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Codon translational machinery to use the correct reading frame. How is the correct reading frame established? The reading frame is set by the initiation codon, which is the first codon of the mRNA to specify an amino acid. After the initiation codon, the other codons are read as successive groups of three nucleotides. No bases are skipped between the codons; so there are no punctuation marks to separate the codons.

The initiation codon is usually AUG, although GUG and UUG are used on rare occasions. The initiation codon is not just a punctuation mark; it specifies an amino acid. In bacterial cells, AUG encodes a modified type of methionine, N-formylmethionine; all proteins in bacteria begin with this amino acid, but the formyl group (or, in some cases, the entire amino acid) may be removed after the protein has been synthesized. When the codon AUG is at an internal

Nucleotide sequence

Nonoverlapping code

Overlapping code

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