The translation machinery decodes only a portion of each mRNA. As we saw in Chapter 'I, and will consider in detail in Chapter 15, the information for protein synthesis is in the form of threu-nucleotide crtdons, which each specify one amino acid. The protein coding re-gion(s) of each mRNA is composed of a contiguous, non-overlapping string of codons called an open-reading frame (commonly known as an OKF). Each ORF specifies a single protein and starts and ends al internal sites within the mRNA. That is. the ends of an ORF are distinct from the ends of the mRNA.
Translation starts at the 5' end of the open-reading frame and proceeds one codon at a time to the 3' end. The first and last codons of an ORF are known as the start and stnp codons. In bacteria, the start codon is usually 5'-AUC-3' but 5'-CUG-3' and sometimes even 5'-UUG-3' are also used. Eukaryotic cells always use 5'-AUG-3' as the start codon. This codon has two important functions. First, it specifies the first amino acid to be incorporated into the growing polypeptide chain. Second, it defines the reading frame for all subsequent codons. Because codons are immediately adjacent to each other and because codons ere three nucleotides long, any stretch of mRNA could he translated in three different reading frames (Figure 14-1). Mowever, once translation starts, each subsequent codon is always immediately adjacent to (but not overlapping) the previous three-base codnn. Thus, by setting the location of the Erst codon, the start codon determines the location of all following codons.
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