A series of amino and thiotetrazole purine derivatives introduced with different alkyl groups in position 9 was synthesized. The structures of the synthesized compounds were characterized using spectroscopic methods. All the synthesized compounds were screened for their antibacterial activities against Gram-positive and Gram-negative bacteria and for their antifungal activities against yeast strains. The effect of the compounds on pBR322 plasmid DNA was studied by gel electrophoretic mobility measurements. The results of antimicrobial activity show that attachment of tetrazole group to purine bases results in disappearance of antimicrobial activity The results of the plasmid DNA interaction and the restriction studies suggest that while aminotetrazole purine derivatives cause DNA damages, thiotetrazole purine derivatives are believed to form a range of interstrand GG adducts with duplex DNA that induce global changes in the DNA conformation.