Overall structure of DNA

The overall structure of DNA is a double helix made of 4 types of nitrogenous base pairs called nucleotides.
The nucleotides are of 2 types purines and pyrimidines. 
A purine on one side of the helix is hydrogen bonded to a pyrimidine on the other side of the helix.
Adenosine and Guanine are purines
Cytosine and Thymine are pyrimidines

Thus A=T  and C=G   are the normal bonding patterns
Groups of 3 nucleotides form a codon that codes for an amino acid on a protein
A gene is enough codons to code for a protein

      

A phosphate residues and a deoxyribose sugar are accessory in the helical shape and form the backbone of the helix and the bases form the rungs across.

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