Which base is substituted for thymine in RNA?

In RNA, the base that substitutes for thymine is uracil. This substitution is significant because it reflects a fundamental difference in the structure of DNA and RNA. While DNA contains thymine to pair with adenine, RNA utilizes uracil instead. This difference plays a crucial role in RNA's function and stability.

Uracil and thymine are structurally similar, but uracil lacks a methyl group present in thymine. This slight difference is important in the context of RNA, which is often more transient and functions in various roles, such as messenger RNA (mRNA) in protein synthesis, transfer RNA (tRNA), and ribosomal RNA (rRNA). The use of uracil allows RNA to be synthesized more rapidly and potentially contributes to its instability compared to DNA, which is necessary for its short-lived roles in cellular processes.

Thus, uracil's role as a substitute for thymine in RNA is foundational to understanding the molecular biology that distinguishes nucleic acid types.