Nucleic Acids To Amino Acids: DNA Specifies Protein - Nature

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A diagram outlines a simplified experimental method used to match an mRNA triplet nucleotide sequence with the amino acid it encodes. The experimental approach is illustrated in a two-step schematic diagram, and test tubes depict mRNA synthesis and protein synthesis steps. Three mRNA triplets matched with their corresponding amino acids are shown at the bottom to illustrate the conclusion.View Full-Size ImageFigure 1 Figure Detail Once the budding molecular biology community was convinced about the triplet code, the race to decode which triplets specified which amino acids began. The simplest way to decipher the code would be to start with an mRNA molecule of known sequence, use it to direct the synthesis of a protein, and then determine the amino acid sequence of the synthesized protein. Then, comparison of the original mRNA sequence with the amino acid sequence of the synthesized protein could provide a means for directly decoding the genetic code (Figure 1).

However, at the time when this decoding project was conducted, researchers did not yet have the benefit of modern sequencing techniques. To circumvent this challenge, Marshall W. Nirenberg and Heinrich J. Matthaei (1962) made their own simple, artificial mRNA and identified the polypeptide product that was encoded by it. To do this, they used the enzyme polynucleotide phosphorylase, which randomly joins together any RNA nucleotides that it finds. Nirenberg and Matthaei began with the simplest codes possible. Specifically, they added polynucleotide phosphorylase to a solution of pure uracil (U), such that the enzyme would generate RNA molecules consisting entirely of a sequence of U's; these molecules were known as poly(U) RNAs. Each poly(U) RNA thus contained a pure series of UUU codons, assuming a triplet code. These poly(U) RNAs were added to 20 tubes containing components for protein synthesis (ribosomes, activating enzymes, tRNAs, and other factors). Each tube contained one of the 20 amino acids, which were radioactively labeled. Of the 20 tubes, 19 failed to yield a radioactive polypeptide product. Only one tube, the one that had been loaded with the labeled amino acid phenylalanine, yielded a product. Nirenberg and Matthaei had therefore found that the UUU codon could be translated into the amino acid phenylalanine. Similar experiments using poly(C) and poly(A) RNAs showed that proline was encoded by the CCC codon, and lysine by the AAA codon.

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