Which nitrogen base is absent in RNA?

In the structure of RNA (ribonucleic acid), thymine is indeed the nitrogen base that is absent. Instead of thymine, RNA contains uracil. Thymine is primarily found in DNA (deoxyribonucleic acid), where it pairs with adenine during the formation of the DNA double helix structure.

The presence of uracil in RNA serves a similar function, pairing with adenine during the processes of transcription and translation. This fundamental difference is crucial, as it contributes to the distinct functions and properties of RNA and DNA. Understanding this base pairing and the differing nitrogen bases is key in studying nucleic acids and their roles in genetics and cellular processes.