The nitrogenous bases that are present in DNA are purine and pyrimidine. The two pyrimidine bases present in DNA are cytosine and thymine. Pyrimidine is a heterocyclic aromatic organic compound. It is a six membered ring that contains four carbon atoms and two nitrogen atoms. Apart from cytosine and thymine, uracil is also a pyrimidine, but it is present in RNA.
Cytosine: contains an amine group and keto group at carbon-4 and carbon-2 position. Cytosine binds with guanine with a triple bond.
Thymine: contains a methyl group (CH3) in the place of the keto group that is at carbon-2. The anime group is present at carbon-4. Thymine bonds with adenine with double bond.
The combination of gates shown below yields

Biomolecules are the most essential organic molecules, which are involved in the maintenance and metabolic processes of living organisms. These non-living molecules are the actual foot-soldiers of the battle of sustenance of life.
There are four major classes of Biomolecules – Carbohydrates, Proteins, Nucleic acids and Lipids.