Which cell state shows both PI and Annexin V positive staining in FACS?
Dual staining with Annexin V FITC and PI resolves apoptosis stages. Early apoptosis shows outer phosphatidylserine with intact membrane, Annexin V positive PI negative. Progression to late apoptosis causes secondary membrane permeabilization while phosphatidylserine remains exposed, allowing both dyes to bind. Thus double positive phenotype indicates late apoptotic cells actively disintegrating but of apoptotic origin. Normal cells are double negative, necrotic cells often PI positive alone. This pattern is essential for quantifying apoptosis kinetics and drug induced cell death via FACS.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.