ORF Finder
Find open reading frames (ORFs) in a DNA sequence.
Ignore ORFs shorter than this length (in amino acids).
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What it does
The ORF finder scans a DNA sequence in all six reading frames and lists every open reading frame it finds, reporting the frame, start and end positions, length in amino acids and the translated protein. A minimum length filter in codons lets you suppress the short ORFs that occur by chance in any sequence. It is the standard first step when you want to know whether a new contig, PCR product or synthetic construct actually contains a plausible coding region.
How it works
An open reading frame runs from an ATG start codon to the first in-frame stop codon, TAA, TAG or TGA, with no stop codon in between. Because the genetic code is read in non-overlapping triplets, a double-stranded sequence has six possible frames: three on the given strand, beginning at offset 0, 1 and 2, and three on its reverse complement. The tool translates each candidate with the standard genetic code, counts length in codons excluding the stop, and reports only ORFs whose stop codon is present within the sequence, so a coding region running off the end of the input is not listed. Positions for reverse frames refer to coordinates on the reverse complement strand, not on the input strand.
Worked example
For the 15-base sequence 5'-ATGAAACCCGGGTAA-3', the tool reports one ORF in frame +1 running from position 1 to position 15. The codons ATG AAA CCC GGG translate to the protein MKPG, a length of 4 amino acids, and TAA is the stop codon that closes the frame. Its reverse complement, TTACCCGGGTTTCAT, contains no ATG at all, so no ORF is reported on the reverse strand.
When to use it
After cloning or assembling a construct, run the insert to confirm the intended ORF is intact and in frame, and that no internal stop codon was introduced by a synthesis error or a frameshift. After codon optimizing a gene for expression, re-scan it to check that optimization did not create a premature stop or a strong alternative ORF on the reverse strand. When screening a novel contig for coding potential, set the minimum length to around 100 codons, since ORFs of that size are unlikely to arise by chance and are far more likely to be genuine genes.
FAQ
- What is an open reading frame in DNA?
- It is a stretch of sequence that begins with a start codon, conventionally ATG, and continues in the same frame to the first stop codon without interruption, so it could in principle be translated into a continuous polypeptide. An ORF is a candidate coding region rather than proof of a gene, since a real gene also needs a promoter, ribosome binding and expression evidence.
- Why do you need to check all six reading frames?
- The triplet code can be read starting at the first, second or third base, giving three frames per strand, and because DNA is double stranded a gene may be encoded on either strand, which doubles that to six. Scanning only the forward strand would miss any gene transcribed in the opposite direction, which is common in compact bacterial and viral genomes.
- What minimum ORF length should I use?
- For gene finding, 100 codons is a common threshold. In random sequence with balanced composition, 3 of the 64 codons are stops, so a stop appears on average roughly every 21 codons, which means short ORFs are abundant by chance while ORFs beyond about 100 codons are rare. Lower the threshold when you are deliberately looking for small peptides.
- Does the ORF finder recognize alternative start codons such as GTG?
- No, it starts only at ATG. Many bacteria use GTG or TTG as alternative initiation codons, and those genes will be reported starting from a downstream internal ATG or missed entirely, so for prokaryotic annotation treat the output as a first pass and confirm the true start with a dedicated gene prediction tool.