answer
It shifts the reading frame, changing every codon after the mutation.
Give one chemical mutagen and one form of mutagenic radiation.
Show answer
Benzo[a]pyrene (in tobacco smoke); ultraviolet light (or X-rays, gamma rays).
Why are somatic mutations not inherited?
Show answer
They are not in germ cells, so they are not passed into gametes.
Frequently asked questions
What is the difference between a substitution and a frameshift mutation?
A substitution replaces one base with another, so only one codon changes and at most one amino acid is altered; it may have no effect because the genetic code is degenerate. A frameshift is caused by inserting or deleting bases that are not a multiple of three, changing every codon afterwards, so the polypeptide usually stops functioning.
What causes gene mutations?
Gene mutations are caused by mutagens and by errors in DNA replication or repair. Chemical mutagens include benzo[a]pyrene and nitrosamines in tobacco smoke. Mutagenic radiation includes ultraviolet light, X-rays and gamma rays. Mutations occur at random anywhere in the genome; no natural mechanism deliberately changes a particular base to change a trait.
Why are mutations important for evolution?
Mutations are important for evolution because gene mutation is the original source of all genetic variation: it is the only process that creates new alleles. Most mutations are harmful or neutral for an individual, but over long periods some new alleles increase survival and reproduction, so natural selection can act on them and species can evolve.
Written and checked against the IB Biology SL specification · Updated October 2026