Practice question

BrdU, 5-bromo-2'-deoxyuridine, is synthetic halogenated analog of thymidine where methyl group at 5 position replaced by bromine atom similar in size allowing Watson-Crick pairing with adenine during DNA synthesis. During S phase, DNA polymerases incorporate BrdU triphosphate from nucleotide pool into nascent strand in place of thymidine without halting polymerase at low micromolar concentrations. Incorporated BrdU detectable by specific anti-BrdU monoclonal antibodies after DNA denaturation exposes epitope, or by mass spectrometry. Assay quantifies fraction of cells actively replicating genome termed labeling index and S phase duration, monitors cell cycle kinetics, measures proliferation inhibition by cytotoxic agents via decreased incorporation, and tracks lineage in developmental biology through pulse-chase retaining label in slow-cycling stem cells while rapidly dividing progenitors dilute signal over successive divisions. Unlike puromycin incorporation marking protein synthesis or BrUTP marking nascent RNA transcripts, BrdU specifically reports DNA replication providing S phase specific information central to cancer and stem cell research.