DNA Structure and History Function

Regarding how DNA can act as a gene, storing information and directing activities of the cell, Watson and Crick wrote,

The phosphate-sugar backbone of our model is completely regular, but any sequence of the pairs of bases can fit into the structure. It follows that in a long molecule many different permutations are possible, and it therefore seems likely that the precise sequence of the bases is the code which carries the genetical information.

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This too is correct, as a series of experiments showed.

Proteins are the workhorses of the cell, controlling the rates of all the reactions within and providing much of the cell's structure as well. Therefore, it was quickly realized, genes must control the production of proteins, and the genetic information carried in the sequence of bases in DNA is a code for the sequence of amino acids in proteins. Proteins are made of twenty amino acids, linked together in varying sequences. The sequence determines the shape and chemical properties of the protein, and so specifying protein sequence is the essential role of DNA.

Since there are four bases and twenty amino acids, a single nucleotide is not enough to specify one amino acid. Even two are not enough, because two nucleotides will only give rise to sixteen unique combinations (AA, AC, AG, and so on). Therefore, it was immediately obvious that each amino acid must be coded for by at least three nucleotides.

Stating this must be so was quite a bit easier than working out the details of how DNA and amino acids interacted to form a protein. Some researchers suggested a solution in which amino acids lined up directly on the surface of DNA; other alternatives were also proposed. A suggestion by Crick that there was some type of adapter between the two was confirmed with the discovery of transfer RNA. In fact, DNA and amino acids never do interact during protein synthesis—instead, an RNA copy of DNA is made (messenger RNA), which links with transfer RNAs that carry amino acids. The code itself was worked out between 1961 and 1967, by several different groups, including Marshall Nirenberg (born 1927), Har Gobind Khorana (born 1922), and Johann Matthaei, who developed pioneering cell-free systems that allowed researchers to work without the complexity and constraints involved with living organisms.

These heroic discoveries marked the beginning of the molecular biology revolution. From them even deeper questions have arisen, about how gene expression is regulated, how genes control development, and how (and whether) genes can be modified to treat disease and improve human life.