PCR: Polymerase Chain Reaction
The nucleotides of DNA are arranged in long strands. DNA is made up of two of these strands that are arranged in a "double helix". The nucleotides on one strand match up with the nucleotides on the other strand. For example, an adenine or A always pairs with a thymine or T. A cytosine or C always pairs with a guanine or G.
To replicate the DNA strands, they are first separated from each other. This process is alled denaturation.
Next, an enzyme is used to make a complentary strand of DNA from each original strand. Now we have two identical double helixes which came from the single original double helix. In this image, the original strands of DNA are colored green. The newly made strands are colored orange/red.
This process can then be repeated numerous times (called cycles) to produce millions of copies of idential DNA double helixes.