X-ray structure of E. coli DNA polymerase I Klenow fragment in complex with a dsDNA. The protein is drawn in ribbon form colored in rainbow order from its N-terminus (blue) to its C-terminus (red). The DNA is shown in stick form with the C atoms of its 10-nt template strand cyan, the C atoms of its 13-nt primer strand lavender, N blue, O red, and P orange, and with successive P atoms in each polynucleotide strand connected by orange rods. A Zn²⁺ ion (magenta sphere) marks the 3' → 5'-exonuclease active site. [Based on an X-ray structure by Thomas Steitz, Yale University. PDBid 1KLN.]