Base substitutions can be silent, missense or nonsense because genetic-code degeneracy changes how codon changes affect proteins, traits and disease phenotypes.
Coverage 2010–2024 · Updated 16 Jul 2026
A single-nucleotide polymorphism (SNP) results from a base substitution, but the substitution may or may not change one amino acid in a polypeptide.
| Codon outcome | Polypeptide consequence |
|---|---|
| Silent | The new codon specifies the same amino acid because the code is degenerate |
| Missense | The new codon specifies a different amino acid |
| Nonsense | The new codon is a stop codon, so translation ends early |
An mRNA codon change from GAA to GAG is silent because both specify glutamate; a change to a stop codon can shorten the polypeptide.
A substitution does not automatically change protein function. First identify the new codon and its amino-acid or stop outcome.
This objective is assessed through structured response, multiple choice, commonly using Describe / Explain / Outline.
Describe / Explain / Outline
Build the answer around this relationship: A substitution changes one base in a DNA sequence.
Stopping at the DNA substitution without tracing the codon and amino acid consequence.
Representative question
Outline how a base substitution leads to sickle cell anemia.
Gene mutations are changes in the base sequence of DNA; main types are substitution, insertion, deletion, and duplication. Base substitutions can create SNPs and change codons; degeneracy can make substitutions silent, missense, or nonsense. Insertions or deletions not in multiples of three cause frameshifts that alter downstream codons and often disrupt protein function. Mutations can arise from replication errors, repair errors, or chromosome damage; mutagens include chemicals, ionizing radiation, and ultraviolet radiation. Mutations occur randomly with respect to organism need or advantage; mutation rate varies with DNA sequence, gene expression, repair, and mutagen exposure. Germ-line mutations can be inherited by offspring; somatic mutations affect only descendant body cells and can contribute to cancer. Mutation is the original source of new alleles and genetic variation; many are neutral or harmful, but variation supplies material for natural selection.
