Thymidine is a nucleoside, which is a building block of DNA. It consists of a thymine base attached to a deoxyribose sugar. Thymidine plays a crucial role in the structure of DNA, where it pairs with adenine during the formation of the double helix. In addition to its structural role, thymidine is important in cellular processes such as DNA replication and repair. It can be synthesized in the body or obtained from dietary sources. Thymidine is also used in research and medical applications, including studies on cancer and viral infections.