The ability to determine the order of the bases within a DNA molecule has been one of the greatest technical contributions to molecular biology. DNA is made by the polymerization of the four deoxynucleotides dATP, dGTP, dCTP, and dTTP. These are joined together when DNA polymerase catalyzes the formation of a phosphodiester link between a free 3'-hydroxyl on the deoxyribose sugar moiety of one nucleotide and a free 5'-phosphate group on the sugar residue of a second nucleotide. However, the artificial dideoxynucleotides ddATP, ddGTP, ddCTP, and ddTTP have no 3'-hydroxyl on their sugar residue (Fig. 7.10), and so if they are incorporated into a growing DNA chain, synthesis will stop. This is the basis of the dideoxy chain termination DNA sequencing technique devised by Frederick Sanger and for which he was awarded a Nobel prize in 1980.
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