In RNA, what base replaces thymine?

In RNA, the base that replaces thymine is uracil. While DNA contains four nitrogenous bases—adenine, thymine, cytosine, and guanine—RNA has a similar but distinct set. Specifically, in RNA, adenine pairs with uracil instead of thymine. This substitution occurs because uracil serves a similar function to thymine in the context of base pairing, maintaining the essential complementary structure needed for the formation of RNA molecules.

Uracil is structurally similar to thymine but lacks a methyl group, which differentiates it from thymine. This modification is significant as it contributes to the different stability and functional characteristics of RNA compared to DNA. The presence of uracil is crucial for various processes such as transcription and translation, where RNA plays a vital role in synthesizing proteins from the genetic code. Understanding this replacement is fundamental to grasping how genetic information is processed and expressed in living organisms.