The Structure of DNA

The 'backbone' of a DNA molecule is made up of only two groups. One is a ribose sugar (that is one with 5 carbon atoms in it), called deoxyribose and the other is a phosphate group.
These alternate and form a long chain.

Sticking out from the backbone is the third group found in DNA. These are the organic bases.
There are four bases :
(Adenine), C ( Cytosine), G (Guanine) and T (Thymine)
Adenine always pairs with thymine.
Cytosine always pairs with guanine.

A to T  and  C to G