<P> In molecular biology, a reading frame is a way of dividing the sequence of nucleotides in a nucleic acid (DNA or RNA) molecule into a set of consecutive, non-overlapping triplets . Where these triplets equate to amino acids or stop signals during translation, they are called codons . </P> <P> A single strand of a nucleic acid molecule has a phosphoryl end, called the 5 ′ - end, and a hydroxyl or 3 ′ - end . These define the 5' → 3' direction . There are three reading frames that can be read in this 5' → 3' direction, each beginning from a different nucleotide in a triplet . In a double stranded nucleic acid, an additional three reading frames may be read from the other, complementary strand in the 5' → 3' direction along this strand . As the two strands of a double - stranded nucleic acid molecule are antiparallel, the 5' → 3' direction on the second strand corresponds to the 3' → 5' direction along the first strand . </P> <P> In general, at the most, one reading frame in a given section of a nucleic acid, is biologically relevant (open reading frame). Some viral transcripts can be translated using multiple, overlapping reading frames . There is one known example of overlapping reading frames in mammalian mitochondrial DNA: coding portions of genes for 2 subunits of ATPase overlap . </P>

The genetic code refers to the sequence of bases in the rna molecule