Nucleic acid sequence based amplification (NASBA) is a method in molecular biology

Molecular biology

Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

 which is used to amplify RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 sequences.

NASBA was developed by J Compton in 1991, who defined it as "a primer-dependent technology that can be used for the continuous amplification of nucleic acids in a single mixture at one temperature."

Immediately after the invention of NASBA it was used for the rapid diagnosis and quantification of HIV-1 in patient sera.

Although RNA can also be amplified by PCR using a reverse transcriptase

Reverse transcriptase

In the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...

 (in order to synthesize a complementary DNA strand as a template), NASBA's main advantage is that it works at isothermic conditions - usually at a constant temperature of 41°C.

NASBA has been introduced into medical diagnostics, where it has been shown to give quicker results than PCR, and it can also be more sensitive.

Explained briefly, NASBA works as follows:

1. RNA template is given to the reaction mixture, the first primer attaches to its complementary site at the 3' end of the template
2. Reverse transcriptase synthesizes the opposite, complementary DNA
   Complementary DNA
   In genetics, complementary DNA is DNA synthesized from a messenger RNA template in a reaction catalyzed by the enzyme reverse transcriptase and the enzyme DNA polymerase. cDNA is often used to clone eukaryotic genes in prokaryotes...
   
    strand
3. RNAse H
   RNase H
   The enzyme RNase H is a non-specific endonuclease and catalyzes the cleavage of RNA via a hydrolytic mechanism. Members of the RNase H family can be found in nearly all organisms, from archaea to bacteria and eukaryota....
   
    destroys the RNA template (RNAse H only destroys RNA in RNA-DNA hybrids, but not single-stranded RNA)
4. the second primer attaches to the 5' end of the DNA strand
5. T7 RNA polymerase
   T7 RNA polymerase
   T7 RNA Polymerase is an RNA polymerase from the T7 bacteriophage that catalyzes the formation of RNA in the 5'→ 3' direction.- Activity :T7 polymerase is extremely promoter-specific and only transcribes DNA downstream of a T7 promoter. The T7 polymerase also requires a DNA template and Mg2+ ion as...
   
   produces a complementary RNA strand which can be used again in step 1, so this reaction is cyclic.

The NASBA technique has been used to develop rapid diagnostic tests for several pathogenic viruses with single-stranded RNA genomes, e.g. influenza A, foot-and-mouth disease virus, severe acute respiratory syndrome (SARS)-associated coronavirus, Human bocavirus(HBoV) and also parasites like Trypanosoma brucei.