Why is thymine in DNA and uracil in RNA?

Since RNA is shorter-lived than DNA, it can get by with the energetically “cheaper” uracil. Also, adding the hydrophobic methyl group changes the shape of the DNA molecule and allows thymine to base-pair only with adenine, whereas uracil would base-pair less selectively. read more

The existence of thymine in DNA instead of uracil is apparently due to evolution process which made DNA more stable. Thymine has greater resistance to photochemical mutation, making the genetic message more stable. read more

RNA stands for ribose nucleic acid. Ribose is a very simple sugar, like glucose, and is easily derived from formaldehyde (HCHO), which is a main product in experiments simulating the primitive earth. read more

from spontaneous deamination of cytosine. Mutant cells that lack this enzyme have a high rate of G≡C to A꞊T mutations. This glycosylase does not remove uracil residues from RNA or thymine residues from DNA. read more