Isol. from seedlings of Phaseolus vulgaris (kidney bean) Deoxythymidine is non-toxic and as part of one of the four nucleotides in DNA it is a naturally occurring compound that exists in all living organisms and DNA viruses. RNA has uridine (uracil joined to ribose) instead. Uracil is chemically very similar to thymine, the latter being 5-methyluracil. Since thymine nucleotides are precursors of DNA, not RNA, the prefix "deoxy" is often left out, i.e., deoxythymidine is often just called thymidine.; In its composition, deoxythymidine is a nucleoside composed of deoxyribose (a pentose sugar) joined to the pyrimidine base thymine.; Iododeoxyuridine is a radiosensitizer and increases the amount of DNA damage received from ionizing radiation.; Thymidine (more precisely called deoxythymidine; can also be labelled deoxyribosylthymine, and thymine deoxyriboside) is a chemical compound, more precisely a pyrimidine deoxynucleoside. Deoxythymidine is the DNA nucleoside T, which pairs with deoxyadenosine (A) in double-stranded DNA. In cell biology it is used to synchronize the cells in S phase.; Thymidine is non-toxic and is a naturally occurring compound that exists in all living organisms and DNA viruses. 25% of DNA is composed of thymidine. RNA does not have thymidine and has uridine instead. Thymidine is a chemical compound which is a pyrimidine nucleoside. Thymidine is the DNA base T, which pairs with adenosine in double stranded DNA.