Insertion

An insertion is a mutation in which one or more nucleotides are added into a DNA sequence. Like deletions, insertions can vary in size - from inserting a single extra base in a gene to inserting a large segment of DNA (for instance, due to a transposable element or viral DNA).

• Insertions that are not in multiples of 3 bases will cause a frameshift in coding sequences, usually destroying the original protein's function by altering every amino acid downstream of the mutation.
• Small insertions (like adding one base) can have big effects (frameshift), whereas an insertion of 3 bases adds one amino acid to a protein which might be better tolerated (unless that insertion disrupts critical regions).
• Transposons (jumping genes) or retroviral insertions are larger-scale insertion events: these DNA pieces can insert into a gene and disrupt it. An example is the insertion of viral DNA into tumor suppressor genes contributing to cancer.

• Expect similar patterns as deletion questions: e.g., a question might show a DNA or mRNA sequence with extra bases in the mutant and ask about the effect (likely frameshift if not a multiple of 3).
• An exam might give a scenario: "A single base addition in a coding sequence results in..." -- the correct answer: a frameshift mutation altering the downstream amino acid sequence and possibly producing a premature stop codon.
• Be aware of terminology: sometimes exam questions lump insertions and deletions together as "indels." Both cause frameshifts if not adding/removing whole codons. If you see "indel" mentioned, it means either an insertion or deletion occurred.