A DNA open reading frame starts with ATG—coding for Met—in most species, and ends with a stop codon (TAA, TAG, or TGA).
In prokaryotes, ORFs are usually the longest sequence without a stop codon (without introns).
In eukaryotes, long ORFs can continue over non-translatable intron gaps, so spliced mRNA must be employed to determine ORFs. Short ORFs can occur outside genes – within the intron, segments of DNA outside genes that were formerly considered ‘junk’ DNA. (see image to left - click to enlarge)
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