Replication fork

Replication fork

The replication fork is a structure that forms during DNA replication. It is created by helicases, which break the hydrogen bonds holding the two DNA strands together. The resulting structure has two branching "prongs", each one made up of a single strand of DNA, that are called the leading and lagging strands. DNA polymerase creates new partners for the two strands by adding nucleotides.

Leading strand

The leading strand is the DNA strand at the opposite side of the replication fork from the lagging strand. It goes from a 5' to 3' direction, because DNA Polymerase can only synthesize a new DNA strand in a 5' to 3' manner. On the leading strand, DNA polymerase III (DNA Pol III) "reads" the DNA and adds nucleotides to it continuously.

Lagging strand

The lagging strand is the DNA strand opposite the replication fork from the leading strand. It goes from a 3' to 5'.

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.

Pol III, the main DNA replication enzyme, cannot work in the 3' to 5' direction of the template strand, and so replication of the lagging strand is more complicated than of the leading strand.

On the lagging strand, primase "reads" the DNA and adds RNA to it in short, separated segments. DNA polymerase III lengthens the primed segments, forming Okazaki fragments. DNA polymerase I then "reads" the fragments, removes the RNA using its flap endonuclease domain, and replaces the RNA nucleotides with DNA nucleotides (this is necessary because RNA and DNA use slightly different kinds of nucleotides). DNA ligase joins the fragments together.

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