Genetic Code

The 1960s were an exciting time for molecular biologists, for it was then that the genetic code was broken. Two possibilities had to be considered for the genetic code. It was possible that the code had gaps, that is, some sort of punctuation mark or a "spacer" nucleotide or nucleotides between coding groups. Second, the code could be either overlapping or nonoverlapping. These possibilities are illustrated in Figures 2 and 3. An overlapping code would have the advantage that more information could be contained in a smaller space.

Figure 2. Any stretch of messenger RNA has three different reading frames, which can be translated to give different amino acid sequences. Only one of them is the correct sequence, dictated by the start point of the first triplet, AUG.

Figure 3. Models of mRNA showing the effect of hypothetical "punctuation marks" separating codons.

However, in overlapping code a mutation that changed one base would lead to the changing of three consecutive amino acids in the protein sequence. Genetic evidence, available even before the code had been deciphered, indicated that a single point mutation, that is, a change in a single nucleotide, affected only one amino acid and thus suggested a nonoverlapping code.

Another possibility was that the code had punctuation marks, that is, a base (indicated by "p" in Figure 3) acting as a comma that would separate each codon. In this situation, if an additional base were inserted into a codon, then only that codon would be affected. In a code without punctuations or gaps, however, insertion of a single nucleotide would result in all codons from that point on being affected. This would in turn change the amino acid sequence in the protein from that point on. Again, genetic evidence ruled out a punctuated code, as base insertions do, in fact, affect the entire protein from the insertion point on, rather than just a single amino acid. This effect is called a frameshift mutation.

In the late 1970s DNA sequencing techniques were developed. A number of proteins had already been sequenced by protein sequencing methods. When the genes for these proteins were cloned and sequenced, the predicted protein sequence could be deduced. Agreement between the DNA and protein sequences confirmed the accuracy of the genetic code.