Figure 2.

. Termination of polymerization by ribonucleotides and 2′-O-methyl ribonucleotides. PCR experiments were conducted with the following polymerases: Pfu (lanes 1–4), Pfu exo(–) (lanes 5–8) and Taq (lanes 9–12). The same ³²P-labeled primer was used as a PCR primer in each experiment. Four different unlabeled primers were used. The four primers were identical except for the inclusion of one or three ribonucleotides or a single 2′-O-methyl ribonucleotide at a particular position (see Fig. 1). A sequencing ladder was used as a size standard so that termination products could be measured with single nucleotide resolution.