Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 21: Which of the following techniques is used for the detection of variation in DNA sequence and gene expression?

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

Explanation: **Explanation:** The correct answer is **Microarray**. This technique is uniquely capable of analyzing thousands of genes simultaneously. It utilizes a solid surface (chip) containing microscopic spots of DNA probes. When labeled sample DNA or cDNA binds to these probes, it allows for the detection of **Single Nucleotide Polymorphisms (SNPs)**—representing variations in DNA sequence—and the quantification of mRNA levels, which reflects **gene expression**. **Analysis of Options:** * **A. Western blot:** Used specifically for the detection and quantification of **proteins** using antibody-antigen interactions. It does not analyze DNA or RNA. * **B. Northern blot:** Used for the detection of specific **RNA** sequences to study gene expression. However, it cannot detect DNA sequence variations (mutations/polymorphisms) and typically analyzes one gene at a time. * **C. Southern blot:** Used for the detection of specific **DNA** sequences (e.g., RFLP). While it detects DNA variations, it cannot measure gene expression (mRNA). * **D. Microarray:** The "high-throughput" nature of microarrays allows for the simultaneous assessment of both structural variations (DNA) and functional activity (Expression). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (SNoW DRoP):** **S**outhern-**D**NA; **N**orthern-**R**NA; **W**estern-**P**rotein. * **Southwestern Blot:** Used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Clinical Use of Microarray:** Widely used in oncology (to categorize tumors based on gene expression profiles) and in prenatal testing to detect submicroscopic chromosomal deletions or duplications.

Question 22: In which type of chromatography is the separation of protein molecules based on their size, shape, and molecular weight?

A. Partition chromatography
B. Gel filtration chromatography (Correct Answer)
C. Ion-exchange chromatography
D. Affinity chromatography

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, shape, and molecular weight**. The stationary phase consists of porous beads (e.g., Sephadex, agarose). * **Mechanism:** Small molecules enter the pores of the beads, increasing their path length and slowing their movement. Large molecules are "excluded" from the pores and travel only through the spaces between beads. * **Result:** Large molecules elute **first**, while smaller molecules elute **later**. **Why the other options are incorrect:** * **A. Partition chromatography:** Separation is based on the differential solubility of solutes between two phases (liquid-liquid or liquid-gas), such as in Paper Chromatography. * **C. Ion-exchange chromatography:** Separation is based on the **net surface charge** of the proteins. It uses a charged stationary phase (anion or cation exchangers) to bind proteins of the opposite charge. * **D. Affinity chromatography:** Separation is based on **high-specificity biological interactions**, such as enzyme-substrate, antigen-antibody, or hormone-receptor binding. It is the most specific method for protein purification. **High-Yield Clinical Pearls for NEET-PG:** * **Elution Order:** In Gel Filtration, the molecule with the **highest molecular weight** elutes first (Void Volume). * **Desalting:** Gel filtration is commonly used to remove small salt molecules from a protein solution. * **Determining MW:** It is a standard technique used to estimate the quaternary structure and molecular weight of native proteins. * **Common Matrices:** Sephadex (Dextran), Sepharose (Agarose), and Bio-Gel (Polyacrylamide).

Question 23: Which preservative is used for blood collection?

A. Sodium fluoride (Correct Answer)
B. Thymol
C. Potassium oxalate
D. No preservative is needed

Explanation: **Explanation:** **Sodium fluoride (NaF)** is the preferred preservative for blood glucose estimation. Its primary role is to inhibit **enolase**, a key enzyme in the glycolytic pathway. Without a preservative, red blood cells continue to metabolize glucose *in vitro*, leading to a decrease in glucose levels by approximately 5–7% per hour. By blocking glycolysis, NaF ensures that the glucose concentration remains stable for up to 48–72 hours. **Analysis of Options:** * **Sodium fluoride (Correct):** Acts as an antiglycolytic agent. It is typically used in combination with potassium oxalate (the anticoagulant) in **grey-topped tubes**. * **Thymol (Incorrect):** This is a preservative used for **24-hour urine collections**, primarily to inhibit bacterial growth and preserve chemical constituents, not for blood collection. * **Potassium oxalate (Incorrect):** While often present in the same tube as NaF, it is an **anticoagulant**, not a preservative. It prevents clotting by precipitating calcium ions. * **No preservative (Incorrect):** If no preservative is used (e.g., plain red-top tube), glycolysis continues, leading to falsely low glucose readings (pseudohypoglycemia). **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** NaF inhibits enolase by forming a complex with magnesium and phosphate. * **Interference:** NaF should **not** be used for samples intended for **Urease-based urea estimation**, as fluoride inhibits the urease enzyme. * **Electrolytes:** NaF can interfere with certain electrolyte assays and enzyme studies (like ALP or Amylase) due to its chelating properties. * **Ratio:** The standard mixture in grey tubes is 1 part NaF to 3 parts Potassium Oxalate.

Question 24: Nephelometry is a technique used in the measurement of:

A. Refraction of light
B. Immunoglobulins (Correct Answer)
C. Size of renal stones
D. Optical density of fluids

Explanation: **Explanation:** **Nephelometry** is a specialized laboratory technique based on the principle of **light scattering**. When a beam of light passes through a solution containing suspended particles (such as antigen-antibody complexes), the light is scattered at various angles. In nephelometry, the intensity of light scattered (usually at a 90-degree angle) is measured, which is directly proportional to the concentration of the particles in the solution. 1. **Why Immunoglobulins (Option B) is correct:** Nephelometry is the "gold standard" for quantifying specific proteins in serum. When specific antibodies are added to a patient's serum, they react with proteins like **Immunoglobulins (IgG, IgA, IgM)**, Complement components (C3, C4), or Acute Phase Reactants (CRP) to form immune complexes. These complexes scatter light, allowing for highly sensitive and rapid quantification. 2. **Why other options are incorrect:** * **Option A (Refraction):** Refraction is the bending of light as it passes from one medium to another. It is measured by a refractometer (e.g., to determine urine specific gravity), not nephelometry. * **Option C (Size of renal stones):** Stone size is measured using imaging modalities like NCCT (Non-Contrast CT) or Ultrasound. * **Option D (Optical Density):** This refers to **Turbidimetry** or Colorimetry, where the *reduction* in light transmission (absorbance) is measured rather than the scattered light. **High-Yield Clinical Pearls for NEET-PG:** * **Nephelometry vs. Turbidimetry:** Nephelometry measures **scattered light** (more sensitive for low concentrations), while Turbidimetry measures **transmitted light** (used for higher concentrations). * **Clinical Uses:** Widely used for measuring **HbA1c**, **Rheumatoid Factor (RF)**, and **Microalbuminuria**. * **Prozone Effect:** Excess antigen can lead to falsely low results in nephelometry; samples must be diluted to ensure they fall within the linear range of the assay.

Question 25: DNA restriction is done by which of the following methods?

A. Paper chromatography
B. Agarose gel electrophoresis (Correct Answer)
C. Spectrophotometry
D. Spectrometry

Explanation: **Explanation:** **Why Agarose Gel Electrophoresis is correct:** DNA restriction involves cutting DNA into fragments using restriction endonucleases. Once cut, these fragments must be separated and analyzed based on their size. **Agarose gel electrophoresis** is the standard technique used for this purpose. DNA molecules are negatively charged (due to the phosphate backbone) and migrate toward the positive electrode (anode) when placed in an electric field. The agarose matrix acts as a molecular sieve; smaller DNA fragments move faster and further through the pores than larger ones, allowing for precise separation and visualization (usually with Ethidium Bromide). **Why other options are incorrect:** * **Paper Chromatography:** This technique is primarily used to separate small soluble molecules like amino acids or sugars based on their solubility and partition coefficients, not large macromolecules like DNA. * **Spectrophotometry:** This is used to **quantify** the concentration and purity of DNA (measuring absorbance at 260 nm and 280 nm) but cannot separate or "restrict" DNA fragments by size. * **Spectrometry (e.g., Mass Spectrometry):** While used for identifying the chemical composition or mass of small molecules and proteins, it is not the standard method for routine DNA restriction analysis in a clinical or research lab. **High-Yield Clinical Pearls for NEET-PG:** * **DNA Charge:** DNA is polyanionic; it always moves from **Negative (Cathode) to Positive (Anode)**. * **Visualization:** **Ethidium Bromide (EtBr)** is the most common intercalating agent used to visualize DNA under UV light. * **Purity Check:** A **260/280 ratio of ~1.8** indicates pure DNA. A lower ratio suggests protein contamination. * **Pulsed-Field Gel Electrophoresis (PFGE):** A variation used to separate extremely large DNA fragments (e.g., whole chromosomes).

Question 26: Which substance can be confused with protein during a biochemical estimation test?

A. Phosphates (Correct Answer)
B. Nitrates
C. Sulphates
D. Bile salts

Explanation: **Explanation:** In biochemical estimation, particularly when using the **Folin-Ciocalteu (Lowry) method** for protein quantification, **Phosphates** are a well-known source of interference. The Lowry method relies on the reduction of the Folin-Ciocalteu reagent (phosphomolybdate and phosphotungstate) by tyrosine and tryptophan residues in proteins. High concentrations of phosphate buffers can react with the reagent to form a precipitate or alter the color intensity, leading to a false overestimation of protein levels. **Analysis of Options:** * **Phosphates (Correct):** They interfere with the formation of the molybdenum blue complex in the Lowry assay. Additionally, in some precipitation tests, phosphates can form insoluble salts that mimic the turbidity seen in protein precipitation (e.g., with sulfosalicylic acid). * **Nitrates:** These are generally inert in standard protein colorimetric assays and do not mimic the chemical behavior of peptide bonds or aromatic amino acids. * **Sulphates:** While high salt concentrations (like ammonium sulphate) are used to "salt out" proteins, they do not typically cause a false-positive colorimetric reaction in the estimation phase itself. * **Bile salts:** These interfere primarily with lipid digestion studies or cause "false positives" in urine dipstick tests for bilirubin/urobilinogen, but they are not a standard confounder for quantitative protein estimation. **High-Yield Clinical Pearls for NEET-PG:** * **Lowry Method:** Most sensitive common method; sensitive to interference by detergents and phosphates. * **Biuret Method:** Depends on **peptide bonds** (requires at least two); less sensitive but less prone to interference by free amino acids. * **Bradford Assay:** Uses **Coomassie Brilliant Blue G-250**; shifts absorbance from 465 to 595 nm when binding to basic/aromatic amino acids. * **Sulfosalicylic Acid (SSA) Test:** A bedside test for proteinuria; false positives can occur with **radiographic contrast media** and high doses of **penicillin/cephalosporins**.

Question 27: Reliability is tested by which of the following?

A. R-chart (Correct Answer)
B. Mean chart
C. Levy Jennings chart
D. Shewhart control chart

Explanation: In clinical biochemistry and laboratory quality control, monitoring the performance of analytical methods is essential for ensuring accurate patient results. ### **Explanation of the Correct Answer** **A. R-chart (Range Chart):** In statistical quality control, **Reliability** refers to the **precision** or reproducibility of a test. The R-chart tracks the difference between the maximum and minimum values (the range) in a series of measurements. Because it monitors the variability or "spread" of data points, it is the specific tool used to assess the **reliability/precision** of a method. If the range increases, the reliability decreases. ### **Explanation of Incorrect Options** * **B. Mean Chart (X-bar Chart):** This chart tracks the average of a set of values over time. It is primarily used to monitor the **accuracy** (closeness to the true value) of a process rather than its reliability. * **C. Levy-Jennings (L-J) Chart:** This is the most common chart used in labs to monitor daily quality control. While it displays both accuracy (mean) and precision (standard deviation), it is a comprehensive tool for detecting **random and systematic errors** rather than a specific test for reliability alone. * **D. Shewhart Control Chart:** This is a broad category of charts (which includes both Mean and R-charts). While technically correct in a general sense, the **R-chart** is the specific component within the Shewhart system designed to test reliability. ### **High-Yield Clinical Pearls for NEET-PG** * **Precision = Reliability:** Measured by Standard Deviation (SD), Coefficient of Variation (CV%), and **R-charts**. * **Accuracy = Trueness:** Measured by the **Mean chart** and Bias. * **Westgard Rules:** These are the rules applied to L-J charts to determine if an analytical run should be accepted or rejected. * **Random Error:** Affects precision (reliability); often caused by bubbles in reagents or temperature fluctuations. * **Systematic Error:** Affects accuracy; often caused by failing light sources or poorly calibrated pipettes.

Question 28: What does a microarray primarily study?

A. Multiple genes (Correct Answer)
B. Diseases
C. Organisms
D. Blood groups

Explanation: **Explanation:** **Microarray technology** is a high-throughput biochemical technique used to analyze the expression levels of thousands of genes simultaneously. It utilizes a solid surface (usually a glass slide or silicon chip) onto which microscopic spots of DNA probes are attached. When a sample of fluorescently labeled cDNA or cRNA is applied, it hybridizes with the complementary probes, allowing researchers to determine which genes are "turned on" or "off" in a specific tissue or under specific conditions. * **Why Option A is correct:** Microarrays are specifically designed for **transcriptomics** and **genomics**. They allow for the simultaneous study of the entire transcriptome (multiple genes) rather than focusing on a single gene (as seen in Northern blotting). * **Why Option B is incorrect:** While microarrays can be used to *diagnose* or research diseases (e.g., cancer subtyping), they primarily study the **genetic expression** underlying the disease, not the disease entity itself. * **Why Option C is incorrect:** Microarrays study molecular components (DNA/RNA) within cells; they do not study whole organisms. * **Why Option D is incorrect:** Blood grouping is typically performed via serological agglutination tests or specific PCR for genotypes, not broad-scale microarray analysis. **High-Yield NEET-PG Pearls:** 1. **Southern Blot:** Detects DNA. 2. **Northern Blot:** Detects RNA. 3. **Western Blot:** Detects Proteins. 4. **Microarray:** Often considered a "multiplex" version of Northern/Southern blotting. 5. **Clinical Use:** Microarrays are the "gold standard" for detecting **Copy Number Variations (CNVs)** in children with developmental delays or congenital anomalies (Chromosomal Microarray).

Question 29: Which of the following is the strongest type of chemical bond?

A. Electrostatic bond (Correct Answer)
B. Hydrogen bond
C. Hydrophobic interaction
D. Van der Waals force

Explanation: ### Explanation **Correct Answer: A. Electrostatic bond** In the context of biochemical interactions, chemical bonds are categorized based on their bond energy (strength). 1. **Why Electrostatic bond is correct:** Electrostatic bonds (also known as **ionic bonds**) are formed by the complete transfer of electrons between atoms, resulting in a strong attraction between oppositely charged ions (e.g., $Na^+$ and $Cl^-$). In biological systems, these occur between charged amino acid side chains (like Lysine and Aspartate). Among the options provided, electrostatic bonds have the highest bond energy (approx. **5–10 kcal/mol** in water, but much higher in hydrophobic environments), making them the strongest non-covalent interaction listed. 2. **Why the other options are incorrect:** * **B. Hydrogen bond:** This is a specific type of dipole-dipole interaction involving Hydrogen and an electronegative atom (O, N, or F). While crucial for DNA base pairing and protein alpha-helices, its strength (approx. **2–5 kcal/mol**) is significantly less than an ionic bond. * **C. Hydrophobic interaction:** This is not a "bond" in the traditional sense but a tendency of non-polar molecules to aggregate in water to increase entropy. It is relatively weak (approx. **1 kcal/mol**). * **D. Van der Waals force:** These are the weakest intermolecular forces (approx. **0.5–1 kcal/mol**) arising from transient dipoles in electron clouds. They only become significant when many atoms are in very close proximity. ### NEET-PG High-Yield Pearls * **Hierarchy of Bond Strength:** Covalent Bond (Strongest, e.g., Peptide bond) > Electrostatic/Ionic > Hydrogen Bond > Hydrophobic Interaction > Van der Waals (Weakest). * **Note:** If "Covalent bond" were an option, it would be the strongest overall. Among **non-covalent** bonds, Electrostatic is the strongest. * **Clinical Relevance:** The stability of the **DNA double helix** is maintained by Hydrogen bonds, while the **tertiary structure of proteins** is stabilized by a combination of all the above, including strong disulfide (covalent) bridges.

Question 30: Karyotyping under light microscopy is done by which banding technique?

A. R banding
B. Q banding
C. G banding (Correct Answer)
D. C banding

Explanation: **Explanation:** **G-banding (Giemsa banding)** is the most common and standard technique used for routine karyotyping under light microscopy. In this method, chromosomes are first treated with **Trypsin** (to partially digest proteins) and then stained with **Giemsa stain**. This produces a characteristic pattern of alternating light and dark bands: * **Dark bands (G-positive):** Represent AT-rich, gene-poor, heterochromatic regions that replicate late. * **Light bands (G-negative):** Represent GC-rich, gene-dense, euchromatic regions that replicate early. **Analysis of Incorrect Options:** * **R-banding (Reverse):** This is the "reverse" of G-banding. Chromosomes are heat-denatured before staining. It is used specifically to study the distal ends (telomeres) of chromosomes. * **Q-banding (Quinacrine):** This was the first banding method developed. It uses quinacrine mustard (a fluorescent stain) and requires a **fluorescence microscope**, not a standard light microscope. * **C-banding (Constitutive Heterochromatin):** This technique specifically stains the centromeres and regions containing constitutive heterochromatin (like chromosomes 1, 9, 16, and Y). **High-Yield Clinical Pearls for NEET-PG:** * **Sample Collection:** For postnatal karyotyping, **Peripheral Blood Lymphocytes** are used. They must be stimulated with a mitogen like **Phytohemagglutinin (PHA)**. * **Cell Cycle Arrest:** Cells are arrested in **Metaphase** using **Colchicine** (which inhibits spindle formation) because chromosomes are most condensed and visible during this stage. * **Resolution:** Standard G-banding identifies roughly 400–550 bands per haploid set, allowing for the detection of numerical and large structural abnormalities.

Practice by Chapter

Spectrophotometry and Colorimetry

Practice Questions

Chromatography Techniques

Practice Questions

Electrophoresis and Applications

Practice Questions

Centrifugation and Ultracentrifugation

Practice Questions

Radioisotope Techniques

Practice Questions

Enzyme-Linked Immunosorbent Assay (ELISA)

Practice Questions

Polymerase Chain Reaction (PCR)

Practice Questions

Blotting Techniques: Southern, Northern, Western

Practice Questions

Mass Spectrometry in Biochemistry

Practice Questions

Recombinant DNA Technology

Practice Questions

DNA Sequencing

Practice Questions

Proteomics and Metabolomics

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free