How many distinct nitrogen-containing bases are found in DNA? Four: adenine, cytosine, guanine, and thymine. These bases are attached to sugar and phosphate groups to form DNA nucleotides.
The four bases belong to two chemical families. Adenine and guanine are purines, with two fused rings, while cytosine and thymine are pyrimidines, with one ring. In the DNA double helix, adenine pairs with thymine and cytosine pairs with guanine.
A common mix-up is counting uracil as a DNA base. Uracil is the corresponding pyrimidine used in RNA, where it replaces thymine. Modified bases can occur in some organisms and viruses, but the standard, canonical bases in ordinary DNA are the same four.
The pairing rules help DNA copy itself accurately: each strand provides a template for a complementary strand. A–T pairs have two hydrogen bonds, while C–G pairs have three, contributing to the stability of the molecule.