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 3' to 5' (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.
See DNA replication