Translational frameshift

Translational frameshifting or ribosomal frameshifting refers to an alternative process of protein translation. A protein is translated from one end of the mRNA to the other, from the 5' to the 3' end. Normally a protein is translated from a template mRNA with consecutive blocks of 3 nucleotides being read as single amino acids. However, certain organisms may exhibit a change or shift in the ribosomes frame by one or two nucleotides when translating the genetic code. This is deemed translational or ribosomal frameshifting. The process can be programmed by the nucleotide sequence of the mRNA and is sometimes affected by the secondary or tertiary mRNA structure. It has been described mainly in viruses (especially retroviruses), retrotransposons and bacterial insertion elements, and also in some cellular genes.

Proteins are translated unidirectionally by reading tri-nucleotides on the mRNA strand also known as codons. Therefore, a shift of any number of nucleotides that is not divisible by 3 in the reading frame will result in subsequent codons to be read differently. This effectively changes the ribosomal reading frame. For example, the following sentence when read from the beginning makes sense to a reader:

However, changing the reading frame by say shifting the first reading up one letter between the T and H on the first word:

Now the sentence makes absolutely no sense. In the case of a translating ribosome, a frameshift can result in nonsense being created after the frameshift or a completely different protein being created after the frameshift. When referring to translational frameshifting, the latter is always implied, the former being usually a result of a point mutation such as a deletion.

...
Wikipedia