Which base is found in DNA but not in RNA?

In DNA, thymine is one of the four nitrogenous bases, alongside adenine, cytosine, and guanine. In RNA, however, thymine is replaced by uracil, which pairs with adenine during RNA synthesis. This key difference in base composition is fundamental in distinguishing the two types of nucleic acids.

Thymine's presence in DNA instead of RNA plays a crucial role in the stability of the DNA double helix structure, as thymine forms strong hydrogen bonds with adenine, ensuring accurate base pairing during DNA replication. Conversely, uracil's incorporation in RNA contributes to its different structural and functional characteristics, where it is typically single-stranded and involved in various roles, including protein synthesis and regulation of gene expression.

The other bases—adenine, cytosine, and guanine—are common to both DNA and RNA, making thymine unique as the base exclusively found in DNA. Thus, recognizing this distinction is fundamental to understanding the molecular biology of nucleic acids.