![]() The commas would be of course, additional nucleotides. That is, perhaps there were the chemical equivalent of commas between the meaningful triplets. One speculation was that the code was punctuated. If only 20 triplets actually encoded amino acids, how would the translation machinery recognize the correct 20 codons to translate? What would prevent the translational machinery from ‘reading the wrong’ triplets, i.e., reading an mRNA out of phase? If for example, if the translation machinery began reading an MRNA from the second or third bases of a codon, it would likely encounter a meaningless 3-base sequence in short order. The next issue was whether there were only 20 meaningful codons and 44 meaningless ones. The protein it made was slightly less active than bacteria with genes with no deletions. On the other hand, bacteria containing a mutant version of the gene in which three bases were deleted were able to make the protein. Likewise, deleting two bases from the gene. They showed that bacteria with a single base deletion in the coding region of a gene failed to make the expected protein. ![]() Sidney Brenner and Frances Crick performed elegant experiments that directly demonstrated the non-overlapping genetic code. The genetic code must be non-overlapping! Therefore, overlapping genetic codes are untenable. Based on amino acid sequences accumulating in the literature, virtually any amino acid could follow another in a polypeptide. A look back at the table of 64 triplet codons quickly shows that only one of 16 amino acids, those that begin with a C can follow the first one in the illustration. However, as attractive as an overlapping codon hypothesis was in achieving genomic economies, it sank of its own weight almost as soon as it was floated! If you look carefully at the example above, you can see that each succeeding amino acid would have to start with a specific base. You can figure out how compressed a gene could get with codons that overlapped by two bases. One idea assumed 44 meaningless and 20 meaningful 3-base codons (one for each amino acid) and 44 meaningless codons, and that the meaningful codons in a gene (i.e., an mRNA) would be read and translated in an overlapping manner.Ī code where codons overlap by one base is shown below. One concern that was raised was whether there is enough DNA in an organism’s genome to fit the all codons it needs to make all of its proteins? Assuming genomes did not have a lot of extra DNA laying around, how might genetic information be compressed into short DNA sequences in a way that is consistent with the colinearity of gene and polypeptide. George Gamow (a Russian Physicist working at George Washington University) was the first to propose triplet codons to encode the twenty amino acids, the simplest hypothesis to account for the colinearity of gene and protein, and for encoding 20 amino acids. ![]() How is the Genetic Code 'Read' to Account for All of an Organisms' Gene? ![]()
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