When DNA is made into RNA by the process called RNA transcription, one of the four bases, thymine, is replaced by uracil. It is hypothesized that uracil actually came first, but it is not very stable. Hence, it is modified to thymine to be more stable in DNA, the blueprint of life. However, uracil is necessary for RNA function, which also leads to protein translation.
More doodles can be found on the Biochemies blog.