Which four bases make up the DNA alphabet in all living organisms?

The four DNA bases are adenine, cytosine, guanine and thymine.

These bases are nitrogen-containing components of nucleotides, the building blocks of DNA. In the double helix, adenine pairs with thymine, while cytosine pairs with guanine. The pairing rules help DNA preserve information when it is copied.

DNA is often described as a four-letter chemical alphabet: A, C, G and T. The order of these letters carries biological instructions, including information used to make proteins and regulate cells. A change in one base is called a substitution, although its effect can range from harmless to medically important.

Uracil is the common source of confusion. It replaces thymine in RNA, another nucleic acid. DNA and RNA also differ in their sugars and typical structures, so uracil is not normally counted as one of DNA's four bases.