Fig. 2.

NASBA. The primer P1, carrying at the 5′ end the T7 RNA polymersase binding site (open box), anneals to the RNA strand and is extended by the reverse transcriptase to synthesize the cDNA. The RNA strand in the RNA/DNA duplex is degraded by the RNAse H. The cDNA is accessible to the second primer, P2 that allows the synthesis of the complementary strand. The resulting double strand DNA has a T7 RNA polymerase binding site where the enzyme can bind and produce many RNA copies. The cycle can be repeated indefinitely and the initial target strand is exponentially amplified