Space-filling models (CPK) of ethidium, ethidium-DNA interactions, NAA and NAA-DNAinteractions, looking at the major groove and the dinucleotides 5'-dTdA-3'; 5'-dTdA-3'. The dinucleotides are oriented with the phosphate backbones (top to bottom) 5'to 3' on the left and3'to 5' on the right.