DNA polymerase structure. The first DNA polymerase structure determined was that of a fragment of E. coli DNA polymerase I called the Klenow fragment. Notice that, like other DNA polymerases, the polymerase unit resembles a right hand with fingers (blue), palm (yellow), and thumb (red). The Klenow fragment also includes an exonuclease domain that removes incorrect nucleotide bases.

[Drawn from 1DPI.pdb.]