RNA Codons

RNA codons are three-nucleotide units that either code for specific amino acids in a protein being synthesized or else signal the synthesis to either start or stop. For example, the codon CUA codes for the amino acid leucine. 

Table 27.4 in the text lists the messenger RNA codons for the various amino acids. The codons for valine and for glutamic acid are ... 

Streptomycin Interferes with pairing between aminoacyl tRNAs and RNA codons, causing misreading. 

This table gives the correspondence between a messenger RNA codon and the amino acid which it specifies. The symbols for bases in the codon are ... 

With degeneracy of the genetic code it is possible to have different base sequences in a gene and yet code for the same sequence of amino acids. In coding for leucine, e.g., the RNA codons for leucine are CUU and CUA which correspond to 33.3% (G -I- C), while its other two RNA codons are CUC and CUG and these are 66.7% (G + C). If the first codon only were used to code for leucine in a gene, and all other amino acids were coded with a similar bias in G -I- C, then the gene would have a G C/A T ratio of 1 2. This extreme situation never actually arises in nature because not all amino acids have four different codons. 

The large (SOS) ribosomal subunit is where protein synthesis, catalyzed formation of peptide bonds, takes place. In light of the discoveries by Thomas Cech and Sidney Altman of ribozymes, RNA molecules that behave as enzymes, the question of catalysis of protein synthesis by RNA or proteins in the ribosome was a major one. The 3 OS ribosomal subunit is where transfer RNAs bind with ribosomal RNA codons. The 50S ribosomal subunit can be further separated into a 23S secondary subunit and a smaller 5S secondary subunit that are normally held together by protein molecules. The 23S subunit includes 3045 nucleotides and 31 proteins. There are six discrete RNA domains in the S23 unit particle. The 5S unit effectively adds a seventh domain. The proteins permeate the exterior of the RNA of S23, but are located over 18 A distant from the active site of catalytic protein bond formation. The structures of complexes of the S23 subunit with two inactive substrate molecules suggest mechanistic similarities to the enzyme chymotrypsin. The S23 structure indicates that an adenine base at the active site plays a role analogous to that of histidine-... 

Alternative RNA Codons for each amino acid residue. 

Fig. 1. The genetic code. Each of the amino acids is shown along with its corresponding RNA codons.

Stop codon (1) The last codon to be translated in a molecule of mRNA, causing the ribosome to release from the mRNA. (2) RNA codons that signal a ribosome to stop translating an mRNA and to release the polypeptide. In the normal genetic code, they are UAG, UGA, and UAA. 

The code for each amino acid in the protein is stored in a three base sequence of DNA which is transcribed in a complementary triplet in the messenger RNA. The information of the messenger RNA triplet is translated into an amino acid by the apposition on the messenger RNA codon of a complementary triplet anticodon of a specific tRNA. It is therefore important that each tRNA be esterified by the proper amino acid. This means that the binding of tRNA to tRNA synthetase must be rigidly specific, and the specificity is believed to reside in the tertiary structure of a critical portion of the tRNA molecule. 

Figure 12.14. The RNA codons for mRNA read from DNA and are installed on the ribosome to be read by tRNA bringing the amino acids to the ribosome for peptide (protein) synthesis. Note that AUG is both the start codon (and thus requires the tRNA anticodon of UAC for the first amino acid, methionine [Met, M]) as well as the codon for methionine (Met, M) that might be found elsewhere, and that UAA, UGA, and UAG are stop codons. Note, too, that this 4 X 16 array, with 64 combinations, is redundant for the 20 amino acids, and thus some amino acids are specified in more than one way, while the start codon is unique. Information in this table was obtained from Nirenberg, M Leder, R Bernfield, M. Brimacombe, R. Trupin, I Rottman, F. O Neal, C. Nat. Acad. Sd. U.S., 1965,53,1161 and subsequent work.

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