The twisted DNA helix is composed of stacked purine and pyrimidine base pairs.

Nucleic acids are one of the four building blocks of cellular life on our planet. Subunits called nucleotides are each made up of a phosphate group, a sugar molecule, and a nitrogenous base. This base is what allows the nucleotide to carry information, and it can be categorized based on its ring shape as either a purine or a pyrimidine. DNA and RNA are both composed of long strings of nucleotides, and half of the possible bases in each are purines.

Nucleotide Purines and Pyrimidines

A purine is composed of a hexagon of nitrogen and carbon fused with a pentagon of nitrogen and carbon, with both rings sharing two carbons. The first standard nucleotide purine is adenine, and it pairs with thymine in DNA and uracil in RNA. The second purine is guanine, and it pairs with cytosine in both DNA and RNA. Thymine, uracil, and cytosine are all pyrimidine bases.