This short review assembles the contributions of the author's laboratory to the structural aspects of DNA. DNA was modified by small ligands and/or substituents. There are three aspects to this work: a) Protonation of guanosine and DNA and the formation of triple- and quadruple-strands of guanosine, its nucleotides, their polymers and DNA. b) Substitution of the 2'-position of deoxyribose by the most polar atom, fluorine: studies on 2'-deoxy-2'-fluoro-nucleosides, -nucleotides and their polymers, studied both by structural and biological methods. c) The effect of introducing the methyl group in the large groove of DNA: NMR studies of oligonucleotides containing N6-methylated adenine residues, and enzymatic and molecular biology work on Dam methylase are reported.