Strand displacement amplification (SDA) is an isothermal in vitro nucleic acid amplification technique.57 Hemi-modified DNA is polymerized by using three conventional dNTPs and one containing a 5'-[alpha-thio]triphosphate. The primer(s) is designed with an RE recognition site in the 5' overhang end. The recognition site is specific for an RE that can nick the unmodified DNA strand at a double-
stranded hemiphosphorothioate recognition site, that is, when the newly synthesized strand incorporates the 5'-[alpha-thio]triphosphate nucleotide in the recognition sequence. DNA polymerase lacking 5' to 3' exonuclease activity is used to extend the 3' end at the nick and displace the downstream strand. Nicking and polymerization with re-formation of the hemiphosphorothioate recognition site continuously cycle, generating complementary copies of the DNA target. Linear amplification (called targetgeneration SDA) occurs when a single primer is used. Exponential amplification (exponential SDA) is achieved by using two primers complementary to opposite DNA strands, with both primers containing RE recognition sites in the 5' overhang end. Strand displacement amplification has been used in a microarray format,58 which may become a clinically useful method to combine amplification of low copy number targets with the multiple features of micro-array in a format that can be automated.
Examples of Applications of SDA
1. Chlamydia trachomatis detection
2. Neisseria gonorrhoeae detection
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