Figure 1. Steps of deoxyribonucleic acid (DNA) polymerization depicting the correct incorporation of thymidine 5′-monophosphate. The ‘thumb’, ‘palm’ and ‘finger’ domains are shown in purple, blue and orange respectively. The green domain represents the ‘PAD’ or ‘little finger’ domain that has thus far been identified only in low-fidelity Y family DNA polymerases. The red transluscent domain (in two of the drawings) represents the 3′5′ exonuclease domain, which is not present in Y family polymerases. ‘†’ represents the transition state for enzyme catalysis. dTTP: thymidine 5′-triphosphate; PPi: pyrophosphate; dTMP: thymidine 5′-monophosphate. The curved black arrows denote movement of the ‘thumb’ and ‘fingers’ domains.