see how DNA is actually copied in living cells.

You're looking at an assembly line of amazing miniature biochemical machines

that are pulling apart the DNA double helix and cranking out a copy of each

strand.

The DNA to be copied enters the production line from the bottom left.

The whirling blue molecular machine is called helicase.

It spins the DNA as fast as a jet engine

as it unwinds the double helix into two strands. One stand is copied continuously

and can be seen spooling off to the right.

Things are not so simple for the other strand

because it must be copied backwards.

It is drawn out repeatedly in loops and copied one section at a time. The end

result is two new DNA molecules.