Protein plays a central role in cell function and cell structure. The genetic information within the nucleotide sequence of DNA is transcribed into a specific nucleotide sequence of an mR NA molecule. Several different classes of RNA combine to direct the synthesis of proteins. Cells must possess the machinery necessary to translate information accurately and efficiently from the nucleotide sequence of an mRNA into the sequence of amino acids of the corresponding specific protein. This process is termed translation, which is the critical step in the expression of genetic information.
Messenger RNAs(mRNA) are single-strand products of the transcription of genomic DNA, with special features that destine them to become attached ribosomes and function properly in translation. Genetic information within mRNA is called genetic code. Since there are only 4 different nucleotides in mRNA, each codon consists of a sequence of 3 nucleotides; it is a triplet code. Three codons do not code for specific amino acids. These have been termed nonsense codons, which are utilized as termination signals; they specify where the polymeriza-tion of amino acids into a protein molecule is to stop. The remaining 61 codons code for 20 amino acids. Thus, there must be “degeneracy” in the genetic code; i. e, multiple codons must decode the same amino acid. The genetic code is non-overlapping. Furthermore, once the reading is commenced at a specific codon, there is no punctuation between codons, and the message is read in a continuing sequence of nucleotide triplets until a nonsense codon is reached. The genetic code is almost, but not quite, universal. Throughout the whole of the prokaryotic, plant, and animal kingdoms the same codons are used for the same amino acids with very few exceptions.
Any cell, prokaryotic or eukaryotic, contains a battery of different types of transfer RNA (tRNA) molecules which is capable of incorporating all 20 amino acids into protein. Each tRNA molecule contains a specific sequence, complementary to a codon, which is termed its anticodon. For a given codon in mRNA, only a single species of tRNA molecules possesses the proper anticodon. Since each tRNA molecule can be charged with only one specific amino acid, each codon therefore specifies only one amino acid. However, some tRNA molecules can utilise the anticodon to recognize more than one codon. The nucleotide in the anticodon that recognises the third (3′-) base of the codon could be less discriminating or nondiscriminating and still manage to insert the proper amino acid. This reduced stringency between the third base of the codon and the complementary nucleotide in the anticodon is referred to as “wobble”.
The adapter function of the tRNA molecules requires the charging of each specific tRNA with its specific amino acid. This recognition must be carried out by a protein molecule capable of recognising both a specific tRNA molecule and a specific amino acid. At least 20 specific enzymes are required for these specific recognition functions and for the proper attachment of the 20 amino acids to specific tRNA molecules. These enzymes are termed aminoa-cyl-tRNA synthetases. The process of recognition and attachment is carried out in two steps. First, the amino acid is activated by ATP to form an aminoacyi adenylate. While bound to the enzyme, this intermediate reacts with the correct tRNA to form the covalent bond and release AMP. It is essential that this matching of amino acid and tRNA be very accurate.