Fortunately, the DNA polymerases of the host cell prefer dTTP, so side effects from the drug are not so severe as might have been predicted. Because AZT has no 3вЂІ -OH group, DNA synthesis by reverse transcriptase halts when AZT triphosphate is incorporated in the growing DNA strand. The reverse transcriptase of the human immunodeficiency virus (HIV) prefers AZT triphosphate to the normal nucleotide (dTTP). AZT (which is also called zidovudine) is taken up by cells where it is converted into the triphosphate. The bottom formula shows the structure of azidothymidine (AZT), a drug used to treat AIDS. For this reason, the dideoxy method is also called the chain termination method.

A dideoxynucleotide (dideoxythymidine triphosphate - ddTTP - is the one shown here) can be added to the growing DNA strand but when it is, chain elongation stops because there is no 3вЂІ -OH for the next nucleotide to be attached to. The dideoxy method gets its name from the critical role played by synthetic nucleotides that lack the -OH at the 3вЂІ carbon atom (red arrow). Each new nucleotide is added to the 3вЂІ -OH group of the last nucleotide added. The top formula shows one of them: deoxythymidine triphosphate (dTTP). The most popular method for doing this is called the dideoxy method or Sanger method (named after its inventor, Frederick Sanger, who was awarded the 1980 Nobel prize in chemistry for this achievment).ДNA is synthesized from four deoxynucleotide triphosphates. DNA sequencing is the determination of the precise sequence of nucleotides in a sample of DNA.

Dna strand sequence