Open reading frame
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An open reading frame or ORF is a portion of an organism's genome which contains a sequence of bases that could potentially encode a protein. The start and stop ends of the ORF are not equivalent to the ends of the mRNA, but they are usually contained within the mRNA. In a gene, ORFs are located between the start-code sequence (initiation codon) and the stop-code sequence (termination codon). ORFs are usually encountered when sifting through pieces of DNA while trying to locate a gene. Since there exist variations in the start-code sequence of organisms with altered genetic code, the ORF will be identified differently. A typical ORF finder will employ algorithms based on existing genetic codes (including the altered ones) and all possible reading frames.
In fact, the existence of an ORF, especially a long one, is usually a good indication of the presence of a gene in the surrounding sequence. In this case, the ORF is part of the sequence that will be translated by the ribosomes, it will be long, and if the DNA is eukaryotic, the ORF may continue over gaps called introns. However, short ORFs can also occur by chance outside of genes. Usually ORFs outside genes are not very long and terminate after a few codons.
Once a gene has been sequenced it is important to determine the correct open reading frame (ORF). Theoretically, the DNA sequence can be read in six reading frames in organisms with double-stranded DNA; three in the forward and three in the reverse direction. The longest sequence without a stop codon usually determines the open reading frame. That is the case with prokaryotes. Eukaryotic mRNA is typically monocistronic and therefore only contains a single ORF. A problem arises when working with eukaryotic pre-mRNA long parts of the DNA within an ORF are not translated (introns). When the aim is to find eukaryotic open reading frames it is necessary to have a look at the spliced messenger RNA mRNA.
For example, if you have 5'-UCUAAAGGUCCA-3' it has 2 out of 3 reading frames possible. This is one of the 2 possible mRNA sequences of the transcript, and we see that it can be reading in the 3 possible ways: 1. UCU AAA GGU CCA 2. CUA AAG GUC etc 3. UAA AGG UCA etc As you can see, the 3rd possibility has a stop codon, thus only 2 out of the 3 reading frames are open (aka have no stop codons).
External links
- Translation and Open Reading Frames - A Site explaining Open Reading Frames
- NCBI ORF finder - A web based interactive tool for predicting and analysing ORFs from nucleotide sequences.
- ORF finder - A web based interactive tool for predicting and analysing ORFs from nucleotide sequences - hosted at bioinformatics.org
- Sequerome - A Sequence profiling tool that links each BLAST record to the NCBI ORF enabling complete ORF analysis of a BLAST reportca:Pauta oberta de lectura
de:Offener Leserahmenfr:Phase ouverte de lecture it:Open reading frame he:מסגרת הקריאה הפתוחה של הגן ja:オープンリーディングフレーム
Acknowledgement and Attribution Regarding Sources of Content
Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .
