What substitution occurs in RNA compared to DNA during transcription?

In RNA, the nucleotide uracil (U) replaces thymine (T), which is found in DNA. During the process of transcription, when DNA is being copied into RNA, adenine (A) in the DNA template strand pairs with uracil in the RNA. This substitution is fundamental to the structure of RNA, as thymine is not present in RNA. Therefore, the correct identification of the substitution—thymine being replaced by uracil during transcription—highlights the key differences between the two nucleic acids. Understanding this substitution is crucial for grasping the mechanisms of gene expression and the role of RNA in protein synthesis.