Explained briefly, NASBA works as follows:
#RNA template added to the reaction mixture, the first primer with the T7 promoter region on its 5 end attaches to its complementary site at the 3 end of the template.
#Reverse transcriptase synthesizes the opposite complementary DNA strand extending the 3' end of the primer, moving upstream along the RNA template.
#RNAse H destroys the RNA template from the DNA-RNA compound (RNAse H only destroys RNA in RNA-DNA hybrids, but not single-stranded RNA).
#The second primer attaches to the 5' end of the (antisense) DNA strand.
#Reverse transcriptase again synthesizes another DNA strand from the attached primer resulting in double stranded DNA.
#T7 RNA polymerase binds to the promoter region on the double strand. Since T7 RNA polymerase can only transcribe in the 3 to 5 direction the sense DNA is transcribed and an anti-sense RNA is produced. This is repeated, and the polymerase continuously produces complementary RNA strands of this template which results in amplification.
#Now a cyclic phase can begin similar to the previous steps. Here, however, the second primer first binds to the (-)RNA
#The reverse transcriptase now produces a (+)cDNA/(-)RNA duplex.
#RNAse H again degrades the RNA and the first primer binds to the now single stranded +(cDNA)
#The reverse transcriptase now produces the complementary (-)DNA, creating a dsDNA duplex
#Exactly like step 6, the T7 polymerase binds to the promoter region to produce (-)RNA, and the cycle is complete.