How are triplet codons determined?

The determination of triplet codons was a landmark achievement in molecular biology, resolved through a combination of elegant theoretical reasoning and decisive experimental work in the early 1960s. The central problem was deciphering the genetic code: how a sequence of four nucleotides in DNA and RNA could specify the sequence of twenty amino acids in proteins. Francis Crick, Leslie Barnett, Sydney Brenner, and others provided the crucial theoretical framework from their work on bacteriophage T4. They demonstrated through frameshift mutation experiments that the code is read in a non-overlapping manner from a fixed starting point in groups of three bases. A mutation that inserted or deleted a single nucleotide could disrupt the entire downstream sequence, but the insertion of three nucleotides could sometimes restore a functional protein. This proved the code was a triplet code, where each amino acid is encoded by a specific group of three nucleotides, or codon, and that these codons are read sequentially without gaps.

The specific assignment of which triplet codon corresponds to which amino acid was accomplished primarily by the biochemical experiments of Marshall Nirenberg, Heinrich Matthaei, and later Har Gobind Khorana. Nirenberg and Matthaei's 1961 experiment was groundbreakingly direct. They created a synthetic RNA polymer composed solely of uracil nucleotides (poly-U) and added it to a cell-free protein-synthesizing system. The system produced a polypeptide chain composed only of phenylalanine, thereby definitively establishing that the RNA codon UUU codes for phenylalanine. This approach formed the core methodology: using other synthetic RNA polymers, like poly-A (which produced lysine) and poly-C (which produced proline), allowed for the initial assignments. However, these homopolymers could only reveal codons for a single repeated nucleotide.

To decipher the more complex mixed codons, two subsequent methodological advances were critical. First, Nirenberg and Philip Leder developed a binding assay where specific triplet RNA sequences (like GUU) could be shown to bind to a corresponding tRNA charged with a specific amino acid (valine in this case), allowing for the rapid testing of numerous synthetic triplets. Second, Khorana's work provided systematic confirmation and completed the code by using chemically synthesized RNA polymers with defined repeating sequences. For example, synthesizing a polymer with the alternating sequence UCUCUC... produced a protein with alternating serine and leucine, allowing the deduction that UCU and CUC were the codons for these amino acids. By combining these techniques—the homopolymer synthesis, the triplet binding assay, and the synthesis of complex repeating copolymers—researchers systematically tested all 64 possible triplet combinations.

The outcome was a complete codon table, revealing key mechanistic features of the genetic code that have profound implications. It was shown to be degenerate, with most amino acids encoded by more than one codon (synonyms), often differing in their third nucleotide. This redundancy provides a buffer against mutational errors. The code also contained specific punctuation marks: stop codons (UAA, UAG, UGA) to terminate translation, and the start codon AUG, which codes for methionine and initiates the process. This universality across nearly all known life forms points to a common evolutionary origin. The determination of the triplet codons was not merely a cataloging exercise; it provided the fundamental rulebook for how genetic information is translated into function, enabling all subsequent research in genetic engineering, biotechnology, and our understanding of genetic disease.