DNA is a nanoscale structure, consisting of a double backbone of phosphate and sugar molecules between which complementary pairs of bases (A and T; C and G) are connected by weak hydrogen bonds (right).

DNA’s most common conformation is B-DNA,  which twists in a right-handed double helix about two nanometers in diameter. One full turn of the helix is about 3.4 nanometers, or 10 base pairs long.

   In special circumstances DNA can form a left-handed double helix called  Z-DNA., which may play an editing role in actively transcribing DNA.

Right handed helix

 Left handed helix

DNA  stick model