what is the principle of PCRfor what purpose it is used,

Polymerase chain reaction operates on the principle that
DNA can be used as a template to produce more of the same DNA. It is used in cloning,
for genetic analysis of DNA, for comparison of DNA amongst species to find evolutionary
relationships and for other purposes. DNA is heated first to separate the double
stranded molecules. Then, either single stranded molecule can serve as a template. DNA
primers help to initiate the process and attach at a particular location on the
strand and DNA polymerase is an enzyme involved in replicating the strand using free
nucleotides that are complementary to the DNA sequence. It is a chain reaction because
you can virtually make as many copies as needed using the original DNA sequence as the
blueprint.