SUMMARY

• 39.1 The Genetic Code Links Nucleic Acid and Protein Information

  Protein synthesis is called translation because information present as a nucleic acid sequence is translated into a different language, the sequence of amino acids in a protein. The genetic code is the relation between the sequence of bases in DNA (or its RNA transcript) and the sequence of amino acids in proteins. Amino acids are encoded by groups of three bases (called codons) starting from a fixed point. Sixty-one of the 64 codons specify particular amino acids, whereas the other 3 codons are signals for chain termination. Thus, for most amino acids, there is more than one code word. In other words, the code is degenerate. The genetic code is nearly the same in all organisms. Messenger RNAs contain start and stop signals for translation, just as genes do for directing where transcription begins and ends.

  Translation is mediated by the coordinated interplay of more than a hundred macromolecules, including mRNA, rRNAs, tRNAs, aminoacyl-tRNA synthetases, and protein factors. Given that proteins typically consist of 100 to 1000 amino acids, the frequency at which an incorrect amino acid is incorporated in the course of protein synthesis must be less than 10^–4. Transfer RNAs are the adaptors that make the link between a nucleic acid and an amino acid. These molecules, single strands of about 80 nucleotides, have an L-shaped structure.

• 39.2 Amino Acids Are Activated by Attachment to Transfer RNA

  Each amino acid is activated and linked to a specific transfer RNA by an enzyme called an aminoacyl-tRNA synthetase. Such an enzyme links the carboxyl group of an amino acid to the 2′- or 3′-hydroxyl group of the adenosine unit of a CCA sequence at the 3′ end of the tRNA by an ester linkage. There is at least one specific aminoacyl-tRNA synthetase and at least one specific tRNA for each amino acid. A synthetase utilizes both the functional groups and the shape of its cognate amino acid to prevent the attachment of the incorrect amino acid to a tRNA. Some synthetases have a separate editing site at which incorrectly linked amino acids are removed by hydrolysis. A synthetase recognizes the anticodon, the acceptor stem, and sometimes other parts of its tRNA substrate. By specifically recognizing both amino acids and tRNAs, aminoacyl-tRNA synthetases implement the instructions of the genetic code.

• 39.3 A Ribosome Is a Ribonucleoprotein Particle Made of Two Subunits

  Protein synthesis takes place on ribosomes—ribonucleoprotein particles (about two-thirds RNA and one-third protein) consisting of large and small subunits. In E. coli, the 70S ribosome (2.7 MDa) is made up of 30S and 50S subunits. The 30S subunit consists of 16S ribosomal RNA and 21 different proteins; the 50S subunit consists of 23S and 5S rRNA and 34 different proteins.

  Proteins are synthesized in the amino-to-carboxyl direction, and the mRNA is read by the ribosome in the 5′ → 3′ direction. Because RNA is transcribed in the 5′ → 3′ direction also, the translation of mRNA in bacteria can begin before transcription is complete.