Practice question

During early apoptosis membrane asymmetry collapses and phosphatidylserine translocates from inner to outer leaflet while plasma membrane remains intact and impermeable. Annexin V is a calcium dependent cellular protein that binds phosphatidylserine with high affinity and specificity. Conjugating it with FITC creates a bright fluorescent probe easily detectable by flow cytometry and fluorescence microscopy. PI, DAPI and Hoechst are DNA intercalating dyes staining nuclei only when membrane integrity is lost. Therefore Annexin V FITC specifically identifies early apoptotic cells before permeabilization occurs in culture.