DNA Replication
  How DNA Makes Copies of Itself

Before a cell divides, its DNA is replicated (duplicated.) Because the two strands of a DNA molecule have complementary base pairs, the nucleotide sequence of each strand automatically supplies the information needed to produce its partner.  If the two strands of a DNA molecule are separated, each can be used as a pattern or template to produce a complementary strand.  Each template and its new complement together then form a new DNA double helix, identical to the original.  

Before replication can occur, the length of the DNA double helix about to be copied must be unwound.  In addition, the two strands must be separated, much like the two sides of a zipper, by breaking the weak hydrogen bonds that link the paired bases.   Once the DNA strands have been unwound, they must be held apart to expose the bases so that new nucleotide partners can hydrogen-bond to them. 
 
The enzyme DNA polymerase then moves along the exposed DNA strand, joining newly arrived nucleotides into a new DNA strand that is complementary to the template. Figure 1 shows the process part way through.