Amino Acids

Marshall Nirenberg and Heinrich Matthei, at the National Institute of Health, sought to create an artificially made m-RNA, and, in 1961, learned that UUU was the codon for phenylalanine. Nirenberg and Matthei produced an m-RNA made entirely of uracil (polyuradilic acid [U]), and, in 1964, were successful in getting ribosomes to accept this artificially produced m-RNA The m-RNA created by Nirenberg and Matthei was structured as shown in figure 6-9. When this m-RNA was run through the ribosomes, the resulting amino acids were always linked in the same manner, creating the polypeptide phenylalanine-phenylalanine-phenylalanine. With the discovery that the triplet UUU represented phenylalanine, a great deal of work followed to determine the codons for all twenty amino acids.

If the sequences of bases (that is, AGCT) on the DNA helix represent code words, how many bases (how many letters) are required to make the code word? The four bases AGCT can be put together in threes sixty-four different ways, more than is needed to yield twenty amino acids. There is also a certain amount of redundancy. For example, UCU, UCG, UCA, AGU, and AGC are all codons for the amino acid serine. Interestingly, several codons do not call for any amino acid but, rather, signal to the ribosome that the end of the molecule has been achieved. These termination codons are UAA, UAG, and UGA. Figure 6-10 shows all the combinations of bases (that is, of nucleotides) and the amino acids they represent.

Figure 6-9 An artificially made m-RNA molecule consisting of only uracil. UUU (uraciluracil- uracil) is a codon for the incorporation of phenylalanine into a protein. Such a chain of uracils was successfully taken up by a cell and passed through the ribosomes, thus producing a long chain of phenylalanines.

Figure 6-10 Matrix showing all the ways that the four bases adenine, guanine, cytosine, and uracil can be assembled in threes to make codons, the RNA triplets of -RNA. The first letters of the triplets are listed in the vertical column at left: the second letters of the triplets are listed horizontally across the top; and the third letters of the triplets are listed in the vertical column at right. The bases indicated by the triplets are abbreviated in the matrix; full names are given below the matrix