- Dideoxyadenosine (ddA) is a synthetic analog of the naturally occurring nucleoside adenosine. It differs structurally from adenosine by lacking both the 2′ and 3′ hydroxyl (–OH) groups on its ribose sugar. This unique feature is critical for its biological role, particularly in molecular biology and antiviral therapy.
- Without a 3′ hydroxyl group, ddA cannot form a phosphodiester bond with the next nucleotide during DNA synthesis, thereby acting as a chain terminator. This property forms the basis for its use in Sanger sequencing—a method of DNA sequencing where ddA is incorporated randomly during DNA replication to terminate the chain at positions where adenine would normally be added. When these terminated fragments are resolved by electrophoresis, they reveal the DNA sequence.
- In the context of Sanger sequencing, ddA is introduced along with the four regular deoxynucleotide triphosphates (dNTPs). DNA polymerase adds nucleotides to the growing DNA strand, but when it incorporates a ddA molecule instead of a normal deoxyadenosine (dA), elongation stops. By using fluorescently labeled or radiolabeled ddA and separating the resulting DNA fragments by size, researchers can determine the positions of adenine in the original DNA strand. This selective termination mechanism is key to the high accuracy of the Sanger method.
- Beyond sequencing, dideoxynucleosides like ddA have also been explored in antiviral research, particularly for their potential to inhibit viral reverse transcriptase enzymes. Since these enzymes also rely on nucleoside incorporation for viral genome replication, the chain-terminating ability of ddA can be leveraged to block viral proliferation. However, more stable and effective dideoxynucleoside analogs, such as zidovudine (AZT) and didanosine (ddI), have been more widely used in clinical settings.
