A famous investigator of the mechanisms of transcription and translation of genetic information is Kenyon alumnus Harvey Lodish. For this reason, we believe RNA evolved first as the genetic material of cells. The base uracil replaces thymine.
Uracil lacks the methyl group of thymine.
Biochemically, thymine is made by methylating uracil. Most RNA in cells is single-stranded. Some RNAs, such as tRNA, make complex intramolecular base pairs, and have modified unusual bases. Some RNA molecules themselves are the final functional product of the gene from which they are transcribed. In eukaryotes, there are other classes of functional RNAs that participate in splicing of messenger RNA and other activities.