6.2.7—Types of mutations

Substitution, insertion and deletion are three ways a gene’s DNA base sequence can change. Their different effects on the triplet grouping explain why some changes are local while others affect many downstream codons and the polypeptide produced.

• Substitution — swap one base: One DNA base is replaced by another. It changes the triplet at that position but is not a frameshift, so it does not regroup all downstream triplets. Because the genetic code is degenerate, the amino-acid sequence may stay the same or may change at that triplet.
• Insertion — add base(s): One or more bases are added to the DNA sequence. The added base changes the local triplet grouping and can shift the reading frame, changing downstream triplets and potentially many amino acids.
• Deletion — remove base(s): One or more bases are removed. Like an insertion, a non-triplet change can shift the reading frame and alter downstream amino-acid instructions.
• Comparison boundary: Substitution is local with respect to the reading frame; insertion/deletion can have a downstream frameshift effect. The actual polypeptide consequence still depends on the sequence position and how the changed codons are read.

The causal chain is DNA edit → altered triplet grouping or codon → possible amino-acid sequence change → possible polypeptide shape/function change. Insertions and deletions can propagate the change through later triplets, whereas a substitution does not automatically do so; the genetic code’s degeneracy means even a changed base need not change the polypeptide.

An insertion or deletion is not automatically the same as a substitution: check whether the reading frame is shifted. A mutation type predicts a mechanism of sequence change, not a guaranteed disease or protein outcome. This card compares the edits; the broader definition of mutation is card 4581 and the context-dependent polypeptide effect belongs to card 4583.