Anticodons, of tRNA

At one end of the loop of tRNA there is a ribonucleotide triplet called anticodon which is complementary to a codon on mRNA. Each codon of mRNA is read in a serial order by an anticodon of tRNA and matched. If matching occurs, the tRNA transfers the desired amino acid to the growing polypeptide chain on the ribosome. When the synthesis of a specific protein is completed, a stop codon signals the end and the synthesized protein is released from the ribosome. 

Nonsense suppressors are produced by tethering a nonnatural amino acid to a stop (or nonsense ) anticodon in tRNA. As a result, the stop codon in an mRNA sequence is converted from a protein synthesis termination site to a site at which the nonnatural amino acid may specifically be inserted. DNA base substitutions that correspond to the stop anticodon of tRNA may thus be made in order to specifically incorporate nonnatural amino acids into proteins. 

Note that synonyms are not distributed haphazardly throughout the genetic code (depicted in Table 5,4). An amino acid specified by two or more synonyms occupies a single box (unless it is specified by more than four synonyms). The amino acids in a box are specified by codons that have the same first two bases but differ in the third base, as exemplified by GUU, GUC, GUA, and GUG. Thus, most synonyms differ only in the last base of the triplet. Inspection of the code shows that XYC and XYU always encode the same amino acid, whereas XYG and XYA usually encode the same amino acid. The structural basis for these equivalences of codons will become evident when we consider the nature of the anticodons of tRNA molecules (Section 29.3.9). 

The Activated Amino Acid and the Anticodon of tRNA Are at Opposite Ends of the L-Shaped Molecule... 

The wobble hypothesis is now firmly established. The anticodons of tRNAs of knovm sequence bind to the codons predicted by this hypothesis. For example, the anticodon of yeast alanyl-tRNA is IGC. This tRNA recognizes the codons... 

During elongation, as mRNA moves through the ribosome in association with die anticodons of tRNA, which site is occupied by the growing protein attached to a tRNA ... 

A priori, the anticodon of tRNA would seem to be a good identifier because each type of tRNA has a different one. 

Ribosomal RNA (rRNA) is a component of the ribosomes, the protein synthesis factories in the cell. rRNA molecules are extremely abundant, making up at least 80 percent of the RNA molecules found in a typical eukaryotic cell. Virtually all ribosomal proteins are in contact with rRNA. Most of the contacts between ribosomal subunits are made between the 16S and 23 S rRNAs such that the interactions involving rRNA are a key part of ribosome function. The environment of the tRNA-binding sites is largely determined by rRNA. The rRNA molecules have several roles in protein synthesis. 16S rRNA plays an active role in the functions of the 308 subunit. It interacts directly with mRNA, with the 508 subunit, and with the anticodons of tRNAs in the P- and A-sites. Peptidyl transferase activity resides exclusively in the 238 rRNA. Finally, the rRNA molecules have a structural role. They fold into three-dimensional shapes that form the scaffold on which the ribosomal proteins assemble. 

The mechanism of translation makes nse of the same complementary base pairing principle nsed in replication and transcription. Each amino acid is associated with a particular tRNA. Transfer RNA is much smaller than DNA and mRNA. It is single-stranded and contains 70-90 ribonucleotides arranged in a cloverleaf pattern (Figure 27.28). Its characteristic shape results from the presence of paired bases in some regions and their absence in others. All tRNAs have a CCA triplet at their 3 terminus, to which is attached, by an ester linkage, an amino acid unique to that particular tRNA. At one of the loops of the tRNA there is a nucleotide triplet called the anticodon, which is complementary to a codon of mRNA. The codons of mRNA are read by the anticodons of tRNA, and the proper amino acids are transferred in sequence to the growing protein. 

Codon Three consecutive nucleotides in mRNA that specify a given amino acid during protein synthesis the anticodon of tRNA is complementary to the mRNA codon. 

The base hypoxanthine is found in the anticodon of tRNA molecules (it is formed by the deamination of an adenine base). Hypoxanthine s role in tRNA is to allow wobble base-pairs to form, as the base hypoxanthine can base pair with adenine, cytosine, or uracil. The wobbling allows one tRNA molecule to potentially form base pairs with three different codons. 

Polypeptide synthesis proceeds at peptidyl iP) and aminoacyl (.4) sites in the ribosome (Fig. 21-11). The synthesis in Fig. 21-11 shows initiation followed by sequential addition of Gly and lie. The 5 end of mRNA, located in the smaller subunit of the ribosome, is prepared to receive the tRNAs for Met and Gly (by matching of codon of mRNA and anticodon of tRNA). The tRNAs for Met and Gly pick up Met and Gly from the cytosol, enter the ribosome, and line up at the P and A sites, respectively, by base pairing of codons of mRNA and anticodons of tRNA (Fig. 21-1 la). The enzyme peptidyl transferase, contained in the large subunit of the ribosome, catalyzes the transfer and... 

All the interactions between nucleic acid molecules that help express genetic information involve base-pairing between complementary sequences. Complementarity is sometimes defined as selective stickiness. Complementary molecules fit together. In the case of nucleic acids, complementarity generally involves base pairing. For example, mRNA is complementary to one strand of DNA, and the anticodon of tRNA is complementary to the codon in mRNA. Replication, transcription, and translation all involve base-pairing at several levels. 

Indirect evidence with synthetic polynucleotides had already suggested that at least two bases—U and A— might be part of the triplet responsible for termination. With most polypeptides used as surrogate messenger, only 10% of the polypeptides synthesized are released however, 60% of the newly made polypeptides are released when the coding polypeptide contains A and U. An important difference between the initiation and termination codon is that initiation requires that an anticodon of tRNA charged with a formyl methionine be lined up, but termination does not require that the codon be coupled with the appropriate anticodon. 

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