Leading strand

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The leading strand is the DNA strand at the opposite side of the replication fork from the lagging strand. It goes from a 3`-5` direction (these numbers indicate the position of the molecule in respect to the carbon atoms it contains).

When replicating, the original DNA splits in two, forming two "prongs" which resemble a fork (i.e. the "replication fork"). DNA has a ladder-like structure; imagine a ladder broken in half vertically, along the steps. Each half of the ladder now requires a new half to match it.

On the leading strand, Pol III "reads" the DNA and adds nucleotides to it continuously. Pol III's role on the lagging strand is slightly different.