DNA is made up of two polynucletide chains or strands. They are parallel but run in opposite direction. They are coiledand form double helix. They appear like a spiral ladder.
The vertical bar of ladder is made up of sugar alternating with phosphate. The horizontal bar of ladder is made up of pair of nitrogen based. Nitrogen based are of two types , purine and pyrimidines . The purines are of two types ,Adenine (A) and Guanine (G). Pyrimidines are of two types ,Cytosine (C) and Thymine (T). Base pairing is complementary. A combined to T with two hydrogen bonds and G combines to C with three hydrogen bonds.