Open reading frame (ORF), in the context of genomics, refers to a section of a DNA sequence that lacks a stop codon and potentially codes for a protein. A codon, which is a sequence of three nucleotides, is the basic unit of genomic information that represents a specific amino acid or signals the termination of protein synthesis. An ORF, usually found in a prokaryotic DNA sequence, spans from the start to stop codons and is bounded by stop codons.
In eukaryotic genes with multiple exons, an ORF only applies to spliced mRNAs, not genomic DNA, due to the presence of introns which can contain stop codons and/or cause shifts between reading frames. Another definition of ORF, useful in transcriptomics and metagenomics, defines it as a sequence that is divisible by three and is bounded by stop codons. In this context, an ORF represents parts of a gene rather than the entire gene.
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