Z-DNA (deoxyribonucleic acid) tetramer, molecular model. DNA is composed of two strands twisted into a double helix. This is a tetramer of the molecule, containing four strands. Each strand consists of a sugar-phosphate backbone attached to nucleotide bases. There are four bases: adenine, cytosine, guanine and thymine. The bases are joined together by hydrogen bonds. The Z form is one of the many possible structures of the DNA double helix. It winds to the left in a zig-zag pattern as opposed to the more common right-handed B-DNA.