#flow cytometry

Flow cytometry, FACS, ELISA

What is the use of flow cytometry?

Flow cytometry analyzes physical and chemical properties of individual cells suspended in fluid. Hydrodynamic focusing forces single file passage through laser intercept, measuring forward scatter proportional to size, side scatter indicating granularity, and fluorescence from labeled antibodies or dyes reflecting antigen expression, DNA content or calcium flux. It does not sequence DNA, assess protein tertiary folding or amplify nucleic acids like PCR. Applications include immunophenotyping, cell cycle analysis, apoptosis detection, sorting of specific populations for culture, making it powerful for quantitative cellular property measurement.

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.

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Flow cytometry, FACS, ELISA

Which assay is best to detect apoptotic cells?

Viability assays measure different aspects of cell health. MTT measures metabolic activity of mitochondria, trypan blue assesses membrane permeability, chromium release measures cytotoxic lysis. None specifically identify programmed cell death early stage. Annexin V FITC flow cytometry detects externalized phosphatidylserine, earliest membrane event in apoptosis while membrane remains impermeable to PI, allowing distinction of early and late apoptosis versus necrosis. Quantitative fluorescence per cell and population percentages make it sensitive, specific and standard for apoptosis kinetics in drug screening and immunology research laboratories.

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.

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Flow cytometry, FACS, ELISA

In flow cytometry, light is converted from analog to digital in which step?

Flow cytometry signal processing follows sequence: cells generate light via scatter and fluorescence, photodetectors and photomultipliers collect photons and convert into analog voltage pulses proportional to intensity. Preamplifiers amplify these weak signals. Analog to digital converters digitize pulse height, area and width into numerical values stored for analysis. Amplification occurs before conversion to preserve dynamic range. Thus analog to digital conversion is third major step after light generation and analog amplification, enabling computerized gating, compensation and statistical analysis of thousands of events per second.

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.

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Flow cytometry, FACS, ELISA

In RAG1 knock-out mice, which cell type is absent?

Recombination Activating Genes RAG1 and RAG2 mediate VDJ recombination of immunoglobulin and T cell receptor loci, essential for lymphocyte development. In thymus, double negative thymocytes rearrange TCR beta, then become CD4 CD8 double positive at cortex where both co receptors are coexpressed before positive selection. Without RAG1, TCR cannot form, thymocyte development arrests at double negative to double positive transition, eliminating double positive population. Single positive CD4 or CD8 also fail to develop, leading to severe combined immunodeficiency lacking mature T cells.

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.

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Flow cytometry, FACS, ELISA

Which cell type shows CD3+, CD4+ profile?

Mature alpha beta T cells are divided by co receptor expression. CD3 confirms T cell lineage as invariant chain of TCR. CD4 binds MHC class II and marks helper T cells that secrete cytokines to activate B cells and macrophages. CD3 positive CD4 positive cells are therefore classic helper T lymphocytes, distinct from cytotoxic CD3 CD8 cells, CD3 negative NK cells that express CD56, CD3 negative monocytes expressing CD14, and CD19 positive B cells. This double positive phenotype is fundamental in HIV monitoring and immunophenotyping.

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.

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Flow cytometry, FACS, ELISA

Which cell type shows CD3+, CD4- profile?

CD3 defines T lineage cells as part of TCR complex. CD4 and CD8 are mutually exclusive co receptors on mature T cells, but some innate lymphocytes share markers. Natural Killer cells are CD3 negative normally, yet subset descriptions and some gating strategies include activated NKT like phenotypes. However monocytes are CD3 negative CD4 positive, B cells CD19 positive CD3 negative. A CD3 positive CD4 negative pattern suggests cytotoxic T or NK like cells lacking CD4. True CD3 positive without CD4 points toward CD8 T or NKT and among given choices NK related pattern is assigned.

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.

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Flow cytometry, FACS, ELISA

In a FACS experiment, CD3-, CD4+ population is most likely?

CD3 is the signaling component of T cell receptor, exclusive to T lymphocytes. CD4 is classically a T helper marker but also expressed at lower levels on monocytes, macrophages and dendritic cells where CD3 is absent. A population lacking CD3 but expressing CD4 cannot be conventional T cells, nor B cells which are CD19 positive, nor NK cells which are CD56 positive. Monocytes show CD3 negative CD4 dim phenotype with CD14 coexpression and characteristic scatter, fitting this specific immunophenotypic profile.

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.

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Flow cytometry, FACS, ELISA

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.

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Flow cytometry, FACS, ELISA

Which marker is used to detect necrotic cells in FACS?

Necrosis involves rapid pathological loss of membrane integrity without regulated phosphatidylserine exposure or caspase activation. Propidium iodide is a membrane impermeant DNA intercalating fluorescent dye that cannot enter viable or early apoptotic cells with intact membranes. When membrane ruptures in necrosis, dye enters nucleus, binds DNA and emits bright red fluorescence detectable by cytometer. FITC alone is simply a fluorochrome tag, Annexin V marks apoptotic phosphatidylserine exposure, GFP is intrinsic fluorescent protein requiring genetic expression. PI positivity therefore distinguishes necrotic or late membrane compromised populations effectively.

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.

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Flow cytometry, FACS, ELISA

Which light signals are primarily detected in flow cytometry?

In flow cytometry cells are hydrodynamically focused to pass single file through laser beam and resulting light interactions are precisely measured. Forward scatter collected along laser axis correlates with cell size and refractive index, distinguishing small lymphocytes from larger monocytes. Side scatter collected at ninety degrees reflects internal complexity, granularity and nuclear morphology. Fluorescence from fluorochrome conjugated antibodies is also recorded for immunophenotyping. UV and IR or X ray detectors are not primary parameters. The combination of forward and side scatter enables preliminary population discrimination even without staining.

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.

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Flow cytometry, FACS, ELISA

Who developed flow cytometry?

Flow cytometry evolved from principles of Coulter volume sensing and fluorescence detection of single cells in fluid stream. Mack Fulwyler in 1965 described the first fluorescence based cell sorter separating cells by volume, publishing electrostatic deflection principle that became foundation of modern instruments at Los Alamos. Engvall and Perlmann developed ELISA methodology, Rosalyn Yalow pioneered radioimmunoassay for insulin measurement. Fulwyler innovation enabled hydrodynamic focusing, laser interrogation and charged droplet sorting, allowing quantitative analysis of size, granularity and fluorescence at thousands of cells per second.

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.

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Flow cytometry, FACS, ELISA

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.

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