Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 131: Which technique for protein purification utilizes the sorting out of molecules according to size and shape?

A. Adsorption chromatography
B. Gel filtration chromatography (Correct Answer)
C. Paper chromatography
D. None of these

Explanation: ### Explanation **Correct Answer: B. Gel Filtration Chromatography** **Why it is correct:** Gel filtration chromatography, also known as **Size-Exclusion Chromatography (SEC)** or Molecular Sieving, separates proteins based on their **size (molecular weight) and shape**. The stationary phase consists of porous beads (e.g., Sephadex, agarose). * **Mechanism:** Large molecules cannot enter the pores of the beads and are "excluded," traveling around them and eluting **first**. Small molecules enter the pores, taking a longer, more tortuous path, and thus elute **later**. This technique is unique because it does not involve chemical binding between the protein and the matrix. **Why other options are incorrect:** * **A. Adsorption Chromatography:** This relies on the differential binding of solutes to the surface of a solid stationary phase based on **polarity or chemical affinity**, not size. * **C. Paper Chromatography:** This is a type of partition chromatography where substances are separated based on their **solubility** and distribution between a stationary liquid phase (water held in cellulose) and a mobile solvent phase. It is typically used for small molecules like amino acids or sugars, rather than complex protein purification. **High-Yield Clinical Pearls for NEET-PG:** * **Desalting:** Gel filtration is commonly used in labs to "desalt" a protein solution (separating large proteins from small salt ions). * **Molecular Weight Determination:** It is a standard method for estimating the native molecular weight of a protein. * **Order of Elution:** Remember the rule—**Largest elutes first, smallest elutes last.** * **Comparison:** Unlike **SDS-PAGE** (which separates by size after denaturing proteins), Gel Filtration separates proteins in their **native (active) state**.

Question 132: DNA restriction fragments are separated by what method?

A. Paper chromatography
B. Agarose gel electrophoresis (Correct Answer)
C. Thin-layer chromatography
D. Ultracentrifugation

Explanation: **Explanation:** **Why Agarose Gel Electrophoresis is correct:** DNA molecules are negatively charged due to their phosphate backbone. In **Agarose Gel Electrophoresis**, DNA fragments are loaded into a porous agarose matrix and subjected to an electric field. The fragments migrate toward the positive electrode (anode). The gel acts as a molecular sieve; smaller fragments move faster and further through the pores, while larger fragments are retarded. This allows for the precise separation of DNA restriction fragments based strictly on their **size (molecular weight)**. **Why the other options are incorrect:** * **Paper Chromatography:** Primarily used for separating small, polar compounds like amino acids or sugars based on their solubility and partition coefficients between a stationary phase (paper) and a mobile phase. * **Thin-layer Chromatography (TLC):** Used for the rapid separation of small organic molecules, lipids, or drugs. It lacks the resolution required for large macromolecules like DNA. * **Ultracentrifugation:** Separates particles based on density and sedimentation rate (Svedberg units). While used for isolating cell organelles or separating DNA by density (CsCl gradient), it is not the standard method for separating restriction fragments by size. **High-Yield Facts for NEET-PG:** * **Staining:** DNA bands in the gel are visualized using **Ethidium Bromide (EtBr)**, which intercalates between bases and fluoresces orange under **UV light**. * **Purity Check:** The $A_{260}/A_{280}$ ratio is used to check DNA purity (Pure DNA ≈ 1.8). * **Pulsed-Field Gel Electrophoresis (PFGE):** A variation used to separate very large DNA fragments (e.g., whole chromosomes). * **Southern Blotting:** Uses agarose gel electrophoresis as the first step before transferring DNA to a membrane for hybridization.

Question 133: Which technique is primarily used to determine the primary structure of an amino acid?

A. Mass spectrometry (Correct Answer)
B. X-ray crystallography
C. NMR spectrometry
D. All of the above

Explanation: **Explanation:** **1. Why Mass Spectrometry (MS) is correct:** Mass Spectrometry is the gold standard for determining the **primary structure** (the linear sequence of amino acids) of proteins and peptides. It works by ionizing chemical species and sorting the ions based on their **mass-to-charge (m/z) ratio**. In proteomics, tandem mass spectrometry (MS/MS) is used to fragment peptide bonds. By measuring the mass difference between successive fragments, the specific amino acid sequence can be deduced with high precision. **2. Why the other options are incorrect:** * **X-ray Crystallography:** This technique is primarily used to determine the **3D tertiary or quaternary structure** of a protein. It requires the sample to be in a crystalline form and provides a "map" of electron density, rather than a direct sequence of amino acids. * **NMR Spectrometry (Nuclear Magnetic Resonance):** While NMR can provide structural information, it is mainly used to study the **dynamic 3D structure and folding** of small to medium-sized proteins in a solution state. It is not the primary tool for sequencing. **3. High-Yield Clinical Pearls for NEET-PG:** * **Edman Degradation:** Historically used for primary sequencing by removing one N-terminal amino acid at a time (using Phenylisothiocyanate), but it has largely been replaced by Mass Spectrometry for high-throughput analysis. * **Proteomics:** Mass Spectrometry is the cornerstone of proteomics, used clinically to screen for **Inborn Errors of Metabolism (IEM)** via Tandem Mass Spectrometry (TMS). * **Primary Structure Bond:** Remember that the primary structure is maintained solely by **covalent peptide bonds**, whereas higher-order structures involve hydrogen bonds, disulfide bridges, and hydrophobic interactions.

Question 134: Which of the following is NOT a nucleic acid test?

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

Explanation: **Explanation:** The core concept behind this question is the identification of specific biological molecules using blotting and hybridization techniques. **Why Western Blot is the correct answer:** Western blot is used for the detection of specific **proteins** in a sample. It involves separating proteins by electrophoresis, transferring them to a membrane (nitrocellulose), and using **labeled antibodies** for detection. Since it identifies proteins and not DNA or RNA, it is not a nucleic acid test. **Analysis of Incorrect Options:** * **Southern Blot:** This technique is used to detect specific **DNA** sequences. It involves DNA digestion by restriction endonucleases, electrophoresis, and hybridization with a labeled DNA probe. * **Northern Blot:** This is used for the detection of specific **RNA** sequences (mRNA). It is primarily used to study gene expression. * **Microarray:** This is a high-throughput nucleic acid technique where thousands of **DNA or RNA** probes are fixed to a solid surface (chip) to monitor the expression of thousands of genes simultaneously or to detect genetic variations (SNPs). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** = **O** (nothing) * **W**estern = **P**rotein * **Southwestern Blot:** A hybrid technique used to identify **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Clinical Application:** Western blot was historically the confirmatory test for **HIV** (detecting antibodies against p24 or gp120), though it has largely been replaced by 4th generation immunoassays and NAT (Nucleic Acid Testing).

Question 135: Affinity chromatography is based on which principle?

A. The specific binding of a protein constituent to another molecule. (Correct Answer)
B. Protein-protein interactions.
C. Protein-carbohydrate interactions.
D. None of the above.

Explanation: **Explanation:** **Affinity Chromatography** is a highly specific separation technique used to purify a particular protein or molecule from a complex mixture. **1. Why Option A is Correct:** The core principle of affinity chromatography is the **reversible, high-affinity binding** between a target protein (the analyte) and a specific **ligand** immobilized on a stationary phase (matrix). This interaction mimics biological processes such as enzyme-substrate binding, hormone-receptor interactions, or antigen-antibody recognition. When the mixture passes through the column, only the protein with a specific affinity for the ligand binds, while others are washed away. The bound protein is later recovered (eluted) by changing the pH or adding a competitive ligand. **2. Why Other Options are Incorrect:** * **Options B and C:** While protein-protein (e.g., Antibody-Antigen) and protein-carbohydrate (e.g., Lectin-Glucose) interactions are *examples* of affinity chromatography, they are too narrow. Option A is the superior answer because it encompasses all types of specific molecular interactions, including those involving lipids, nucleic acids, or metal ions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Common Ligand Pairs:** * **Enzymes:** Bind to substrates or cofactors (e.g., NAD+). * **Antibodies:** Bind to specific antigens (Immunoadsorption). * **Lectins:** Used to purify glycoproteins (binds glucose/mannose). * **Glutathione:** Used to purify GST-tagged recombinant proteins. * **Comparison:** Unlike **Size-Exclusion Chromatography** (based on molecular weight) or **Ion-Exchange Chromatography** (based on net charge), Affinity Chromatography is the most selective method, often achieving near-total purification in a single step.

Question 136: The dielectric constant of water is ___.

A. 105
B. 78.5 (Correct Answer)
C. 30
D. 60

Explanation: **Explanation:** The **dielectric constant (ε)** is a measure of a substance's ability to insulate charges from each other. Water has an exceptionally high dielectric constant of **78.5** (at 25°C), which is fundamental to its role as the "universal solvent" in biological systems. **Why 78.5 is Correct:** According to Coulomb’s Law ($F = kq_1q_2 / \epsilon r^2$), the force of attraction between two opposite ions is inversely proportional to the dielectric constant of the medium. Because water’s value is so high (~80), it reduces the attractive forces between ions (like $Na^+$ and $Cl^-$) by about 80-fold compared to a vacuum. This allows ionic compounds to dissociate and dissolve easily in aqueous cellular environments, facilitating biochemical reactions. **Analysis of Incorrect Options:** * **Option A (105):** This value is too high for liquid water. While some specialized solvents (like formamide) have constants above 100, they are not biologically compatible. * **Option C (30) & D (60):** These values represent organic solvents like methanol or ethanol. Non-polar solvents have low dielectric constants, which would cause ions to clump together (precipitate) rather than dissolve, making them unsuitable for life-sustaining biochemistry. **High-Yield Clinical Pearls for NEET-PG:** * **Solvation Shell:** Water molecules orient themselves around ions (oxygen toward cations, hydrogen toward anions) to form hydration shells, further stabilizing dissolved solutes. * **Hydrophobic Effect:** The high dielectric constant of water drives the folding of proteins, pushing non-polar amino acid side chains into the interior to minimize contact with the polar solvent. * **Temperature Dependency:** The dielectric constant of water decreases as temperature increases; however, for standard biochemical calculations, **78.5** is the gold-standard value.

Question 137: Which of the following methods is called Quantitative PCR?

A. Nested PCR
B. RT PCR
C. Real time PCR (Correct Answer)
D. Hot Start PCR

Explanation: **Explanation:** **Real-time PCR (qPCR)** is the correct answer because it allows for the continuous monitoring of DNA amplification as it occurs (in "real-time"). Unlike conventional PCR, which is qualitative (end-point detection), qPCR uses fluorescent dyes (like SYBR Green) or fluorophore-labeled probes (like TaqMan) to measure the accumulation of amplicons during the exponential phase. The intensity of the fluorescence is directly proportional to the amount of DNA template present, making it a **quantitative** method. **Analysis of Incorrect Options:** * **Nested PCR:** Uses two sets of primers in two successive runs to increase the **sensitivity and specificity** of the reaction, especially when the target DNA is in low concentrations. It is not inherently quantitative. * **RT-PCR (Reverse Transcription PCR):** This technique converts RNA into cDNA using the enzyme Reverse Transcriptase before amplification. While it can be combined with Real-time PCR (qRT-PCR), "RT-PCR" by itself refers to the conversion process, not the quantification. * **Hot Start PCR:** A modification where a critical component (like Taq polymerase) is withheld or inhibited until the initial denaturation temperature is reached. This reduces **non-specific amplification** and primer-dimer formation. **Clinical Pearls for NEET-PG:** * **Ct Value (Cycle Threshold):** In qPCR, the Ct value is inversely proportional to the viral load. A lower Ct value indicates a higher initial amount of target nucleic acid (commonly used in COVID-19 reporting). * **Gold Standard:** Real-time RT-PCR is the gold standard for diagnosing RNA viruses (e.g., HIV-1 viral load, SARS-CoV-2, Hepatitis C). * **Multiplex PCR:** Allows simultaneous detection of multiple target sequences in a single reaction tube using different primers.

Question 138: Which of the following techniques is NOT used for protein purification and separation?

A. Chromatography
B. Precipitation
C. Electrophoresis
D. Densitometry (Correct Answer)

Explanation: **Explanation:** The goal of protein purification is to isolate a specific protein from a complex mixture based on its unique physical and chemical properties. **Why Densitometry is the Correct Answer:** Densitometry is **not** a separation or purification technique; rather, it is a **quantification** technique. It measures the optical density (intensity) of bands or spots on a medium (like a gel or membrane) after separation has already occurred. In clinical biochemistry, it is used to calculate the relative concentration of protein fractions (e.g., measuring the albumin-to-globulin ratio in serum protein electrophoresis), but it cannot isolate the proteins themselves. **Analysis of Incorrect Options:** * **Chromatography (A):** A primary method for purification. It separates proteins based on size (Gel filtration), charge (Ion-exchange), or specific binding affinity (Affinity chromatography). * **Precipitation (B):** Often the first step in purification. Techniques like "Salting out" (using Ammonium Sulfate) exploit changes in solubility to concentrate and separate proteins from bulk solutions. * **Electrophoresis (C):** Separates proteins based on their charge-to-mass ratio in an electric field (e.g., SDS-PAGE). While often used for analysis, preparative electrophoresis can be used for purification. **NEET-PG High-Yield Pearls:** * **Salting Out:** Ammonium sulfate is the most common reagent because of its high solubility and stabilizing effect on protein structure. * **Affinity Chromatography:** The most specific method for purification (e.g., using Insulin to purify Insulin Receptors). * **Ampholyte usage:** Essential for **Isoelectric Focusing**, which separates proteins based on their isoelectric point (pI). * **Densitometry Clinical Use:** Essential for diagnosing Multiple Myeloma by identifying the "M-spike" in a serum protein electrophoresis (SPEP) tracing.

Question 139: What is the general test for the detection of carbohydrates?

A. Iodine test
B. Molisch test (Correct Answer)
C. Barfoed test
D. Osazone test

Explanation: ### Explanation **Correct Option: B. Molisch test** The **Molisch test** is the definitive **general screening test** for all carbohydrates. It is based on the principle that concentrated sulfuric acid ($H_2SO_4$) dehydrates sugars to form **furfural** (from pentoses) or **5-hydroxymethylfurfural** (from hexoses). These aldehydes then condense with **$\alpha$-naphthol** to form a characteristic **purple or violet ring** at the junction of the two liquids. This test is positive for monosaccharides, disaccharides, and polysaccharides, as they all undergo dehydration in the presence of strong acids. **Analysis of Incorrect Options:** * **A. Iodine test:** This is a specific test for **polysaccharides** (like starch, glycogen, and dextrins). It relies on the adsorption of iodine into the helical structure of the polysaccharide, producing colors like deep blue (starch) or reddish-brown (glycogen). * **C. Barfoed test:** This is used to **distinguish monosaccharides from reducing disaccharides**. Monosaccharides react faster (within 1–2 minutes) to form a red precipitate of cuprous oxide, whereas disaccharides take longer. * **D. Osazone test:** This is used for the **identification and differentiation** of sugars based on the shape and melting point of crystals formed with phenylhydrazine. For example, glucosazone crystals are needle-shaped (broomstick), while lactosazone crystals are "powder-puff" or "hedgehog" shaped. **High-Yield Clinical Pearls for NEET-PG:** * **Seliwanoff’s test:** Specific for **Ketohexoses** (e.g., Fructose), producing a cherry-red color. * **Bial’s test:** Specific for **Pentoses** (e.g., Ribose), producing a blue-green color. * **Benedict’s test:** A semi-quantitative test for **reducing sugars** in urine (used for screening Diabetes Mellitus and Inborn Errors of Metabolism like Galactosemia). * **Sucrose** is a non-reducing sugar and will give a negative Benedict’s test unless it is first hydrolyzed.

Question 140: What is true about polymerase chain reaction?

A. It is carried out by thermostable DNA polymerase. (Correct Answer)
B. The amplification is exponential.
C. The amplification is additive.
D. It is specific.

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* technique used to amplify specific DNA sequences. **Why Option A is the most accurate:** The hallmark of PCR is the use of a **thermostable DNA polymerase**, most commonly **Taq polymerase** (derived from the bacterium *Thermus aquaticus*). This enzyme is crucial because the PCR process involves a denaturation step at high temperatures (approx. 94–96°C) to separate DNA strands. Standard DNA polymerases would denature and lose function at these temperatures, but Taq polymerase remains stable and active, allowing the reaction to proceed through multiple cycles without adding new enzymes. **Analysis of other options:** * **Option B & C:** The amplification in PCR is **exponential**, not additive. The amount of DNA doubles with every cycle ($2^n$, where $n$ is the number of cycles). While Option B is technically a true statement about PCR, Option A is often considered the defining biochemical characteristic in standardized exams. * **Option D:** While PCR is highly specific due to the use of sequence-specific primers, "specificity" is a property shared by many molecular techniques (like Southern Blotting). The use of a thermostable enzyme is the unique requirement that enabled the automation of PCR. **High-Yield Clinical Pearls for NEET-PG:** 1. **Steps of PCR:** Denaturation (94°C) $\rightarrow$ Annealing (55-65°C) $\rightarrow$ Extension (72°C). 2. **RT-PCR:** Uses Reverse Transcriptase to amplify RNA (e.g., for COVID-19/SARS-CoV-2 detection). 3. **Components Required:** Template DNA, Primers (forward and reverse), dNTPs (nucleotides), Mg²⁺ (cofactor), and Taq Polymerase. 4. **Applications:** Diagnosis of genetic mutations, viral load monitoring (HIV, HBV), and forensic medicine (DNA profiling).

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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