DNA Has Four Bases. Some Viruses Swap in a Fifth
31st July 2021
But some exceptions have cropped up. In 1977, for instance, researchers in the Soviet Union found something peculiar while looking at a virus that infects photosynthetic bacteria: All the A’s in the genome had been replaced with an alternative base, 2-aminoadenine, which was later dubbed Z. Usually, C pairs with G and T pairs with A to form double-stranded DNA. But in this virus, with no A’s to be found, T paired with Z. (During gene transcription, T-Z was still treated as though it were T-A.)
The Z base looks like a chemical modification of A; it’s an adenine nucleotide with an extra attachment. But that modest change allows Z to form a triple hydrogen bond with T, which is more stable than the double bond that holds together A-T.