Quoting Richard Feynman from Chapter 3 of his book Six Easy Pieces, when he talks about DNA:
Attached to each sugar along the line, and linking the two chains together, are certain parts of cross-links. However, they are not all of the same kind; there are four kinds, called adenine, thymine, cytosine, and guanine, but let us call them A, B, C, and D.
Next comes the question, precisely how does the order of the A, B, C, D units determine the arrangement of the amino acids in the protein? This is the central unsolved problem in biology today.
Feynman taught his lectures during 1961-1963. Do we now know how does the order of adenine, thymine, cytosine and guanine affect the order of the amino acids in the DNA?