Biology / Unit 6 Bio 1221 short lesson, about 20 seconds
DNA Polymerase's 5'-to-3' Directionality
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The idea
DNA polymerase adds new nucleotides only to a primer's or growing strand's 3' end, so synthesis always proceeds in the 5'-to-3' direction.
In practice
DNA polymerase extends an existing 3'-OH end one nucleotide at a time, never building a new strand from its 5' end.
Where people slip
This one-directional restriction, not any other property of the enzyme, forces the leading and lagging strands to synthesize differently.
Your place on the trail
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