Novel organisms, including DNA construct host cell combinations, are disclosed. The organisms comprise a transcription unit (e.g. operon) comprising DNA sequences encoding for enzymes which promote the supply of single carbon units for the conversion of dUMP to dTMP. Examples include: dihydrofolate reductase genes e.g. T4 frd; Serine Hydroxymethyltransferase genes e.g. glyA; 3-phosphoglycerate dehydrogenase genes e.g. serA; and THF synthase genes e.g. ADE3. The organisms are used in a biological method of producing thymidine with significantly reduced levels of uridine.