of dsDNA can be “read” in six possible ways.
The diagram shows a single dsDNA molecule: either
strand could be the sense strand read in the 5' 3'
direction, and on either possible sense strand there are three
possible start points, beginning at the 1st, 2nd,
or 3rd nucleotides. Each of these 2 x 3 = 6
possibilities is called a reading
Because three of the 64 possible DNA triplets correspond to mRNA stop codons, a DNA sequence read at random will have 3-letter stop triplets approximately once in every 20 triplets. The occurrence of multiple stops in a particular reading frame indicates that it does not code for a polypeptide: this is a "closed" reading frame. In contrast, an Open Reading Frame (ORF) can be read through several hundred triplets without encountering a stop sequence. ORFs are therefore candidates for protein-coding exon regions: the inferred amino-acid sequence can be compared with GenBank to identify possible analogous proteins.
example above, the longest open runs in reading frames ##1, 2, 3, 4, & 6 [gold boxes] are
very short runs, typically < 100 bases of amino acids
without stops. Reading
frame #5 , read 5'3' right to left, includes
an ORF [blue box] of
more than 2,000 nucleotides, corresponding to a protein of
more than 600 amino acids.
Figure © 2000 by Griffiths et al. ; text © 2013 by Steven M. Carr