Which quadrant in FACS analysis represents early apoptotic cells?
Apoptosis analysis by flow cytometry typically uses Annexin V FITC on one axis and PI on the other, dividing dot plot into four quadrants. Live cells remain double negative. Early apoptotic cells expose phosphatidylserine but retain intact membranes, becoming Annexin V positive and PI negative. Conventional layout places this population in lower right quadrant, designated quadrant IV. Late apoptotic cells are double positive in upper right, necrotic cells PI only positive in upper left. Quadrant gating thus distinguishes stage specific membrane changes.
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.