![]() ![]() ![]() Nirenberg reasoned that he could use a man-made stand-in for RNA that had a repeating known sequence (the same codon over and over) to produce proteins consisting of only one amino acid. Because RNA, not DNA, is what the cell reads directly to make proteins, Dr. It was known at the time that DNA is “transcribed” into a template RNA that interacts with ribosomes in the cell to produce proteins. Nirenberg and his colleagues set about trying to figure out which three-base combinations represented each amino acid. If each codon was three bases however, that would result in 64 possible codons (4^3 =64), plenty of codons to represent each of the 20 amino acids separately. If it was only two bases long, that would allow for only 16 different combinations of the four bases (4^2 = 16). The structure suggested that the order of the bases formed a code representing the order in which amino acids should be joined to produce different kinds of proteins.īut what was the code? What order of bases made up the “code words” or "codons” DNA used to represent each of the 20 amino acids? Researchers hypothesized that each codon for amino acid would be three bases long. ![]() The structure of DNA was also known- a sort of twisted ladder shape known as double helix where the “side rails” consisted of a sugar phosphate backbone and the “rungs” were made of paired nucleic acid bases (represented by A, T, G, C). It was prepared by Nirenberg to keep track of which codons stood for which amino acids.īy the late 1950s, scientists understood that DNA was the molecule containing the instructions for life. Nirenberg's signature is visible at the top of the chart. All entries on the chart are handwritten and some sections of the graph are circled or outlined in red. The 64 nucleotide codons, the specific segments of DNA that code for amino acids, are on the vertical axis. The chart, made from several sheets of graph paper taped together, shows the twenty amino acids in columns across the top of the chart. Marshall Nirenberg, a scientist who won the 1968 Nobel Prize in Physiology or Medicine for his work in helping to “crack the genetic code,” or to understand the way DNA codes for the amino acids that are linked to build proteins. This chart was used in the National Institutesof Health lab of Dr. ![]()
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