A primer is a strand of nucleic acid that serves as a starting point for DNA replication. They are required because the enzymes that catalyze replication, DNA polymerases, can only add new nucleotides to an existing strand of DNA.The primers are to be designed correctly in thr amplification of a single DNA fragment corresponding to the target reagion of the template molecule.The primer should be complementary to its template strand at the 5' and 3' ends of the DNA region. in order for hybridization to occur . Amplification can occur when mismatching primers are close enough together on opposite strands of DNA - and an unwanted sequence is produced with ends that precisely match the primers. If such an 'incorrect' fragment is synthesized in the early cycles of a PCR, it will be efficiently amplified on subsequent cycles.