Microscopy

Microscopy

Which technique uses colorimetric detection but not enzyme?

Colorimetric immunoassays fundamentally rely on enzyme-substrate chemistry to generate visible product. In ELISA, horseradish peroxidase or alkaline phosphatase linked to antibody catalyzes conversion of TMB or pNPP into colored compound, intensity proportional to antigen amount; no enzyme means no color. Western blot chromogenic detection also uses enzyme-conjugated antibodies acting on BCIP/NBT system to deposit insoluble colored precipitate on membrane. Immunofluorescence, by contrast, depends on fluorochrome excitation and emission, detected as fluorescence under specific filters, not as absorbable color. Therefore no listed method combines colorimetric readout without enzymatic amplification.

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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Which antibody fragment cannot precipitate?

Precipitation requires formation of large lattice through cross-linking where bivalent or multivalent antibodies bridge polyvalent antigens. Intact IgG with two Fab arms and F(ab')2 dimer retain bivalency, enabling network extension and insoluble aggregate generation. Whole antibody adds Fc but remains precipitable. In contrast, Fab fragment obtained by papain digestion is monovalent, possessing single binding site. It can bind epitope and block it, but cannot connect two antigen molecules to build lattice. Resulting complexes stay small and soluble, invisible in gel. Therefore Fab is used to inhibit agglutination or precipitation, not promote it.

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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Which agglutination is used in blood group testing?

Blood grouping depends on detecting antigens naturally present on erythrocyte membrane such as A, B and RhD. When specific antiserum is mixed with whole blood, IgM antibodies bind directly to intrinsic surface antigens without requiring artificial carrier coupling. This direct agglutination occurs because red cells themselves act as particulate antigen, forming visible clumps without intermediate coating step. Passive agglutination involves antigens attached to latex or red cells secondarily, useful for soluble antigens. Fluorescent or double variants are not standard for routine ABO typing. Hence direct method remains reference for transfusion compatibility.

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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Which substrate-enzyme pair gives yellow product in alkaline pH?

Para-nitrophenyl phosphate serves as artificial substrate for alkaline phosphatase, which catalyzes hydrolysis of phosphate ester at alkaline pH optimum around 9.5. Cleavage liberates para-nitrophenol, which under basic conditions exists as para-nitrophenolate anion exhibiting intense yellow color with absorbance near 405 nm. Reaction is widely used for quantitative ELISA because product remains soluble and obeys Beer-Lambert law. Horseradish peroxidase with TMB generates blue intermediate. Luciferase-luciferin pair emits light rather than yellow. Alkaline phosphatase with NBT produces purple precipitate. Hence pNPP uniquely yields yellow signal in phosphatase systems.

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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Why is GFP unique in detection?

Green fluorescent protein isolated from jellyfish Aequorea victoria contains intrinsic chromophore formed autocatalytically from tripeptide Ser-Tyr-Gly cyclization and oxidation within beta-barrel structure. No external cofactor, substrate or enzymatic activation is required beyond molecular oxygen for maturation. Once folded, it absorbs blue light near 395 nm and emits green photons near 509 nm. This self-contained fluorescence distinguishes it from luciferase which needs luciferin and ATP or peroxidase requiring luminol. Consequently GFP can be expressed as genetic fusion tag enabling live cell imaging, lineage tracing and biosensor construction without exogenous chemistry.

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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Which chromogenic substrate gives purple product with ALP?

Alkaline phosphatase paired with nitroblue tetrazolium chloride chemistry generates insoluble formazan precipitate through reduction. NBT accepts electrons, converting from yellow soluble tetrazolium to dark purple-blue diformazan that deposits at enzymatic site. Often combined with 5-bromo-4-chloro-3-indolyl phosphate which yields indigo intermediate, enhancing localization. TMB is substrate for horseradish peroxidase, not alkaline phosphatase, giving blue. ONPG is hydrolyzed by beta-galactosidase to yellow ortho-nitrophenol. Para-nitrophenyl phosphate with phosphatase produces yellow soluble product, not purple. Thus NBT deposition indicates phosphatase activity in blotting and in situ assays.

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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Which ELISA is best to detect minor antigen concentration differences?

Competitive ELISA excels for low molecular weight haptens and subtle concentration variations because signal is inversely proportional to analyte and competition amplifies small differences. Limited antibody sites create steep competition curve around IC50, so slight rise in sample antigen displaces labeled competitor measurably. Sandwich formats depend on two epitopes and are better for larger proteins but suffer less resolution at low range because excess antigen increases signal linearly. Direct and indirect formats lack competition step and are less sensitive to minor changes. Hence for steroid hormones, toxins and drug monitoring, competitive design provides superior discrimination of minute differences.

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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A test that measures soluble antibody with soluble antigen in gel is called:

When both antibody and antigen are soluble proteins, interaction can be visualized by allowing them to diffuse and form insoluble lattice in semi-solid medium. Immunoprecipitation encompasses methods where soluble immune complexes exceed solubility threshold and separate as precipitate, often facilitated by gel to stabilize band. Agglutination refers to particulate antigens like cells or latex beads, not soluble molecules. Radioimmunoassay employs radioactive labels for quantification, and flow cytometry measures cell surface fluorescence in suspension. Therefore detection of soluble-soluble reaction in gel, such as Ouchterlony, Mancini radial diffusion or immunoelectrophoresis, falls under precipitation category.

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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Ouchterlony method is best for:

Double immunodiffusion developed by Ouchterlony involves diffusion of antigens and antibodies from separate wells toward each other in agar gel. Precipitin lines form at equivalence, and their fusion patterns indicate antigenic relatedness. Complete fusion suggests shared epitopes and identity, spur formation indicates partial cross-reactivity, while crossing lines denote distinct antigens. This pattern analysis makes technique ideal for comparing antisera potency, identifying strain relatedness among microbial isolates and testing antigenic similarity of toxin variants. It does not quantify hormones, detect immunoglobulin class specifically or amplify nucleic acids, which require other analytical platforms.

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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Which is true about Fab region?

Antibody molecule comprises Fab and Fc segments after papain cleavage. Fab includes entire light chain paired with variable and first constant domain of heavy chain. Hypervariable complementarity determining regions within variable domains fold to create paratope complementary to epitope shape, charge and hydrophobicity. This interaction determines specificity, affinity and neutralization capacity. Fc region mediates effector functions like complement fixation and Fc receptor engagement, not antigen recognition. All immunoglobulin isotypes and monoclonal preparations possess Fab, and it lacks intrinsic enzymatic activity, functioning solely as antigen-binding unit guiding humoral immunity and diagnostic immunoassays.

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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What kind of antibody is essential for agglutination?

Agglutination requires cross-linking of particulate antigens such as erythrocytes or bacteria into clumps visible macroscopically. IgM is pentameric comprising five immunoglobulin monomers joined by J chain, presenting ten antigen-binding sites arranged in star-like plane. This multivalency provides high avidity and spans greater intercellular distance, efficiently bridging cells despite electrostatic repulsion and zeta potential. IgG is bivalent and smaller, less capable of overcoming repulsive forces without enhancement media. IgE and IgA have other effector roles. Clinically, anti-ABO isohemagglutinins are often IgM, explaining potent blood group agglutination.

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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Which antigen-antibody ratio leads to max precipitation?

Precipitation reaction is governed by lattice theory described by Marrack. Small soluble complexes form when one component dominates, remaining invisible. At equivalence zone, ratio of antigen to antibody valencies matches, allowing extensive alternating cross-linking into large three-dimensional network. These macromolecular aggregates become insoluble and precipitate efficiently, yielding maximal turbidity and fastest sedimentation. Antigen excess or antibody excess each saturates binding sites with same species, preventing bridging. Therefore laboratories titrate reactants to approach equivalence for immunodiffusion, quantitative precipitin curves and antisera standardization to achieve strongest visible precipitation.

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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