Resolution of a Paradox
Hybridization is possible between nucleic acids of complementary sequence. Of course, there is a lower limit to the length of the partici - pating polymers. A single adenine in solution is not normally seen to base pair with a single thymine. The lower length limit for specific hybridization between two nucleic acid molecules in typical buffers containing 0.001 M to 0.5 M salt at temperatures between 10° C and 70°C is about ten bases. How then can protein synthesis have any degree of accuracy since only three base pairs form between the codon and anticodon?
One part of the answer to how triplet base pairing can provide accuracy in protein synthesis is simply that additional binding energy is provided by contacts other than base pairing. A second part of the answer is that by holding the codon and anticodon rigid and in comple-mentary shapes, binding energy is not consumed in bringing all three bases of the codon and anticodon into correct positions (Fig. 7.16). Once pairing occurs at the first base, the second and third bases are already
Figure 7.16 Under the same conditions, complementary anticodons in tRNAmolecules will hybridize whereas the same isolated trinucleotides will not.
in position to pair. When two complementary trinucleotides bind to each other, this is not the case. Even after one base pair has formed, the second and third bases of each polymer must be brought into the correct positions before they can base pair. Additional binding energy is con-sumed in properly orienting them. This is another example of the chelate effect.
A simple hybridization experiment with tRNA provides a direct demonstration of the consequences of correctly positioning and orient-ing bases. Two tRNA molecules with self-complementary anticodons will hybridize! Since the structure of the entire tRNA molecule serves to hold the anticodon rigid, the binding energy derived from forming the second and third base pairs need not go into holding these in the proper position. They already are in the correct position. As a result, the two tRNA molecules will hybridize via their anticodons whereas self-complementary trinucleotides will not hybridize.