Reading frame

A reading frame is a way of breaking a sequence of nucleotides (in DNA or RNA) into three letter codons which can be translated into amino acids. There are 3 possible reading frames in an mRNA strand, each one corresponding to starting at a different alignment. Double-stranded DNA has six possible reading frames 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 and there may be many of these in bacteria. Some viruses use several overlapping genes in different reading frames.

In rare cases a translating ribosome may shift from one frame to another in a translational frameshift. It is distinct from a frameshift mutation as the nucleotide sequence 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, a subsequent region which usually has a length which is a multiple of 3 nucleotides, and a stop codon at its end.