thymidine

Thymidine, also called deoxythymidine (dThd), is a deoxyribonucleoside composed of the pyrimidine base thymine linked to a deoxyribose sugar. Its chemical formula is C10H14N2O5. In DNA, thymidine is one of the four standard deoxyribonucleosides and pairs with adenine through two hydrogen bonds.

When phosphorylated, thymidine exists as dTMP, dTDP, and dTTP, which serve as the nucleotide building blocks required for DNA replication and repair. dTTP is the triphosphate form used by DNA polymerases to extend the growing DNA strand.

Biosynthesis occurs via two pathways. In de novo synthesis, thymidylate synthase converts deoxyuridine monophosphate (dUMP) to dTMP using methylene tetrahydrofolate as a cofactor. In salvage pathways, thymidine kinase phosphorylates thymidine to form dTMP. Cells rely on both routes to maintain adequate dTMP/dTTP pools for DNA synthesis.

Thymidine and its metabolism are targets in medicine. In cancer therapy, inhibitors of thymidylate synthase, such as 5-fluorouracil, deplete dTMP and disrupt DNA synthesis; folate antagonists like methotrexate have related effects. AZT, a thymidine analog (3′-azido-3′-deoxythymidine), is used as an antiretroviral agent against HIV by inhibiting reverse transcription.

Thymidine is present in the DNA of all organisms and is widely used in molecular biology and cell culture as a substrate for DNA synthesis and in various nucleotide assays.