Any piece
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 frame.
Because
three of the 64 possible DNA
triplets correspond to
mRNA stop codons, a DNA sequence read at random will have 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.
In the
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 ©2014 by Steven M. Carr