Insertion, deletion and substitution, described in full

A recurring Achieved-level requirement: describing what each mutation TYPE actually does to the DNA sequence itself, before discussing its effect on the protein.

What each mutation type does to the sequence

SUBSTITUTION: one base is SWAPPED for a different one. The total NUMBER of bases in the gene stays the SAME — only that one position changes.
INSERTION: one or more EXTRA bases are ADDED into the sequence, increasing the gene's length.
DELETION: one or more bases are REMOVED from the sequence, decreasing the gene's length.
Both insertion and deletion change the TOTAL LENGTH of the DNA base sequence, and both cause a FRAMESHIFT — every codon downstream of the change is read differently, since the three-base grouping has shifted.

Worked example Naming which point mutation is LEAST likely to change the protein, and why Name the type of point mutation unlikely to change the protein, and explain why. THE TYPE: a SILENT (same-sense) substitution. WHY: due to the DEGENERACY of the genetic code, most amino acids are coded for by MORE THAN ONE codon — so a substitution can change the specific codon's bases while it still codes for the exact SAME amino acid. RESULT: the amino acid sequence of the protein is UNCHANGED, so the protein's structure and function are also unchanged. ✓ Naming "degeneracy of the genetic code" explicitly, not just "sometimes mutations don't matter", is what this question is checking for. ✕

Saying insertions/deletions and substitutions both change gene length

Only insertions and deletions change the TOTAL LENGTH of the DNA base sequence — a substitution swaps one base for another WITHOUT changing how many bases are present. Confusing "changes the sequence" (true of all three) with "changes the length" (true only of insertion/deletion) is a precise, commonly-tested distinction.

Mutation type	Changes gene LENGTH?	Causes a FRAMESHIFT?
Substitution	No	No — affects at most one codon
Insertion	Yes	Yes — every codon downstream shifts
Deletion	Yes	Yes — every codon downstream shifts

Practice for this standard covers the whole topic — flashcards and quiz.