Key Concepts of Section 5.3

Key Concepts of Section 5.3

The Decoding of mRNA by tRNAs

  • Genetic information from DNA is carried by mRNA in the form of a non-overlapping, degenerate triplet code.

  • Each amino acid is encoded by one or more three-nucleotide sequences (codons) in mRNA. Each codon specifies one amino acid, but most amino acids are encoded by multiple codons (see Table 5-1).

  • The AUG codon for methionine is the most common start codon, specifying the amino acid at the amino-terminus of a protein chain. Three codons (UAA, UAG, UGA) function as stop codons and specify no amino acids.

  • A reading frame, the uninterrupted sequence of codons in mRNA from a specific start codon to a stop codon, is translated into the linear sequence of amino acids in a polypeptide chain.

  • Decoding of the nucleotide sequence in mRNA into the amino acid sequence of proteins depends on tRNAs and aminoacyl-tRNA synthetases.

  • All tRNAs have a similar three-dimensional structure that includes an acceptor stem for attachment of a specific amino acid and a stem-loop with a three-base anticodon sequence at its end (see Figure 5-20). The anticodon can base-pair with its corresponding codon in mRNA.

  • Because of nonstandard interactions, a tRNA may base-pair with more than one mRNA codon; conversely, a particular codon may base-pair with multiple tRNAs. In each case, however, only the proper amino acid is inserted into a growing polypeptide chain.

  • Each of the 20 aminoacyl-tRNA synthetases recognizes a single amino acid and covalently links it to a cognate tRNA, forming an aminoacyl-tRNA (see Figure 5-19). This reaction activates the amino acid so that it can participate in peptide bond formation