What a gene mutation is

• Gene mutations are structural changes to genes at the molecular level.

• The syllabus requires you to distinguish between substitutions, insertions and deletions.

• A mutation changes the base sequence of DNA, which may change the mRNA codons produced and therefore the amino acid sequence of a polypeptide.

• Not all mutations change the phenotype: effects can be neutral, harmful, or occasionally beneficial.

Types of gene mutation

• Substitution = one base is replaced by another.

• Insertion = one or more bases added to the sequence.

• Deletion = one or more bases removed from the sequence.

• Substitutions often affect one codon only.

• Insertions and deletions are more likely to cause a frameshift mutation if the number of bases added/removed is not a multiple of 3.

This diagram shows how a base substitution can produce silent, missense or nonsense outcomes depending on how the altered codon changes translation. It is useful for linking mutation type to protein-level consequence. Source

This image shows how an insertion or deletion can shift the reading frame, changing all downstream codons. It helps explain why frameshifts usually have a much larger effect on protein structure and function than a single-base substitution. Source

Consequences of substitutions

• Single-nucleotide polymorphisms (SNPs) are the result of base substitution mutations.

• Because of the degeneracy of the genetic code, a substitution may not change the amino acid coded for.

• A substitution may be silent if the altered codon still codes for the same amino acid.

• A substitution may change one amino acid in a polypeptide, which may alter protein structure and therefore function.

• In exams, always link: DNA base change → mRNA codon change → amino acid change (or no change) → altered protein function/phenotype.

This diagram highlights a single nucleotide polymorphism (SNP) within a DNA sequence. It is useful for visualizing that a SNP is a single-base difference that may or may not affect the amino acid sequence of a protein. Source

Consequences of insertions and deletions

• Insertions and deletions often cause frameshift changes.

• A frameshift changes the grouping of bases into codons from the mutation onward.

• This usually changes many amino acids downstream, so the polypeptide is likely to cease functioning.

• Major insertions or deletions can also disrupt the gene so severely that the final protein is non-functional.

• Exam tip: if asked which type is most likely to produce a severe effect, explain why frameshift mutations usually affect multiple codons, not just one.

Causes of gene mutation

• Mutations can be caused by mutagens.

• Mutations can also arise from errors in DNA replication.

• Mutations can also arise from errors in DNA repair.

• Examples of chemical mutagens should be known in general terms, for example chemicals that alter bases or interfere with correct pairing.

• Examples of mutagenic radiation include forms of radiation that damage DNA, such as ionizing radiation and ultraviolet radiation.

• In exam answers, state clearly that mutation is not always caused by an external mutagen; it can happen spontaneously during replication or repair.

Randomness in mutation

• Mutations are random with respect to the needs of the organism.

• They can occur anywhere in the genome, although some bases have a higher probability of mutating than others.

• There is no natural mechanism known that deliberately changes a specific base in order to improve a trait.

• This is important for evolution questions: mutations arise first, then natural selection acts on the resulting variation.

Germ cell vs somatic cell mutations

• A mutation in a germ cell can be inherited by offspring.

• A mutation in a somatic cell is not normally inherited by offspring.

• Somatic mutations can lead to cancer if they affect genes controlling the cell cycle.

• Germline mutations matter most in questions about inheritance and evolution.

• Somatic mutations matter most in questions about disease within an individual.

Mutation as a source of variation

• Gene mutation is the original source of all genetic variation.

• Most mutations are neutral or harmful to the individual.

• However, mutations are essential in the long term because they create the new alleles needed for evolution by natural selection.

• Sexual reproduction reshuffles alleles, but mutation creates new alleles.

• Strong exam phrase: mutation produces variation; natural selection changes allele frequencies.

This diagram outlines how knockout organisms can be generated and bred for research. It supports the idea that gene knockout helps scientists infer gene function by comparing normal and altered phenotypes. Source

This diagram shows how Cas9 cuts DNA and how cells repair that cut, producing either disruption of a gene or a more precise edited sequence. It is ideal for explaining why CRISPR is a targeted gene-editing tool rather than a random mutational process. Source

Exam links and common traps

• Do not confuse SNP with frameshift: a SNP is a single-base substitution.

• Do not assume every mutation changes a protein; because of degeneracy, some substitutions are silent.

• Do not say mutations happen because the organism “needs” them — mutations are random.

• Distinguish inherited germline mutation from somatic mutation causing cancer.

• Distinguish random natural mutation from targeted gene editing using CRISPR-Cas9.