DNA (Deoxyribonucleic acid) is a polynucleotide, a macromolecule made of units called nucleotides. Each nucleotide consists of three sub units:
A base and a sugar combine to form a nucleoside, while it becomes a nucleotide when a phosphate group gets attached to the nucleoside.
Base + sugar = nucleoside
Base + sugar + Phosphate = nucleotide
So there are four nucleotides in DNA formed of sugar and nitrogenous base and phosphate.
The four nucleotides are not present in equal amounts in a DNA molecule. But the amount of purines (A + G) and that of pyrimidines (T + C) is always equal.
A = T and G = C
A DNA molecule is three dimensional and made of two strands helically coiled around each other. Franklin and Wilkins first showed through X-ray diffraction studies of DNA that it is a double helix.
In 1953, James Watson and Francis Crick were awarded the Nobel Prize for working out the structure of DNA. According to the Watson and Crick model:
There are two hydrogen bonds between A and T and three hydrogen bonds between G and C. A and T are complementary bases and so are G and C.
In the DNA helix, a complete helical turn occurs after 3.4 nm (or 34Å). This complete turn encloses 10 base pairs. Each base pair lies 0.34 nm (3.4 Å) apart. The diameter of the double helical DNA molecule is 2.0 nm.