Why DNA Contains Thymine Instead of Uracil

Chapter 27 covers the chemistry and structures of nucleosides, nucleotides, and nucleic acids (RNA and DNA). You will see—mechanistically—why ATP is the universal carrier of chemical energy, how nucleotides are liked to form nucleic acids, why DNA contains thymines instead of uracils, and how the genetic messages encoded in DNA are transcribed into mRNA and then translated into proteins. Also explained are how the sequence of bases in DNA is determined and how DNA with specific base sequences can be synthesized. 

Uracil DNA glycosylases, for example, found in most cells, specifically remove from DNA the uracil that results 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. The capacity to distinguish thymine from uracil, the product of cytosine deamination—necessary for the selective repair of the latter—may be one reason why DNA evolved to contain thymine instead of uracil (p. 293). 

Nucleic acids (DNA and RNA) contain substituted purines and substituted pyrimidines (Section 26.1) DNA contains adenine, guanine, cytosine, and thymine (abbreviated A, G, C, and T) RNA contains adenine, guanine, cytosine, and uracil (A, G, C, and U). Why DNA contains T instead of U is explained in Section 26.10. Unsubstituted purine and pyrimidine are not found in nature. Guanine (a hydroxypurine) and cytosine, uracil, and thymine (hydroxypyrimidines) are more stable in the keto form than in the enol form, so the keto forms are shown here. We will see that the preference for the keto form is crucial for proper base pairing in DNA (Section 26.3). 

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