* RNA actually uses a different base than T. This different base is abbreviated as U. We’re leaving that out of the main text for clarity, but since you’re dorky enough to read the footnote, we thought we’d let you know. U is chemically very similar to T, so you might wonder why RNA bothers using something different. We can’t be certain, but there’s a good reason why you might prefer U (for “uracil”) in RNA and T (for “thymine”) in DNA. In short: U is a bit crappier than T. Inside a cell, U takes less energy to make, but is more likely to degrade. This is okay for RNA, which is short-lived. But DNA is the master copy for RNA, so it needs to be more robust.