Biochemical Techniques Practice Questions

Q: Which of the following methods is not antibody-dependent?

SDS-PAGE. **Explanation:** The correct answer is **SDS-PAGE** (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis). **Why SDS-PAGE is the correct answer:** SDS-PAGE is a technique used to separate proteins based solely on their **molecular weight**. The process involves denaturing proteins with SDS (an anionic detergent) which imparts a uniform negative charge to the proteins. They are then separated by size as they migrate through a polyacrylamide gel toward the anode. This process is purely physical and chemical; it does **not** require the specificity of an antibody-antigen interaction. **Analysis of Incorrect Options:** * **ELISA (Enzyme-Linked Immunosorbent Assay):** This is the gold standard for antibody-dependent assays. It utilizes specific antibodies (primary and secondary) to detect and quantify antigens or antibodies in a sample. * **Chromatin Immunoprecipitation (ChIP) assay:** As the name suggests ("Immuno"), this technique uses specific antibodies to isolate DNA-binding proteins (like transcription factors or histones) along with the DNA sequences they are bound to. * **Flow Cytometry:** This technique frequently uses **fluorescently labeled antibodies** to identify specific cell surface markers (CD markers) or intracellular proteins to characterize cell populations. **High-Yield Clinical Pearls for NEET-PG:** * **Western Blot:** Often confused with SDS-PAGE. While SDS-PAGE separates the proteins, the subsequent "blotting" and detection phase **is** antibody-dependent. * **Southern Blot:** Detects DNA (Mnemonic: **S**outhern-**D**NA). * **Northern Blot:** Detects RNA (Mnemonic: **N**orthern-**R**NA). * **Western Blot:** Detects Protein (Mnemonic: **W**estern-**P**rotein). * **SDS-PAGE Utility:** It is the first step in Western Blotting and is used to determine protein purity and estimate molecular weight.

Q: What color does a positive Fouchet's test yield?

Green. **Explanation:** **Fouchet’s test** is a qualitative biochemical method used to detect the presence of **bilirubin** in urine. This is a crucial diagnostic tool for identifying obstructive jaundice or hepatocellular jaundice, where conjugated bilirubin is excreted in the urine (bilirubinuria). 1. **Why Green is Correct:** The test relies on the oxidation of bilirubin. When Fouchet’s reagent (which contains trichloroacetic acid and ferric chloride) is added to urine pre-treated with barium chloride, the ferric chloride acts as an oxidizing agent. It oxidizes the yellow-colored **bilirubin** into **biliverdin**, which is **green** in color. A positive result is indicated by the appearance of a green or bluish-green precipitate. 2. **Analysis of Incorrect Options:** * **Red:** This is characteristic of a positive **Rothera’s test** (detecting ketones) or a positive **Benzidine test** (detecting blood/hemoglobin). * **Violet:** A violet or purple ring is seen in the **Hopkins-Cole test** (for tryptophan) or the **Biuret test** (for proteins). * **Yellow:** This is the baseline color of normal urine and the color of bilirubin itself before oxidation. **High-Yield Clinical Pearls for NEET-PG:** * **Principle:** Oxidation of bilirubin to biliverdin. * **Reagent Components:** Trichloroacetic acid (precipitates proteins) and Ferric chloride (oxidizing agent). * **Clinical Significance:** Bilirubin appears in urine only when it is **conjugated** (water-soluble). Therefore, Fouchet’s test is positive in obstructive and hepatic jaundice but **negative in hemolytic jaundice** (where unconjugated bilirubin predominates). * **Pre-step:** Barium chloride is added first to concentrate the bilirubin by adsorbing it onto the barium sulfate precipitate.

Q: Which enzyme is commonly used in ELISA?

Alkaline phosphatase. **Explanation:** ELISA (Enzyme-Linked Immunosorbent Assay) is a biochemical technique used to detect the presence of an antigen or antibody in a sample. The core principle involves an enzyme conjugated to an antibody which reacts with a colorless substrate to produce a measurable colored product (chromogenic reaction). **Why Alkaline Phosphatase (ALP) is correct:** Alkaline Phosphatase and **Horseradish Peroxidase (HRP)** are the two most commonly used enzymes in ELISA. ALP is preferred because of its high catalytic turnover rate, stability at room temperature, and the availability of sensitive substrates like p-nitrophenyl phosphate (pNPP), which yields a distinct yellow color upon reaction. **Why other options are incorrect:** * **Acid Phosphatase:** Unlike ALP, this enzyme functions at an acidic pH, which is not compatible with the physiological buffers (like PBS) required to maintain the structural integrity of antibodies and antigens during the assay. * **Glucosidase:** While used in some metabolic studies, it lacks the high sensitivity and rapid signal amplification required for standard diagnostic ELISA. * **Glycosyl transferase:** This is a biosynthetic enzyme involved in carbohydrate chain formation; it does not produce the easily measurable colorimetric change necessary for a detection assay. **High-Yield Clinical Pearls for NEET-PG:** * **Most common enzymes in ELISA:** Horseradish Peroxidase (HRP) > Alkaline Phosphatase (ALP) > β-galactosidase. * **Substrate for ALP:** p-nitrophenyl phosphate (pNPP). * **Substrate for HRP:** TMB (Tetramethylbenzidine) or OPD (o-phenylenediamine). * **Application:** ELISA is the **screening test** of choice for HIV (Western Blot is the confirmatory test).

Q: Match the following blotting techniques with the type of biomolecule they detect: Blotting Technique Detects A. Southern blot 1. RNA B. Northern blot 2. Lipids C. Western blot 3. DNA D. Eastern blot 4. Protein

A-3, B-1, C-4, D-2. ***A-3, B-1, C-4, D-2*** - **Southern blot** is named after Edwin Southern and is the foundational technique used to detect specific **DNA** sequences. - **Northern blot** detects specific **RNA** sequences, while **Western blot** targets specific **Proteins** using antibodies. ***A-1, B-3, C-2, D-4*** - This option incorrectly matches Southern blot (A) with RNA (1) and Northern blot (B) with DNA (3). - The standard nomenclature links Southern with **DNA** and Northern with **RNA** due to their evolutionary development from the original technique. ***A-4, B-2, C-1, D-3*** - This option incorrectly assigns Southern blot (A) to Protein (4) and Northern blot (B) to Lipids (2). - Western blot (C) detects **Protein**, not RNA (1), as suggested here. ***A-2, B-4, C-3, D-1*** - This option incorrectly matches Southern blot (A) with Lipids (2) and Western blot (C) with DNA (3). - **Eastern blot** (D) is a technique designed to detect **post-translational modifications** (like lipids and carbohydrates) on proteins, making the D-2 match plausible, but the other matches are incorrect (i.e., **Southern blot** detects DNA).

Q: What is the technique shown in the graph that demonstrates antibody-antigen interaction and response kinetics?

Surface plasmon resonance for affinity. ***Surface plasmon resonance for affinity*** - **Surface Plasmon Resonance (SPR)** is the only technique among these capable of measuring biological interactions **in real time**, displaying a sensorgram that maps response units versus time, indicating association and dissociation phases. - The curve shown depicting the binding (association rate, $\text{k}_{on}$) and unbinding (dissociation rate, $\text{k}_{off}$) kinetics is characteristic of SPR, which allows for the accurate calculation of **binding affinity ($\text{K}_{D})$** derived from these kinetic constants. ***Competitive ELISA for antibody affinity*** - **ELISA** is an endpoint assay; it records the final quantity of bound product rather than the **real-time kinetic** curves (association and dissociation) shown in the graph. - While affinity can be approximated using specialized competitive ELISA formats, it relies on static equilibrium measurements, not the detailed kinetic analysis provided by $\text{k}_{on}$ and $\text{k}_{off}$. ***Competitive ELISA for antibody concentration*** - This technique is typically used for **quantitation** (determining concentration) by comparing an unknown sample to a standard curve at a single time point (endpoint). - It cannot generate the characteristic graph showing dynamic changes in the **association and dissociation profile** over time, which are essential for kinetic analysis. ***Surface plasmon resonance for concentration*** - While SPR can be used for quantitation, the primary purpose of generating and analyzing the complex **full kinetic curve** (both association and dissociation phases) is to precisely determine the binding affinity ($\text{K}_{D}$). - Simple concentration measurements using SPR often involve a brief contact time or steady-state analysis, not the detailed monitoring of the specific **dissociation phase** necessary to calculate $\text{k}_{off}$ and thus affinity.

Question 1

What is the primary purpose of flow cytometry?

Accepted Answer

To detect and quantify specific cell populations based on surface or intracellular markers

Answer

To measure blood flow velocity in the brain

Answer

To determine the net oxygen supply to tissues

Answer

To assess the amount of oxygen bound to hemoglobin

Question 2

The ability of amino acids/proteins to behave like zwitterions forms the basis for separating them using which of the following techniques?

Accepted Answer

Isoelectric focusing

Answer

Gel filtration chromatography

Answer

Ion exchange chromatography

Answer

Mass spectrometry

Question 3

Which of the following methods is not antibody-dependent?

Accepted Answer

SDS-PAGE

Answer

ELISA

Answer

Chromatin Immunoprecipitation assay

Answer

Flow cytometry

Question 4

Which of the following statements is true regarding fluorescence?

Accepted Answer

Release of light of a longer wavelength after absorbing light of a shorter wavelength.

Answer

Spontaneous illumination in the dark.

Answer

Release of light of a shorter wavelength after absorbing light of a longer wavelength.

Answer

Continuous emission of lights of different wavelengths.

Question 5

What color does a positive Fouchet's test yield?

Accepted Answer

Green

Answer

Red

Answer

Violet

Answer

Yellow

Question 6

Which enzyme is commonly used in ELISA?

Accepted Answer

Alkaline phosphatase

Answer

Acid phosphatase

Answer

Glucosidase

Answer

Glycosyl transferase

Question 7

Nephelometry is based on the principle of what?

Accepted Answer

Light attenuated in intensity by scattering

Answer

Refraction of light

Answer

Reduced transmission of light

Answer

Filtration of solutes by kidney

Question 8

Which amino acid migrates fastest on paper chromatography on methylcellulose medium?

Accepted Answer

Valine

Answer

Aspartic acid

Answer

Lysine

Answer

Glycine

Question 9

Match the following blotting techniques with the type of biomolecule they detect: Blotting Technique Detects A. Southern blot 1. RNA B. Northern blot 2. Lipids C. Western blot 3. DNA D. Eastern blot 4. Protein

Accepted Answer

A-3, B-1, C-4, D-2

Answer

A-4, B-2, C-1, D-3

Answer

A-2, B-4, C-3, D-1

Answer

A-1, B-3, C-2, D-4

Question 10

What is the technique shown in the graph that demonstrates antibody-antigen interaction and response kinetics?

Accepted Answer

Surface plasmon resonance for affinity

Answer

Competitive ELISA for antibody concentration

Answer

Surface plasmon resonance for concentration

Answer

Competitive ELISA for antibody affinity

Biochemical Techniques — MCQs

Biochemical Techniques — MCQs

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