<prompt> Zidovudine and didanosine used in HAA act by:Inhibitory effects on viralDNA Nucleoside reverse transcriptase inhibitionInhibit the synthesis of gp41Protease inhibition  <response>: Understanding the biology of HIV infection and availability of several potent drugs belonging to different classes has mandated ;highly active antiretroviral therapy; HAA with combination of 3 or more drugs whenever indicated. Monotherapy is contraindicated. Zidovudine :It is a thymidine analogue azidothymidine, AZT, the prototype NI. After phosphorylation in the host cell-zidovudine triphosphate selectively inhibits viral reverse transcriptase RNA-dependent DNA polymerase in preference to cellular DNA polymerase. Didanosine ddl: It is a purine nucleoside analogue which after intracellular conversion to didanosine triphosphate competes with ATP for incorporation in viral DNA, inhibits HIV reverse transcriptase and terminates proviral DNA. Antiretroviral activity of didanosine is equivalent to AZT ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:770,771 