Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 101: All of the following techniques can be used to determine protein structure, EXCEPT:

A. High-performance liquid chromatography (HPLC) (Correct Answer)
B. Mass spectrometry
C. X-ray crystallography
D. Nuclear magnetic resonance (NMR) spectrometry

Explanation: **Explanation:** The determination of protein structure involves analyzing the spatial arrangement of atoms (tertiary and quaternary structure). **Why HPLC is the correct answer:** **High-performance liquid chromatography (HPLC)** is primarily a **separation and purification technique**. It separates proteins or peptides based on their physical properties such as size, charge, or hydrophobicity. While it can be used for quantitative analysis or to check purity, it cannot provide information regarding the three-dimensional folding or atomic coordinates of a protein. **Analysis of incorrect options:** * **X-ray Crystallography:** This is the "gold standard" for determining the 3D structure of proteins at atomic resolution. It requires the protein to be crystallized and uses diffraction patterns to map electron density. * **Nuclear Magnetic Resonance (NMR) Spectrometry:** This technique is used to determine the structure of smaller proteins in **aqueous solution**, making it ideal for studying protein dynamics and folding in a physiological-like state. * **Mass Spectrometry (MS):** While traditionally used for sequencing (primary structure) and determining molecular weight, advanced MS techniques (like Hydrogen-Deuterium Exchange) are now integral in studying protein conformation and folding. **High-Yield NEET-PG Pearls:** * **X-ray Crystallography** requires crystals; **NMR** works for proteins in solution. * **Cryo-electron microscopy (Cryo-EM)** is an emerging high-yield technique for visualizing large macromolecular complexes that are difficult to crystallize. * **Circular Dichroism (CD)** is a common technique used specifically to determine the **secondary structure** (alpha-helices and beta-sheets) of proteins. * **Sanger’s Reagent** (1-fluoro-2,4-dinitrobenzene) is used for N-terminal amino acid sequencing.

Question 102: The Biuret test is used for the detection of which of the following?

A. Carbohydrate
B. Cholesterol
C. Protein (Correct Answer)
D. Steroid

Explanation: **Explanation:** The **Biuret test** is a chemical assay used to detect the presence of **peptide bonds**, making it the standard qualitative and quantitative test for **proteins**. **Why Protein is Correct:** The reaction occurs when copper (II) ions ($Cu^{2+}$) in an alkaline solution react with the nitrogen atoms of peptide bonds. This results in the formation of a **violet or purple-colored coordination complex**. For a positive Biuret test, a substance must contain at least **two peptide bonds** (three amino acids). Therefore, while proteins and long polypeptides give a positive result, individual amino acids (except for histidine in some conditions) and dipeptides do not. **Why Other Options are Incorrect:** * **Carbohydrates:** Detected using tests like **Benedict’s** (for reducing sugars), **Molisch’s** (general carbohydrate test), or **Iodine** (for starch). * **Cholesterol/Steroids:** Detected using the **Salkowski test** or the **Libermann-Burchard reaction**, which produce characteristic color changes in the presence of sterol rings. **High-Yield Clinical Pearls for NEET-PG:** * **Composition of Biuret Reagent:** It contains Copper sulfate ($CuSO_4$), Sodium potassium tartrate (**Rochelle salt** to stabilize the cupric ions), and Potassium hydroxide ($KOH$). * **Intensity of Color:** The intensity of the purple color is directly proportional to the number of peptide bonds present, allowing it to be used in spectrophotometry for protein quantification. * **Clinical Application:** It is commonly used in clinical laboratories to measure **total serum protein** levels. * **Note:** Free amino acids are detected by the **Ninhydrin test**, not the Biuret test.

Question 103: Which of the following is not required for Polymerase Chain Reaction (PCR)?

A. Primer
B. DNA fragments
C. DNA polymerase
D. Radio-labeled DNA probe (Correct Answer)

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. The process mimics natural DNA replication but occurs in a thermal cycler through three repeating steps: Denaturation, Annealing, and Extension. **Why Option D is the Correct Answer:** A **Radio-labeled DNA probe** is used in **Southern Blotting** or **In Situ Hybridization** to detect a specific DNA sequence after amplification or separation. It is **not** a component of the PCR reaction mixture itself. PCR produces millions of copies of DNA; visualization is typically done via gel electrophoresis with ethidium bromide or real-time fluorescence, rather than radioactive probes. **Why the other options are incorrect:** * **A. Primer:** Essential. These are short, synthetic oligonucleotides (usually 18–25 base pairs) that provide a 3'-OH group for DNA polymerase to initiate synthesis. Two primers (forward and reverse) are required. * **B. DNA fragments:** Essential. This is the **template DNA** containing the target sequence that needs to be amplified. * **C. DNA polymerase:** Essential. A heat-stable enzyme, typically **Taq Polymerase** (derived from *Thermus aquaticus*), is required to extend the primers and synthesize new DNA strands at high temperatures. **High-Yield Clinical Pearls for NEET-PG:** * **Taq Polymerase:** Optimum temperature is **72°C**. It lacks 3' to 5' exonuclease activity (no proofreading). * **RT-PCR:** Used for RNA viruses (like SARS-CoV-2); involves converting RNA to cDNA using **Reverse Transcriptase** before PCR. * **Components of PCR Mix:** Template DNA, Primers, Taq Polymerase, **dNTPs** (Deoxynucleotide triphosphates), and **Magnesium ions ($Mg^{2+}$)** which act as a cofactor for the polymerase.

Question 104: The molecular weight of a protein can be determined by which of the following methods?

A. Native polyacrylamide gel electrophoresis (PAGE)
B. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) (Correct Answer)
C. Isoelectric focusing
D. Ion exchange chromatography

Explanation: **Explanation:** The molecular weight of a protein is primarily determined by **SDS-PAGE** because it separates proteins based solely on their **mass**. **1. Why SDS-PAGE is correct:** In this technique, the anionic detergent **Sodium Dodecyl Sulfate (SDS)** denatures proteins and imparts a uniform **negative charge** proportional to their length. This masks the protein's intrinsic charge, ensuring a constant mass-to-charge ratio. When an electric field is applied, proteins migrate through the polyacrylamide gel matrix; smaller proteins move faster, while larger ones are retarded. By comparing the migration distance to known standards (molecular weight markers), the molecular weight can be accurately estimated. **2. Why other options are incorrect:** * **Native PAGE:** Separates proteins in their folded state based on a combination of **charge, size, and shape**. Since the charge-to-mass ratio is not uniform, it cannot be used to determine molecular weight. * **Isoelectric Focusing (IEF):** Separates proteins based on their **isoelectric point (pI)**—the pH at which the protein has no net charge. It does not provide information about size. * **Ion Exchange Chromatography:** Separates proteins based on their **net surface charge** at a specific pH using charged resin beads. **High-Yield Clinical Pearls for NEET-PG:** * **2-D Electrophoresis:** Combines **IEF** (1st dimension) and **SDS-PAGE** (2nd dimension) to separate proteins by both pI and molecular weight. * **Beta-mercaptoethanol:** Often added to SDS-PAGE to break **disulfide bonds**, ensuring complete denaturation into individual polypeptide subunits. * **Western Blot:** Uses SDS-PAGE followed by membrane transfer and antibody tagging to identify specific proteins.

Question 105: DNA restriction fragments are separated by which technique?

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

Explanation: **Explanation:** **Agarose gel electrophoresis** is the standard technique used to separate DNA restriction fragments based on their **molecular size**. DNA molecules are negatively charged (due to the phosphate backbone); when placed in an electric field, they migrate toward the positive electrode (anode). The agarose matrix acts as a molecular sieve: smaller fragments move faster and further through the pores, while larger fragments are retarded. **Analysis of Incorrect Options:** * **Paper Chromatography (A):** Primarily used for separating small polar molecules like amino acids or sugars based on their solubility and partition coefficients. * **Thin-layer Chromatography (C):** Used for rapid analysis of small organic molecules, drugs, or lipids. It is not suitable for large macromolecules like DNA. * **Ultracentrifugation (D):** Separates particles based on density or sedimentation rate (Svedberg units). While used to isolate whole organelles or DNA types (e.g., CsCl density gradient), it is not the standard method for separating specific restriction fragments. **High-Yield Facts for NEET-PG:** * **Visualization:** DNA bands in the gel are visualized using **Ethidium Bromide (EtBr)**, which intercalates between bases and fluoresces orange under **UV light**. * **Pulsed-Field Gel Electrophoresis (PFGE):** A variation used to separate very large DNA fragments (e.g., whole chromosomes). * **Southern Blotting:** After electrophoresis, DNA is transferred to a membrane for hybridization with a specific probe to identify particular sequences. * **Charge-to-Mass Ratio:** DNA has a constant charge-to-mass ratio, which is why separation depends solely on length/size.

Question 106: What gel or gels are used in electrophoresis?

A. Agarose gel
B. Polyacrylamide plain gel
C. Polyacrylamide SDS (Sodium dodecyl sulphate) impregnated Polyacrylamide gel
D. All of the above (Correct Answer)

Explanation: **Explanation:** Electrophoresis is a fundamental biochemical technique used to separate macromolecules (DNA, RNA, and proteins) based on their size, charge, and shape. The choice of gel depends on the nature of the molecule and the desired resolution. 1. **Agarose Gel:** Derived from seaweed, it has a large pore size. It is primarily used for separating large molecules, specifically **DNA and RNA fragments**. It is the standard medium for horizontal electrophoresis. 2. **Polyacrylamide Plain Gel (Native PAGE):** Polyacrylamide has a much smaller and more controllable pore size than agarose. "Plain" or Native gels separate proteins based on their **intrinsic charge and size** simultaneously, preserving the protein's natural conformation and biological activity. 3. **SDS-Polyacrylamide Gel (SDS-PAGE):** In this method, the detergent Sodium Dodecyl Sulphate (SDS) is added to denature proteins and impart a uniform negative charge proportional to their mass. This allows proteins to be separated **strictly based on their molecular weight**, masking the effects of native charge and shape. Since all three types are standard matrices used in clinical and research laboratories for electrophoresis, **Option D** is the correct answer. **High-Yield NEET-PG Pearls:** * **Agarose:** Best for DNA (e.g., PCR product analysis). * **PAGE:** Best for small proteins and sequencing DNA due to high resolving power. * **SDS-PAGE:** Most common method for determining protein molecular weight. * **Ethidium Bromide (EtBr):** The most common fluorescent dye used to visualize DNA in agarose gels (intercalating agent). * **Southern Blotting:** Uses agarose gel to separate DNA before transferring to a membrane.

Question 107: Flow cytometry is performed to determine which of the following parameters?

A. Rapid cell shrinkage (Correct Answer)
B. Blood flow to the brain
C. Net oxygen supply to tissues
D. Amount of oxygen bound to hemoglobin

Explanation: **Explanation:** **Flow cytometry** is a sophisticated laser-based technology used to analyze the physical and chemical characteristics of particles or cells in a fluid suspension. **Why Option A is Correct:** Flow cytometry measures two primary physical properties as cells pass through a laser beam: 1. **Forward Scatter (FSC):** Correlates with **cell size**. 2. **Side Scatter (SSC):** Correlates with **internal complexity or granularity**. **Rapid cell shrinkage** is a hallmark of **apoptosis** (programmed cell death). During this process, cells lose water and ions, leading to a reduction in volume. In flow cytometry, this is detected as a **decrease in Forward Scatter (FSC)**. Therefore, flow cytometry is a gold-standard technique for quantifying cell size changes and identifying apoptotic populations. **Why the Other Options are Incorrect:** * **Option B & C:** Blood flow to the brain and net oxygen supply to tissues are hemodynamic parameters typically measured using imaging modalities like Functional MRI (fMRI), PET scans, or Doppler ultrasound. * **Option D:** The amount of oxygen bound to hemoglobin is measured via pulse oximetry (SpO2) or arterial blood gas (ABG) analysis. **High-Yield Clinical Pearls for NEET-PG:** * **CD Markers:** Flow cytometry is the primary tool for **Immunophenotyping** (e.g., diagnosing Leukemias and Lymphomas by identifying CD3, CD4, CD8, CD19, etc.). * **HIV Monitoring:** It is used to calculate the absolute **CD4+ T-cell count** to monitor disease progression. * **DNA Analysis:** It can determine **Ploidy** (DNA content) and cell cycle phases using fluorescent dyes like Propidium Iodide. * **Fetal-Maternal Hemorrhage:** The **Kleihauer-Betke test** is being replaced by flow cytometry to quantify fetal RBCs in maternal circulation.

Question 108: Protein purification and separation can be achieved by which of the following methods, except?

A. Chromatography
B. Centrifugation
C. Electrophoresis
D. None of the above (Correct Answer)

Explanation: ### Explanation The correct answer is **None of the above** because all the listed techniques—Chromatography, Centrifugation, and Electrophoresis—are standard, fundamental methods used for the purification and separation of proteins based on their physical and chemical properties. **1. Why the correct answer is "None of the above":** Protein purification is a multi-step process designed to isolate a single protein from a complex mixture (like serum or cell lysate). Since options A, B, and C are all valid methodologies for this purpose, none of them can be excluded. **2. Analysis of Options:** * **Chromatography (Option A):** This is the most versatile tool for purification. It separates proteins based on specific characteristics: **Size** (Size-exclusion/Gel filtration), **Charge** (Ion-exchange), or **Binding affinity** (Affinity chromatography). * **Centrifugation (Option B):** Specifically **Ultracentrifugation**, separates proteins based on their **molecular weight and density** (Svedberg units). It is often the initial step (differential centrifugation) to separate organelles or large protein complexes from the cytosol. * **Electrophoresis (Option C):** Techniques like **SDS-PAGE** separate proteins primarily based on their **molecular mass**, while **Isoelectric Focusing (IEF)** separates them based on their **isoelectric point (pI)**. These are essential for both analytical and preparative separation. **High-Yield Clinical Pearls for NEET-PG:** * **Salting Out:** A common initial purification step using **Ammonium Sulfate** to precipitate proteins based on solubility. * **Dialysis:** Used to remove salts or small molecules from a protein solution using a semi-permeable membrane. * **Specific Activity:** During purification, as the protein becomes purer, its *total protein* content decreases, but its *specific activity* (units of enzyme/mg of protein) increases. * **Molecular Weight Determination:** SDS-PAGE is the most common lab method, while Ultracentrifugation and Gel Filtration can also be used.

Question 109: Which of the following is/are applications of FISH?

A. Gene mapping
B. Study of 3D chromosome organization in interphase nuclei
C. Monitoring the success of bone marrow transplantation
D. All the above (Correct Answer)

Explanation: **Explanation:** **Fluorescence In Situ Hybridization (FISH)** is a cytogenetic technique that uses fluorescent probes that bind to only those parts of a nucleic acid sequence with a high degree of sequence complementarity. It bridges the gap between molecular biology and cytogenetics. **Why "All the Above" is Correct:** * **Gene Mapping (Option A):** FISH is a primary tool for physical mapping of genes. By using specific fluorescent probes, scientists can visualize the exact location of a gene on a specific chromosome. * **3D Chromosome Organization (Option B):** Unlike traditional karyotyping, FISH can be performed on **interphase nuclei**. This allows researchers to study the spatial arrangement and "territories" of chromosomes within the nucleus, which is crucial for understanding gene regulation. * **Monitoring Bone Marrow Transplantation (Option C):** In sex-mismatched transplants (e.g., male donor to female recipient), FISH for X and Y chromosomes is used to assess **chimerism**. This helps determine if the donor cells have successfully engrafted or if the recipient's cells are returning (relapse). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Speed:** FISH is faster than traditional karyotyping because it does not require cell culture (can be done on non-dividing interphase cells). * **Resolution:** It has a higher resolution than G-banding, detecting microdeletions (e.g., **22q11 deletion in DiGeorge Syndrome**). * **Common Applications:** * **Aneuploidy:** Rapid screening for Trisomy 13, 18, 21. * **Cancer Genetics:** Detecting the **BCR-ABL fusion** (Philadelphia chromosome) in CML or **HER2/neu** amplification in breast cancer. * **Limitation:** You must know the "target" sequence beforehand to select the correct probe; it is not a "blind" screening tool like a full karyotype.

Question 110: Which of the following techniques is used to study protein-protein interactions?

A. Western blot
B. Affinity electrophoresis (Correct Answer)
C. Thin-layer chromatography
D. None of the above

Explanation: **Explanation:** **1. Why Affinity Electrophoresis is correct:** Affinity electrophoresis is a specialized technique used to study the **binding interactions** between molecules. It works on the principle that the electrophoretic mobility of a protein changes when it interacts with another molecule (ligand). When a protein binds to another protein (or a specific ligand) within the gel matrix, its migration speed is altered. By observing these shifts in mobility, researchers can determine the **binding constant** and the specificity of **protein-protein interactions**. **2. Why the other options are incorrect:** * **Western Blot:** While it is a gold-standard technique for protein analysis, it is used for the **detection and quantification** of a specific protein in a sample using antibodies. It identifies the presence of a protein but does not inherently measure the interaction between two different proteins in their native state. * **Thin-layer Chromatography (TLC):** This is a separation technique based on differential partitioning between a stationary phase and a mobile phase. It is primarily used for identifying **small molecules** like amino acids, lipids, or drugs, rather than complex macromolecular interactions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Yeast Two-Hybrid System:** Another high-yield technique frequently asked for studying protein-protein interactions *in vivo*. * **Co-Immunoprecipitation (Co-IP):** The "gold standard" for identifying protein complexes from cell lysates. * **Southern Blot = DNA; Northern Blot = RNA; Western Blot = Protein** (Mnemonic: **SNOW DROP**). * **Southwestern Blot:** Used specifically to study **Protein-DNA interactions**.

Practice by Chapter

Spectrophotometry and Colorimetry

Practice Questions

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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