The Reading Frame and Initiation Codons

Findings from early studies of the genetic code indicated that it is generally nonoverlapping. An overlapping code is one in which a single nucleotide is included in more than one codon, as shown in 4 Figure 15.14. Usually, however, each nucleotide sequence of an mRNA specifies a single amino acid. A few overlapping codes are found in viruses; in these cases, two different proteins may be encoded within the same sequence of mRNA.

For any sequence of nucleotides, there are three potential sets of codons—three ways that the sequence can be read in groups of three. Each different way of reading the sequence is called a reading frame, and any sequence of nucleotides has three potential reading frames. The three reading frames have completely different sets of codons and therefore will specify proteins with entirely different amino acid sequences. Thus, it is essential for the

Table 15.2 The wobble rules, indicating which bases in the third position (3' end) of the mRNA codon can pair with bases at the first (5' end) of the anticodon of the tRNA

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