Terminology

De novo Pathway - A term used to describe biochemical synthetic reactions where products are made completely from raw materials, as opposed to the situation where products are made by "rebuilding" a substrate resulting from breakdown of the product. The latter are called salvage pathways. Figure 22.1.
HAT Medium - A cell culture medium augmented by hypoxanthine, aminopterin, and thymidine, which selects for cells that have a functional purine salvage pathway.
Hyperuricemia - A condition in about 0.3% of the human population involving chronic elevation of blood uric acid levels, well beyond normal limits.
Mutator Phenotypes - Cell lines that show increased rates of spontaneous mutataion at all genetic loci tested.
Nucleoside - A molecule that, upon complete hydrolysis, yields at least 1 mole per mole of a purine or pyrimidine base and a sugar.
Nucleoside Diphosphate - A nucleoside with two phosphates attached to it.
Nucleoside Monophosphate - A nucleoside with one phosphate attached to it.
Nucleoside Triphosphate - A nucleoside with three phosphates attached to it.
Nucleotide - A molecule that, upon complete hydrolysis, yields at least 1 mole per mole of a purine or pyrimidine base, a sugar, and inorganic phosphate.
purR - An E. coli repressor protein controlling expression of genes in de novo purine biosynthesis. It binds either hypoxanthine or guanine. The resulting complex binds to DNA sites near several genes of purine (and pyrimidine) synthesis, inhibiting their transcription.
Salvage Pathway - A term used to describe biochemical synthetic reactions where products are made by rebuilding a substrate resulting from breakdown of the product (or a related product).
Synchronizing Cell Cultures - Laborotory procedure of manipulating cells to bring all of the them to the same phase of the cell cycle. It is accomplished by a thymidine block.
Thymidine Block - Mechanism for synchronizing cells in which thymidine is added to cells to inhibit DNA synthesis. This prevents passage of the cells from the G1 to S phase of the cell cycle. Subsequent transfer of cells to a medium lacking thymidine reverses the inhibition and allows them to initiate DNA synthesis synchronously.