Figure 2. Generalized schematic of polymerase enzyme activity. The ellipse represents the enzyme moving from left to right along a single-stranded DNA template below it. DNA subunits, or dNTPs, free in solution, are bound by the polymerase and added to the primer (shaded) to form the complementary strand. All known polymerases require a free 3' hydroxyl group and therefore will only polymerize in the 5' to 3' direction (left to right in the example above).

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