Question

Why are adenine and guanine called substituted purines and thymine and cytosine are called substituted primitives?

Answer 1

A purine is a heterocyclic aromatic organic compound that consists of a pyrimidine ring fused to an imidazole ring. Below is the structure of a basic purine solution:

Adenine and guanine are called substituted purines as they have a structure similar to purine and they also have substitutions.

Thymine and cytosine are pyrimidine derivative, with a heterocyclic aromatic rind and substituents attached. (Thymine)
(Cystosine)