Complementary base pairing codon-anticodon interactions

Translation begins as each ribosome binds an mRNA molecule and proceeds to convert its base sequence into a polymer of amino acids linked by peptide bonds. Each amino acid is specified by a code word, called a codon, that consists of three sequential bases. The actual transfer of information occurs when each mRNA codon interacts and forms complementary base pairs with a three-base sequence in a transfer RNA (tRNA) molecule called an anticodon. 

The translation process that occurs at the site of the ribosomes in the cytoplasm requires that the mRNA designate the codons that then specify the amino acid sequence for the protein. The codons on the mRNA must interact with the anticodons on the charged tRNA molecules, which bring to the site the specific amino acid residues. Watson-Crick complementary base pairing provides the specificity for this interaction. 

Each tRNA has an anticodon sequence that allows it to pair with the codon for its cognate amino acid in the mRNA. Because base pairing is involved, the orientation of this interaction wEl be complementary and antiparallel as shown in Figure T4-6. The arg-tRNA 8 has an anticodon sequence, UCG, allowing it to pair with the arginine codon CGA. 

The pioneering molecular biologists recognized that, because amino acids cannot bind directly to the sets of three nucleotides that form their codons, adapters are required. The adapters were found to be tRNA molecules. Each tRNA molecule contains an anticodon and covalently binds a specific amino acid at its 3 -end (see Chapters 12 and 14). The anticodon of a tRNA molecule is a set of three nucleotides that can interact with a codon on mRNA (Fig. 15.2). To interact, the codon and anticodon must be complementary (i.e., they must be able to form base pairs in an antiparallel orientation). Thus, the anticodon of a tRNA serves as the link between an mRNA codon and the amino acid that the codon specifies. 

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. 

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