Chemical Structure Of Purines & Pyrimidines

Nitrogenous bases of nucleic acid Nitrogenous bases of nucleic acids are heterocyclic , nitrogen

containing organic compounds They are 2 kinds

1. Purines2. Pyrimidines

Commonest purines of DNA & RNA 1. Adenine ( A )2. Guanine ( G )

Pyrimidines of DNA & RNA1. Cytosine ( C )2. Thymine( T )3. Uracil ( U ) in RNA Thymine is replaced by

Uracil

Pyrimidines and Purines Pyrimidine and purine are the names of the parent

compounds of two types of nitrogen-containing heterocyclic aromatic compounds.

N

N

N

N

N

NH

Pyrimidine Purine

Benzene- type ring

Imidazol ring

Purines

Purines are dicyclic ( double ringed ) molecules . Formed by the fusion between a six membered benzene – type ring &

a five membered imidazole ring. Carbon atoms in positions 4 & 5 are common for both rings. The dicyclic ring has N in positions 1 , 3 , 7 & 9. In nucleic acid N-9 of the purines links with the C-1 of sugar through

glycosidic bonding and form a purine nucleoside.

Adenine is a 6- amino purine ( -NH2 group at position 6 ) Guanine is a 2- amino,6- hydroxyl purine ( -NH2 group at 2nd

position , and an –OH group at 6th position.

Important Purines Adenine and guanine are the principal purines of both

DNA and RNA.

Adenine

N

N

NH2

N

NH

Guanine

OH

HN

NH

N

N

H2N

Pyrimidines

Pyrimidines are monocyclic ( single ringed ) molecule Formed 6 membered benzene – type ring N-3 of the pyrimidine links with the C-1 of sugar through

glycosidic bonding and form a pyrimidine nucleoside.

Cytosine is a 2- hydroxyl, 6- amino pyrimidine Thymine is a 2,6- dihydroxy , 5- methyl pyrimidine Uracil is a 2,6 – dihydroxy pyrimidine

One of the fundamental properties of nucleic acid bases is their ability for hydrogen bonding between themselves.

It is also important that thymine and uracil can exist in the keto and enol tautomeric forms.

Tautomerism refers to the migration and rearrangement of hydrogen atoms within the molecule .

The tautomeric forms are responsible for the ionic properties of bases.

Thank you

byAnand P P