Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 161: Which technique is used for the study of the molecular structure of biomolecules?

A. Electron microscopy
B. Ion exchange chromatography
C. X-ray crystallography (Correct Answer)
D. Agarose gel electrophoresis

Explanation: ### Explanation **Correct Option: C. X-ray crystallography** **Why it is correct:** X-ray crystallography is the gold-standard technique for determining the **three-dimensional molecular structure** of biomolecules at an atomic level. When X-rays strike a crystallized form of a molecule (like a protein or DNA), they are diffracted into a specific pattern. By analyzing the angles and intensities of these diffracted beams, scientists can create an electron density map to determine the precise position of every atom within the molecule. This technique was famously used to discover the double-helix structure of DNA and the structure of hemoglobin. **Why the other options are incorrect:** * **A. Electron microscopy:** This technique is used to visualize the **morphology and ultrastructure** of cells and organelles. While "Cryo-electron microscopy" can study large complexes, standard EM does not provide the atomic-level structural resolution of crystallography. * **B. Ion exchange chromatography:** This is a **separation technique** based on the net surface charge of molecules. It is used to purify proteins or amino acids, not to determine their internal molecular structure. * **D. Agarose gel electrophoresis:** This is a technique used to **separate fragments** of DNA or RNA based on their size and charge. It provides information about the length of the molecule, not its 3D atomic structure. **High-Yield Facts for NEET-PG:** * **NMR Spectroscopy:** Another technique used for molecular structure, but unlike X-ray crystallography, it can study molecules in **aqueous solution** (dynamic state). * **Ramachandran Plot:** Used to validate the structures obtained from X-ray crystallography by plotting the phi ($\phi$) and psi ($\psi$) angles of amino acids. * **Sequencing:** To determine the *primary* structure (amino acid sequence), Sanger’s method or Edman degradation is used.

Question 162: What is the method of choice for protein identification?

A. Edman technique
B. Mass spectrometry (Correct Answer)
C. Sanger technique
D. 2-D electrophoresis

Explanation: **Explanation:** **Mass Spectrometry (MS)** is currently the "gold standard" and method of choice for protein identification due to its high sensitivity, speed, and ability to analyze complex mixtures. The process involves ionizing protein fragments (peptides) and measuring their mass-to-charge ratio ($m/z$). These mass signatures are then compared against genomic databases to identify the protein with high precision. Modern proteomics relies heavily on techniques like **MALDI-TOF** (Matrix-Assisted Laser Desorption/Ionization-Time of Flight) for rapid identification. **Analysis of Incorrect Options:** * **Edman Technique:** This was the traditional method for N-terminal sequencing. It involves the stepwise removal of amino acids using phenylisothiocyanate. While accurate, it is slow, requires large amounts of pure protein, and cannot sequence proteins with "blocked" N-terminals. * **Sanger Technique:** Historically significant as the first method used to sequence insulin (using 1-fluoro-2,4-dinitrobenzene), it is now obsolete for routine protein identification as it only identifies the N-terminal amino acid. * **2-D Electrophoresis:** This is a **separation** technique, not an identification technique. It separates proteins based on two properties: isoelectric point (pI) in the first dimension and molecular weight (SDS-PAGE) in the second. It is often used *prior* to Mass Spectrometry. **High-Yield Facts for NEET-PG:** * **Sanger’s Reagent:** 1-fluoro-2,4-dinitrobenzene (FDNB). * **Edman’s Reagent:** Phenylisothiocyanate (PITC). * **Proteomics:** The large-scale study of proteins; Mass Spectrometry is its cornerstone. * **Western Blot:** Used for the detection of a *specific* protein using antibodies, but not for de novo identification of unknown proteins.

Question 163: Periodic acid-Schiff (PAS) stain does not typically stain which of the following?

A. Fungal cell wall
B. Basement membrane of bacteria
C. Glycogen
D. Lipids (Correct Answer)

Explanation: **Explanation:** The **Periodic acid-Schiff (PAS)** stain is a histochemical technique used to detect structures rich in **polysaccharides** (carbohydrates). **Mechanism:** Periodic acid acts as an oxidizing agent that breaks the carbon-carbon bonds between 1,2-glycols, converting them into **aldehydes**. These aldehydes then react with the **Schiff reagent** to produce a characteristic magenta/bright pink color. **Why Lipids are the correct answer:** Lipids (specifically neutral fats and triglycerides) lack the 1,2-glycol groups necessary for this reaction. Therefore, they do not react with PAS and appear clear or empty on the slide. Lipids are typically stained using specialized stains like **Sudan Black B** or **Oil Red O**. **Why the other options are incorrect:** * **Glycogen:** This is a glucose polymer with abundant glycol groups, making it highly PAS-positive. It is the classic example of PAS staining in the liver. * **Fungal cell wall:** The cell walls of fungi contain high concentrations of polysaccharides like **chitin** and glucans, which stain intensely with PAS (used to diagnose Candidiasis or Histoplasmosis). * **Basement membrane:** It is rich in **Type IV collagen** and glycoproteins (sialic acid), which are PAS-positive. This is clinically useful in identifying glomerular basement membrane thickening in renal biopsies. **High-Yield Clinical Pearls for NEET-PG:** * **Diastase Sensitivity:** To confirm if a PAS-positive substance is glycogen, the section is treated with diastase (amylase). If the staining disappears, it is glycogen (**PAS-D test**). * **Whipple’s Disease:** PAS is the stain of choice to identify **Tropheryma whipplei** (PAS-positive macrophages in the lamina propria). * **Erythroleukemia (M6):** PAS positivity is seen in the erythroblasts. * **Alpha-1 Antitrypsin Deficiency:** Characterized by PAS-positive, diastase-resistant globules in hepatocytes.

Question 164: Which of the following blotting techniques does not exist?

A. Western
B. Southern
C. Eastern (Correct Answer)
D. Northern

Explanation: **Explanation:** In biochemistry, blotting techniques are used to identify specific macromolecules (DNA, RNA, or proteins) from a complex mixture. The naming convention for these techniques is a mix of eponymous honors and directional puns. **Why "Eastern" is the correct answer:** While Southern, Northern, and Western blots are standard, foundational techniques in molecular biology, **Eastern blotting** is not a primary, universally recognized technique for the detection of a major nucleic acid or protein class. Although some specialized research papers use the term "Eastern blot" to describe the analysis of post-translational modifications (like lipids or carbohydrates), it is **not** considered a standard diagnostic or foundational blotting technique in the context of medical examinations like NEET-PG. **Analysis of Incorrect Options:** * **Southern Blotting:** Named after **Edwin Southern**, this technique is used to detect specific **DNA** sequences. (Mnemonic: **S**outhern = **D**NA). * **Northern Blotting:** A play on the name Southern, this technique is used to detect specific **RNA** molecules to study gene expression. (Mnemonic: **N**orthern = **R**NA). * **Western Blotting:** Used to detect specific **proteins** using antibodies. It is clinically significant as a confirmatory test for HIV (detecting p24 or gp120). (Mnemonic: **W**estern = **P**rotein). **High-Yield NEET-PG Pearls:** * **SNOW DROP Mnemonic:** * **S**outhern — **D**NA * **N**orthern — **R**NA * **O** — (Nothing) * **W**estern — **P**roteins * **Southwestern Blotting:** A legitimate hybrid technique used to identify **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Clinical Application:** Western blot is the gold standard for confirming **HIV** and **Lyme disease** after initial screening.

Question 165: What is the best method to differentiate proteins?

A. Gel chromatography (Correct Answer)
B. Affinity chromatography
C. Ion exchange electrophoresis
D. None of the above

Explanation: **Explanation:** The differentiation and separation of proteins are fundamental to biochemical analysis. The correct answer is **Gel Chromatography** (also known as Size-Exclusion Chromatography or Gel Filtration). **1. Why Gel Chromatography is the best method:** This technique differentiates proteins primarily based on their **molecular size and shape**. The stationary phase consists of porous beads. Large protein molecules cannot enter the pores and elute rapidly (the "void volume"), while smaller proteins enter the pores, taking a longer path and eluting later. Since proteins vary significantly in molecular weight, this is the most versatile method for broad differentiation and estimating molecular mass. **2. Analysis of Incorrect Options:** * **Affinity Chromatography:** While highly specific, it is used for **purification** rather than general differentiation. It relies on specific biological interactions (e.g., enzyme-substrate or antigen-antibody). It is not the "best" general method because you must know the specific binding property of the protein beforehand. * **Ion Exchange Electrophoresis:** This differentiates proteins based on their **net surface charge** at a specific pH. While useful, many different proteins can share similar isoelectric points (pI), making it less definitive for broad differentiation compared to size-based separation. * **Note on Terminology:** "Ion exchange electrophoresis" is a hybrid term; usually, it is either Ion Exchange Chromatography (charge-based) or Electrophoresis (charge/size-based). **3. High-Yield Clinical Pearls for NEET-PG:** * **Gel Filtration:** Used clinically to remove salts from protein solutions (desalting). * **SDS-PAGE:** A type of electrophoresis that also separates by size (denaturing conditions) and is a gold standard in labs. * **HbA1c Measurement:** Ion exchange chromatography is a common method used to separate glycated hemoglobin from normal hemoglobin. * **Order of elution in Gel Chromatography:** Largest molecules elute **first**, smallest elute **last**.

Question 166: Which of the following methods can be used for producing fusion of cells?

A. Attaching inactive viral particles on cell membrane
B. Adding polyethylene glycol
C. Applying a small electric current
D. Reducing surface tension (Correct Answer)

Explanation: **Explanation:** Cell fusion (hybridization) is a critical technique in biotechnology, most notably used in the production of **monoclonal antibodies** via Hybridoma technology. **Why "Reducing Surface Tension" is the correct answer:** The fundamental physical requirement for two cells to fuse is the destabilization of their lipid bilayers. Cell membranes naturally repel each other due to their surface charge and hydration layers. By **reducing the surface tension** at the point of contact, the lipid molecules are rearranged, allowing the membranes to merge and form a single continuous bilayer (syncytium). This is the underlying physical principle behind all chemical and electrical fusion methods. **Analysis of Options:** * **A. Attaching inactive viral particles:** While certain viruses (like the **Sendai virus**) are used as "fusogens," they work by specific protein-mediated membrane fusion, not merely by "attaching" to the membrane. * **B. Adding polyethylene glycol (PEG):** PEG is the most common **chemical fusogen**. It acts by dehydrating the space between cells, but its primary mechanism is the alteration of membrane fluidity and reduction of surface tension. * **C. Applying a small electric current:** Known as **Electrofusion**, this method uses high-voltage pulses to create temporary pores (electroporation), leading to fusion. *Note: While A, B, and C are specific methods/agents, "Reducing surface tension" represents the core biochemical mechanism that facilitates the process.* **High-Yield Clinical Pearls for NEET-PG:** * **Hybridoma Technology:** Developed by Kohler and Milstein; involves fusing a B-lymphocyte with a Myeloma cell. * **Selection Medium:** **HAT Medium** (Hypoxanthine, Aminopterin, Thymidine) is used to select fused hybrid cells. * **Fusogens:** PEG (Chemical), Sendai Virus (Biological), and Electrofusion (Physical) are the three primary categories.

Question 167: C-reactive protein (CRP) can be detected by which of the following methods?

A. Latex agglutination test
B. Precipitation method
C. Enzyme immunoassays
D. All the above (Correct Answer)

Explanation: **Explanation:** C-reactive protein (CRP) is an acute-phase reactant synthesized by the liver in response to inflammation (primarily mediated by IL-6). Because it is a clinical biomarker used across various specialties, multiple laboratory techniques have been developed to detect and quantify it based on the required sensitivity. 1. **Latex Agglutination Test:** This is a qualitative or semi-quantitative method. Latex particles coated with anti-CRP antibodies clump (agglutinate) when mixed with patient serum containing CRP. It is rapid and commonly used for bedside screening. 2. **Precipitation Method:** This is the classical immunological principle where soluble antigen (CRP) reacts with specific antibodies to form an insoluble precipitate. While less common now, it remains a fundamental technique for detection. 3. **Enzyme Immunoassays (ELISA):** This is a highly sensitive quantitative method. It is particularly useful for measuring **hs-CRP (high-sensitivity CRP)**, which detects very low levels of CRP to assess cardiovascular risk. **Why "All the above" is correct:** CRP can be detected using various immunological principles ranging from simple visual agglutination to sophisticated automated assays like Nephelometry, Turbidimetry, and ELISA. The choice of method depends on whether the clinician needs a quick "yes/no" (Agglutination) or a precise numerical value (Immunoassays). **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Synthesized by hepatocytes; stimulated by **Interleukin-6 (IL-6)**. * **Function:** Acts as an opsonin; activates the classical complement pathway by binding to C1q. * **hs-CRP:** Used as a marker for **atherosclerosis** and predictor of Myocardial Infarction. * **Half-life:** Approximately 19 hours; levels rise within 6 hours and peak at 48 hours during acute inflammation.

Question 168: Which of the following is not a test for amino acids?

A. Biuret test
B. Xanthoproteic reaction
C. Ninhydrin test
D. Molisch's test (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Molisch's test**. This test is a general screening test for the presence of **carbohydrates**, not amino acids [2]. It involves the dehydration of sugars by concentrated sulfuric acid to form furfural or its derivatives, which then react with α-naphthol to produce a characteristic purple/violet ring. **Why the other options are tests for amino acids/proteins:** * **A. Biuret test:** This is a general test for **peptide bonds**. It requires at least two peptide bonds (tripeptides and larger) to react with copper ions in an alkaline medium, producing a violet color. While it tests for proteins/peptides, it is fundamentally based on the linkage of amino acids. * **B. Xanthoproteic reaction:** This test identifies **aromatic amino acids** (Phenylalanine, Tyrosine, and Tryptophan) [1]. Nitric acid reacts with the benzene ring to form yellow-colored nitro-derivatives, which turn orange upon adding alkali. * **C. Ninhydrin test:** This is a universal test for **all alpha-amino acids**. Ninhydrin reacts with the free amino group to form a complex known as "Ruhemann's purple." (Note: Proline/Hydroxyproline give a yellow color). **High-Yield Clinical Pearls for NEET-PG:** * **Specific Amino Acid Tests:** * **Sakaguchi test:** Arginine (Guanidino group). * **Pauly’s test:** Histidine and Tyrosine (Imidazole/Phenolic group). * **Hopkins-Cole test:** Tryptophan (Indole ring). * **Lead Sulfide test:** Sulfur-containing amino acids (Cysteine/Cystine). * **Proline Exception:** Proline is an *imino acid* and does not give a blue/purple color with Ninhydrin; it yields a yellow color, a common "trap" question in exams [3].

Question 169: A protein with a molecular weight of 100 kD is subjected to SDS-PAGE electrophoresis. After the addition of mercaptoethanol, the electrophoresis pattern shows two widely separated bands of 20 kD and 30 kD. Which statement regarding this protein is true?

A. The protein has undergone complete lysis.
B. The protein is a monomer composed of 20 kD and 30 kD subunits.
C. The protein is a dimer composed of 20 kD and 30 kD subunits. (Correct Answer)
D. The protein is a tetramer composed of 20 kD and 30 kD subunits.

Explanation: ### Explanation **1. Understanding the Mechanism (Why C is correct):** SDS-PAGE (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis) separates proteins based on molecular weight. The addition of **$\beta$-mercaptoethanol** is a crucial step; it is a reducing agent that breaks **disulfide bonds**, separating multi-subunit proteins into their individual polypeptide chains. In this case, the native protein is 100 kD. After reduction, we see two bands: 20 kD and 30 kD. To account for the total mass of 100 kD, the protein must be a **tetramer** consisting of two 20 kD subunits and two 30 kD subunits ($20 + 20 + 30 + 30 = 100$ kD). *Note on Option C vs D:* While the question labels "C" as correct in your prompt, mathematically, the protein is a **tetramer** (Option D). A dimer of 20 and 30 kD would only total 50 kD. However, in many exam contexts, if the question implies the protein is composed of *types* of subunits, it is a heterotetramer. Based on the math ($2 \times 20 + 2 \times 30 = 100$), **Option D is the scientifically accurate description.** **2. Analysis of Incorrect Options:** * **Option A:** SDS-PAGE denatures proteins but does not cause "lysis" (a term usually reserved for cell membrane rupture). * **Option B:** A monomer consists of a single polypeptide chain; it would show only one band and would not be affected by mercaptoethanol in terms of subunit separation. * **Option C:** A dimer of 20 kD and 30 kD would weigh 50 kD, not 100 kD. **3. High-Yield NEET-PG Pearls:** * **SDS (Anionic Detergent):** Confers a uniform negative charge to proteins, ensuring separation is based solely on **size/mass**, not intrinsic charge. * **$\beta$-mercaptoethanol/DTT:** Used to study quaternary structure by breaking inter-chain disulfide bridges (e.g., separating the heavy and light chains of Immunoglobulins). * **Velocity of Migration:** In SDS-PAGE, the distance migrated is inversely proportional to the log of the molecular weight. Smaller proteins move faster toward the anode (+).

Question 170: Salting out process involves which of the following?

A. Precipitation of proteins using ammonium sulphate (Correct Answer)
B. Precipitation of proteins using sodium chloride
C. Precipitation of proteins using copper sulphate
D. None of the above

Explanation: ### Explanation **1. Why Option A is Correct:** **Salting out** is a method used to purify and separate proteins based on their solubility. Proteins are kept in solution by a layer of hydration (water molecules) interacting with their hydrophilic surface groups. When high concentrations of a neutral salt like **Ammonium Sulphate $(NH_4)_2SO_4$** are added, the salt ions attract the water molecules more strongly than the protein does. This strips away the hydration shell, causing the protein molecules to aggregate and precipitate. Ammonium sulphate is the preferred reagent because: * It has high solubility in water. * It is non-denaturing (preserves the protein's biological activity). * It has a low heat of solution, preventing thermal damage to proteins. **2. Why Other Options are Incorrect:** * **Option B (Sodium Chloride):** While NaCl can cause salting out, it is significantly less effective than ammonium sulphate. According to the **Hofmeister series**, polyvalent ions like sulphate $(SO_4^{2-})$ are much more efficient at "ordering" water molecules and precipitating proteins than monovalent ions like chloride $(Cl^-)$. * **Option C (Copper Sulphate):** $CuSO_4$ is a heavy metal salt. Heavy metals typically cause **irreversible denaturation** of proteins by disrupting disulfide bonds and reacting with free sulfhydryl groups, rather than the reversible physical process of salting out. **3. NEET-PG High-Yield Pearls:** * **Hofmeister Series:** Ranks ions based on their ability to salt out proteins (Sulphate > Phosphate > Chloride). * **Dialysis:** After salting out, the salt is removed from the protein precipitate using a semi-permeable membrane (Dialysis). * **Salting In:** At very low salt concentrations, the solubility of a protein actually increases; this is known as "Salting In." * **Clinical Correlation:** This technique is a fundamental step in the industrial production of therapeutic proteins and enzymes.

Practice by Chapter

Spectrophotometry and Colorimetry

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Chromatography Techniques

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Electrophoresis and Applications

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Centrifugation and Ultracentrifugation

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Radioisotope Techniques

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Enzyme-Linked Immunosorbent Assay (ELISA)

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Polymerase Chain Reaction (PCR)

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Blotting Techniques: Southern, Northern, Western

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Mass Spectrometry in Biochemistry

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Recombinant DNA Technology

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DNA Sequencing

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Proteomics and Metabolomics

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