The structure of Pol I. (a) Crystal structure of Thermus aquaticus Pol I bound to DNA. All DNA polymerases are shaped like a right hand and have domains referred to as fingers, thumb, and palm. The 3′→5′ exonuclease is in a separate domain from the polymerase active site. E. coli Pol I (not shown) also has a 5′→3′ exonuclease. (b) The 3′ terminus of the DNA binds to the palm, but neither the duplex DNA nor the template 5′ single-stranded DNA enters the cleft between the fingers and thumb. The dNTP-binding site is located in the fingers. (c) A mispaired 3′ terminus frays by about four nucleotides to insert into the 3′→5′ exonuclease site.