Practice question

HAT medium selection developed by John Littlefield in 1964 exploits differential nucleotide anabolism pathways to isolate hybrid cells from mixture of parental populations. De novo purine and pyrimidine synthesis requires tetrahydrofolate-dependent enzymes, specifically aminopterin, a folate analog, competitively inhibits dihydrofolate reductase DHFR blocking de novo pathway. Alternative salvage pathway recycles hypoxanthine via hypoxanthine-guanine phosphoribosyltransferase HGPRT converting to IMP and thymidine via thymidine kinase TK to TMP, bypassing blocked de novo routes. Myeloma partners engineered to lack HGPRT through 8-azaguanine selection or TK deficiency via bromodeoxyuridine selection cannot utilize salvage and therefore die in HAT containing hypoxanthine, aminopterin and thymidine. B lymphocytes naturally express HGPRT and TK but are mortal in culture, senescing within days. Only hybrid cells inheriting immortal growth from myeloma and salvage enzymes from B cell survive after 10 to 14 days culture. Subsequent cloning by limiting dilution at 0.5 cell per well ensures monoclonality. This biochemical selection remains gold standard because de novo dependence versus salvage provides elegant genetic selection mechanism.