Mutations are changes in DNA sequence, and they are a major source of genetic variation. Some mutations have little or no effect, while others change proteins, disrupt cell function, or contribute to disease. Understanding mutation types helps explain inheritance, evolution, cancer, antibiotic resistance, and genetic disorders.
In a gene sequence, even one changed base can matter because DNA is read in three-base codons.
Key Facts
- Substitution: one DNA base is replaced by another, such as GAG becoming GTG.
- Insertion: one or more bases are added to a DNA sequence, such as ATG CCA becoming ATG ACC A.
- Deletion: one or more bases are removed from a DNA sequence, such as ATG CCA becoming ATC CA.
- Frameshift mutation: an insertion or deletion not in multiples of 3 changes the codon reading frame.
- Silent mutation: a codon changes but the amino acid stays the same, such as GAA and GAG both coding for glutamic acid.
- Nonsense mutation: a codon changes into a stop codon, causing early termination of translation.
Vocabulary
- Mutation
- A mutation is a change in the nucleotide sequence of DNA.
- Codon
- A codon is a group of three mRNA bases that specifies an amino acid or a stop signal during translation.
- Frameshift
- A frameshift is a change in the reading frame caused by an insertion or deletion that is not a multiple of three bases.
- Point mutation
- A point mutation is a change affecting a single nucleotide or base pair in DNA.
- Chromosomal mutation
- A chromosomal mutation is a large-scale change in chromosome structure or number.
Common Mistakes to Avoid
- Calling every substitution a frameshift is wrong because a substitution changes one codon but usually does not shift the grouping of all later codons.
- Assuming every mutation is harmful is wrong because many mutations are neutral, some are silent, and a few can be beneficial in certain environments.
- Forgetting that codons are read in groups of three is wrong because insertion or deletion effects depend on whether the number of bases changed is a multiple of 3.
- Confusing gene mutations with chromosomal mutations is wrong because gene mutations affect a DNA sequence within a gene, while chromosomal mutations affect large chromosome segments or whole chromosomes.
Practice Questions
- 1 A coding DNA strand reads ATG GAA TTT CCG. A substitution changes GAA to GTA. How many codons are changed directly, and is this a substitution, insertion, or deletion?
- 2 A DNA sequence has 30 bases. A deletion removes 2 bases near the beginning of the sequence. How many complete codons could be read from the remaining bases, and would the mutation cause a frameshift?
- 3 A mutation changes a codon from one amino acid codon to a stop codon near the middle of a gene. Explain how this affects the protein and why it is usually more serious than a silent mutation.