Harpers

360 / CHAPTER 38Table 38–2. Features of the genetic code.• Degenerate• Unambiguous• Nonoverlapping• Not punctuated• Universaloften need to deduce mRNA structure from the primarysequence of a portion of protein in order to synthesizean oligonucleotide probe and initiate a recombinantDNA cloning project. The main features of thegenetic code are listed in Table 38–2.AT LEAST ONE SPECIES OF TRANSFERRNA (tRNA) EXISTS FOR EACH OF THE20 AMINO ACIDStRNA molecules have extraordinarily similar functionsand three-dimensional structures. The adapter functionof the tRNA molecules requires the charging of eachspecific tRNA with its specific amino acid. Since thereis no affinity of nucleic acids for specific functionalgroups of amino acids, this recognition must be carriedout by a protein molecule capable of recognizing both aspecific tRNA molecule and a specific amino acid. Atleast 20 specific enzymes are required for these specificrecognition functions and for the proper attachment ofthe 20 amino acids to specific tRNA molecules. Theprocess of recognition and attachment (charging)proceeds in two steps by one enzyme for each of the 20amino acids. These enzymes are termed aminoacyltRNAsynthetases. They form an activated intermediateof aminoacyl-AMP-enzyme complex (Figure 38–1).The specific aminoacyl-AMP-enzyme complex thenrecognizes a specific tRNA to which it attaches theaminoacyl moiety at the 3′-hydroxyl adenosyl terminal.The charging reactions have an error rate of less than10 −4 and so are extremely accurate. The amino acid remainsattached to its specific tRNA in an ester linkageuntil it is polymerized at a specific position in the fabricationof a polypeptide precursor of a protein molecule.The regions of the tRNA molecule referred to inChapter 35 (and illustrated in Figure 35–11) now becomeimportant. The thymidine-pseudouridine-cytidine(TΨC) arm is involved in binding of the aminoacyl-tRNAto the ribosomal surface at the site ofprotein synthesis. The D arm is one of the sites importantfor the proper recognition of a given tRNA speciesby its proper aminoacyl-tRNA synthetase. The acceptorarm, located at the 3′-hydroxyl adenosyl terminal, is thesite of attachment of the specific amino acid.The anticodon region consists of seven nucleotides,and it recognizes the three-letter codon in mRNA (Figure38–2). The sequence read from the 3′ to 5′ directionin that anticodon loop consists of a variablebase–modified purine–XYZ–pyrimidine–pyrimidine-5′. Note that this direction of reading the anticodon is3′ to 5′, whereas the genetic code in Table 38–1 is read5′ to 3′, since the codon and the anticodon loop of themRNA and tRNA molecules, respectively, are antiparallelin their complementarity just like all other intermolecularinteractions between nucleic acid strands.The degeneracy of the genetic code resides mostly inthe last nucleotide of the codon triplet, suggesting thatthe base pairing between this last nucleotide and thecorresponding nucleotide of the anticodon is not strictlyATPPP iAMP + EnzOOHOOCHCREnz • Adenosine O P O C CH RH 2 NOHNH 2Amino acid (aa)Enzyme (Enz)AMINOACYLtRNASYNTHETASEEnz•AMP-aa(Activated amino acid)Aminoacyl-AMP-enzymecomplextRNAtRNA-aaAminoacyl-tRNAFigure 38–1. Formation of aminoacyl-tRNA. A two-step reaction, involving the enzymeaminoacyl-tRNA synthetase, results in the formation of aminoacyl-tRNA. The first reaction involvesthe formation of an AMP-amino acid-enzyme complex. This activated amino acid is nexttransferred to the corresponding tRNA molecule. The AMP and enzyme are released, and the lattercan be reutilized. The charging reactions have an error rate of less than 10 –4 and so are extremelyaccurate.