Starting in 1953 the paradigm shifts from function-to-structure to structure-to-function. People had been observing inheritance and found that there must be a defining principle in the structure of a gene. Now the hope was turned to what might appear when the structure of DNA was elucidated at the molecular level.
James Watson (born 1928), Francis Crick (1916-2004) and Maurice Wilkins (1916-2004) received the 1962 Nobel Prize in Physiology and Medicine for "their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material". Rosalind Franklin should also be mentioned. The structure of DNA had been solved with the help of X-ray crystallography. It consisted of a double helix of two intertwined strands complementary to each other, a positive and a negative.
It was earlier known, 1950, that the four nucleotides in DNA was not present in stable proportions, they varied, but that adenine seemed to be present in the same amount as thymine and guanine in the same amount as cytosine by the work of Erwin Chargaff (1905-2002). These pairs form through space bonds in the double helix. It could be concluded now that DNA carries a specific sequence of base pairs. Information must reside in this sequence thus explaining the heredity principle of a gene, a sequence of base-pairs in a double helix.
It now became clear that during a cell division the two strands split and served as a precursor for the next double helix to form. In 1941 it was demonstrated that genes code for proteins. It now remained to explain how the information of the protein was encoded in the DNA.
It could be concluded that the "codon", ie, the smallest unit in the code, should be a triplet since one needs at least three bases to form all used 20 amino acids from four bases (4^3). The first elucidation of a codon was performed by Marshall Nirenberg and Heinrich J. Matthaei in 1961 at the NIH in Bethesda. It was possible to use a cell free extract to produce phenylalanine from a poly-uracil strand of RNA, since DNA forms via RNA. The codon for phenylalanine was then UUU. Similar techniques were used to elucidate all 64 codons.
Needless to say, these results yielded a revolution of scientific activity called molecular biology and resulted in the actual sequencing of the complete human DNA in 2001.
Data from Wikipedia