the formation of new DNA molecules.

Copying occurs at a localized region called the replication fork,

which is a y-shaped structure where new DNA strands are synthesized by a

multi-enzyme complex. Here,

the DNA to be copied enters the complex from the left.

One new strand is leaving at the top of frame and the other new strand is

leaving at the bottom. The

first step in DNA replication is the separation of the two strands by an enzyme called

helicase.

This spins the incoming DNA to unravel it

at ten thousand rpm in the case of bacterial systems.

the separated strands are called three prime

and five prime,

distinguished by the direction in which the component nucleotides join up.

The three prime DNA strand, also known as the leading strand, is diverted to a

DNA polymerase and is used as a continuous template for the synthesis of

the first daughter DNA helix.

The other half of the DNA double helix, known as the lagging strand, has

the opposite three prime to five prime orientation and consequently requires

a more complicated copying mechanism.

As it emerges from the helicase, the lagging strand is organized into

sections called Okazaki fragments

these are then presented to a second DNA polymerase emzyme in the preferred

five prime to three prime orientation.

These sections are then effectively synthesized backwards.

When the copying is complete, the finished section is released, and the

next loop is drawn back for replication.

Intricate as this mechanism appears,

numerous components have been deliberately left out to avoid complete

confusion.

The exposed strands of single DNA are covered by protective binding proteins

and in some systems multiple Okazaki fragments may be present.