MRNA codon

Among the 76 nucleotides of tRNA are two sets of three that are especially important The first is a group of three bases called the anticodon, which is comple mentary to the mRNA codon for the ammo acid being transferred Table 28 3 lists two mRNA codons for phenylalanine UUU and UUC (reading m the 5 3 direction) Because base pairing requires the mRNA and tRNA to be antiparallel the two anticodons are read m the 3 5 direction as AAA and AAG... 

As described in the preceding sections protein synthesis involves transcription of the DNA to rtiRNA followed by translation of the mRNA as an amino acid sequence In addition to outlining the mechanics of transcription we have described the relationship among mRNA codons tRNA anticodons and ammo acids... 

FIGURE 28 12 Translation of mRNA to an ammo acid sequence of a protein starts at an mRNA codon for methionine Nucleophilic acyl substitution transfers the N formylmethionme residue from Its tRNA to the ammo group of the next ammo acid (shown here as alanine) The process converts an ester to an amide... 

From what DNA sequences were each of the mRNA codons in Problem 28.21 transcribed ... 

Draw the complete structure of the deoxyribonucleotide sequence from which the mRNA codon in Problem 28.24 was transcribed. 

A charged tRNA binds in the A site. The particular aminoacyl-tRNA is determined by the mRNA codon aligned with the A site. 

Figure 13.3 The process of protein synthesis on the ribosome. The strand of mRNA is shown associated with the small subunit of the ribosome. The aminoacyl-tRNA molecules are shown associated with the large subunit of the ribosome and base-paired with mRNA codons. A peptide bond is in the process of formation between the two associated amino acids, extending the growing polypeptide chain by one unit. On the left, a tRNA is shown leaving the ribosome, having donated its amino acid to the growing chain. On the right, an aminoacyl-tRNA molecule is shown entering the ribosome. It is next in line to contribute its amino acid to that chain.

The ribosome is both the site of protein synthesis and an active participant in the process. The eukaryotic ribosome is constructed from two subunits the smaller 40S subunit and the larger 60S subunit. Basically, the 40S subunit binds the mRNA and monitors the recognihon between the mRNA codon and tRNA anticodon. The 60S subunit has the binding sites for aminoacyl-tRNAs and catalyzes the formation of peptide bonds. Remarkably, the catalytic entity for peptide bond formahon in the 60S subunit is the RNA component, not the protein component. Therefore, the 60S subunit acts as a ribozyme. 

The actual information transfer is based on the interaction between the mRNA codons and another type of RNA, transfer RNA (tRNA see p. 82). tRNAs, of which there are numerous types, always provide the correct amino acid to the ribosome according to the sequence information in the mRNA. tRNAs are loaded with an amino acid residue at the 3 end. Approximately in the middle, they present the triplet that is complementary to each mRNA codon, known as the anticodon (GAA in the example shown). If the codon UUC appears on the mRNA, the anticodon binds a molecule of Phe-t-RNA to the mRNA (5) and thus brings the phenylalanine residue at the other end of the molecule into a position in which it can take over the growing polypeptide chain from the neighboring tRNA (6). 

The aminoglycosides decrease the fidelity of translation by binding to the 30S subunit of the ribosome. This permits the formation of the peptide initiation complex but prohibits any subsequent addition of amino acids to the peptide. This effect is due to the inhibition of polymerization as well as to the failure of tRNA and mRNA codon recognition. Aminoglycosides are ototoxic (i.e., may produce partial deafness), damaging the auditory nerve. Kanamycin is less toxic. Since aminoglycosides are concentrated in the kidney, they may occasionally cause kidney damage. 

It is bactericidal drug and exerts its action by combining with bacterial ribosome and induces misreading of mRNA codons. Also in sensitive bacteria, disruption of cytoplasmic membrane occurs resulting in leakage of amino acids, ions, leading to bacterial death. 

Transfer RNAs base-pair with mRNA codons at a three-base sequence on the tRNA called the anticodon. The first base of the codon in mRNA (read in the 5 —>3 direction) pairs with the third base of the anticodon (Fig. 27-8a). If the anticodon triplet of a tRNA recognized only one codon triplet through Watson-Criclc base pairing at all three positions, cells would have a different tRNA for each amino acid codon. This is not the case, however, because the anticodons in some tRNAs include the nucleotide inosinate (designated I), which contains the uncommon base hypoxanthine (see Fig. 8-5b). Inosinate can form hydrogen bonds with three different nucleotides (U, C, and A Fig. 27-8b), although... 

The first two bases of an mRNA codon always form strong Watson-Criclc base pairs with the corresponding bases of the tRNA anticodon and confer most of the coding specificity. 

Amino acids are specified by mRNA codons consisting of nucleotide triplets. Translation requires adaptor molecules, the tRNAs, that recognize codons and insert amino acids into their appropriate sequential positions in the polypeptide. 

Stage 2 Initiation The mRNA bearing the code for the polypeptide to be made binds to the smaller of two ri-bosomal subunits and to the initiating aminoacyl-tRNA. The large ribosomal subunit then binds to form an initiation complex. The initiating aminoacyl-tRNA base-pairs with the mRNA codon AUG that signals the beginning of the polypeptide. This process, which requires GTP, is promoted by cytosolic proteins called initiation factors. 

Each tRNA has an amino acid arm with the terminal sequence CCA(3 ) to which an amino acid is esterified, an anticodon arm, a Ti//C arm, and a D arm some tRNAs have a fifth arm. The anticodon is responsible for the specificity of interaction between the aminoacyl-tRNA and the complementary mRNA codon. 

Complementary, antiparallel bindinc of the anticodon for methionyl-tRNA (CAU) to the mRNA codon for methionine (AUG). 

Binding of the tRNA anticodon to the mRNA codon follows the rules of complementary and antiparallel binding, that is, the mRNA codon is "read" 5 ->3 by an anticodon pairing in the "flipped" (3 —>5 ) orientation (Figure 31.9). [Note When writing the sequences of bolh codons and anticodons, the nucleotide sequence must ALWAYS be listed in the 5 —>3 order.]... 

Translation involves transfer RNA. (a) The structure of a transfer RNA molecule, with an anticodon at one end and an amino acid attachment site at the other end. (b) A highly simplified symbol for tRNA. The anticodon is a series of three nucleotides that complement the mRNA codon and code for a specific amino acid at the amino acid attachment site. 

As the mRJSlA leaves the cell nucleus in which it was created and enters the cytoplasm, it binds with specialized structures called ribosomes, as shown in Figure 13.36. Ribosomes are microscopic complexes of rRNA and proteins, and they are the site where proteins are built. As the mRNA is scrolled sequentially over the ribosome, the anticodon end of a free tRNA molecule binds to an mRNA codon. In this manner, tRNA molecules and their tag-along amino acids are placed adjacent to one another along the mRNA strand. The amino acids then chemically bond with one another, forming a long polypeptide chain that breaks away from the tRNA as it forms. This process continues until a stop mRNA codon, for which there are no tRNA anticodons, is encountered. At this point, the primary structure of a new protein has been built. 

Figure 25-28 Peptide-bond formation in protein biosynthesis showing how the amino-acid sequence is determined by complementary basepairing between messenger RNA and transfer RNA, The peptide chain is bound to tRNA, which is associated with mRNA through three bases in mRNA (codon) and three bases in tRNA (anticodon). In the diagram, the next codon A-A-G codes for lysine. Hence, Lys-tRNA associates with mRNA by codon-anticodon base-pairing and, under enzyme control, couples to the end of the peptide chain.

Table 6.1 shows the relationship between the codon sequence in mRNA and its corresponding amino acid in the new protein. Because there are 64 (43) different anticodon combinations and only 20 encoded amino acids, some different anticodon sequences encode for the same amino acid. Generally, all the anticodons matching a given amino acid will have the same first two nucleotides. Exceptions are arginine, serine, and isoleucine. For example, the codon for proline will always start with CC, but the arginine codon may start with either AG or CG. The 3 end of the tRNA anticodon pairs with the 5 end of the mRNA codon. In other words, the codon and anticodon align and bind in an antiparallel fashion. 

TABLE 6.1 Translation of mRNA codons to amino acids3... 

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