What are the three reading frames of a gene?

In biology, a reading frame is a way of breaking a sequence of nucleotides in DNA or RNA into three letter codons which can be translated in amino acids. There are 3 possible reading frames in an mRNA strand: each reading frame corresponds to a different starting alignment. Double stranded DNA has six different reading frames per molecule due to the two strands from which transcription is possible—three of them reading forward and three of them reading backwards.

The existence of multiple reading frames leads to the possibility of overlapping genes; there may be many of these in bacteria and fungi. Some viruses, e.g. Hepatitis B virus and BYDV, use several overlapping genes in different reading frames.

In rare cases a translating ribosome may shift from one frame to another, a translational frameshift. This is distinct from a frameshift mutation, as the nucleotide sequence (DNA or RNA) is not altered—only the frame in which it is read.

An open reading frame (ORF) is a reading frame that contains a start codon, and a subsequent region which usually has a length which is a multiple of 3 nucleotides, but does not contain a stop codon in a given reading frame.