<P> In molecular genetics, an open reading frame (ORF) is the part of a reading frame that has the potential to be translated . An ORF is a continuous stretch of codons that contain a start codon (usually AUG) and a stop codon (usually UAA, UAG or UGA). An ATG codon within the ORF (not necessarily the first) may indicate where translation starts . The transcription termination site is located after the ORF, beyond the translation stop codon . If transcription were to cease before the stop codon, an incomplete protein would be made during translation . In eukaryotic genes with multiple exons, ORFs span intron / exon regions, which may be spliced together after transcription of the ORF to yield the final mRNA for protein translation . </P> <P> One common use of closed reading frames (CRF) is as one piece of evidence to assist in gene prediction . Long ORFs are often used, along with other evidence, to initially identify candidate protein - coding regions or functional RNA - coding regions in a DNA sequence . The presence of an CRF does not necessarily mean that the region is never translated . For example, in a randomly generated DNA sequence with an equal percentage of each nucleotide, a stop - codon would be expected once every 21 codons . A simple gene prediction algorithm for prokaryotes might look for a start codon followed by an open reading frame that is long enough to encode a typical protein, where the codon usage of that region matches the frequency characteristic for the given organism's coding regions . By itself even a long open reading frame is not conclusive evidence for the presence of a gene . On the other hand, it has been proven that some short ORFs (sORFs) that lack the classical hallmarks of protein - coding genes (both from ncRNAs and mRNAs) can produce functional peptides . 5'NTR of about 50% of mammal mRNAs are known to contain one or several sORFs. 64 - 75% of experimentally found translation initiation sites of sORFs are conserved in the genomes of human and mouse and may indicate that these elements have function . However, sORFs can often be found only in the minor forms of mRNAs and avoid the selection; the high conservatism of initiation sites may be connected with their location inside promoters of the relevant genes . Such kind of situation is characteristic of SLAMF1 gene, for example . </P> <P> Since DNA is interpreted in groups of three nucleotides (codons), a DNA strand has three distinct reading frames . The double helix of a DNA molecule has two anti-parallel strands; with the two strands having three reading frames each, there are six possible frame translations . </P>

How many potential open reading frames are present in a dna sequence