insertion

An insertion is a mutation that adds one or more nucleotides into a DNA sequence, like wedging extra letters into the middle of a word. The sequence grows longer than the original, and everything downstream is pushed along by the new material.

Insertions range in size from a single base to long stretches of DNA, including whole transposable elements that copy themselves into new locations. Small insertions often arise when the copying machinery slips on a repetitive run of bases; larger ones can come from errors during recombination or from mobile genetic elements.

The consequence of an insertion in a protein-coding region depends heavily on its length. If the number of inserted bases is not a multiple of three, the reading frame shifts and every codon beyond the insertion is misread, usually destroying the protein. An insertion of a multiple of three may simply add extra amino acids while leaving the rest of the protein intact.