=Base Deletion / Insertion:= As we saw earlier the genetic code is read three bases at a time. Inserting or deleting a single base will disrupt the ‘''reading frame’''. To illustrate this look at the following sentence, read letters at a time: THE BIG FAT '''R'''ED CAT SAT FOR TEN MIN If a single letter, for instance the R (in bold) is deleted, the sentence will read as follows: THE BIG FAT '''EDC ATS ATF ORT ENM IN''' Everything ‘downstream’ of the deletion is affected. The same is true for Base Deletions and Insertions; the sequence of every amino acid downstream of the mutation is affected. For example: [image:] Extra for Experts: The above example illustrates how a frame shift results in a severe missense mutation (the sequence of amino acids is drastically altered). However a frame shift will usually result in a STOP Codon somewhere downstream of the initial insertion / deletion. Thus Insertions / deletions resulting in a frame shift are usually nonsense mutations. ==Partial Reading Frame Shift:== If a single base is inserted and another is deleted at a different location, this only causes a localised frame shift. This is a type of mis-sense mutation as only the amino acids coded for by the triplets within the effected region are altered. This is known as a balanced mutation as the reading frame is restored. [image:] Extra for Experts: Note that that if three bases are deleted, the reading frame is unaffected (essentially one amino acid is removed). In fact, any multiple of three insertions or deletions will be balanced. If there are a combination of both insertions and deletions, the insertions can be denoted with a +1 and deletions with a -1. If the sum total for the insertions & deletions is 0 or a multiple of three, the mutation is balanced.