**2.4 Primers**

To achieve selective amplification of nucleotide sequences from a DNA extract by PCR, it is essential to have least one pair of oligonucleotides. These oligonucleotides, which will serve as primers for replication, are synthesized chemically and must be the best possible complementarity with both ends of the sequence of interest that one wishes to amplify. One of the primers is designed to recognize complementarily a sequence located upstream of the fragment 5′–3′ strand DNA of interest; the other to recognize, always by complementarity, a sequence located upstream complementary strand (3′–5′) of the same fragment DNA. Primers are single-stranded DNAs whose hybridization on sequences flanking the sequence of interest will allow its replication so selective. The size of the primers is usually between 10 and 30 nucleotides in order to guarantee a sufficiently specific hybridization on the sequences of interest of the matrix DNA [1–5].
