Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 61: Northern blotting is used for the separation of:

A. DNA
B. RNA (Correct Answer)
C. Proteins
D. None of the above

Explanation: **Explanation:** **Northern blotting** is a molecular biology technique specifically designed for the detection and separation of specific **RNA** molecules (mRNA, tRNA, or rRNA) within a sample. The process involves separating RNA fragments by size using gel electrophoresis, transferring them to a membrane (nitrocellulose or nylon), and identifying specific sequences using a labeled complementary nucleic acid probe. This technique is crucial for studying **gene expression** levels in different tissues. **Analysis of Options:** * **Option A (DNA):** DNA fragments are separated and identified using **Southern blotting**. This is used for applications like DNA fingerprinting and detecting genetic mutations. * **Option C (Proteins):** Proteins are separated by SDS-PAGE and detected using antibodies in a process called **Western blotting**. This is clinically used as a confirmatory test for HIV (detecting p24 or gp120 antibodies). * **Option D:** Incorrect, as RNA is the specific target for Northern blotting. **NEET-PG High-Yield Pearls:** To remember the blotting techniques, use the mnemonic **SNOW DROP**: * **S**outhern — **D**NA * **N**orthern — **R**NA * **O** — **O** (Placeholder) * **W**estern — **P**rotein **Additional Related Techniques:** * **South-Western Blotting:** Used to identify proteins that bind to DNA (e.g., transcription factors). * **Eastern Blotting:** Used to detect post-translational modifications of proteins (e.g., carbohydrates or lipids).

Question 62: Which of the following techniques for protein separation is based on molecular size?

A. Ion exchange chromatography
B. Gel filtration chromatography (Correct Answer)
C. Affinity chromatography
D. Hydrophobic interaction chromatography

Explanation: **Explanation:** Protein separation techniques are fundamental in biochemistry for isolating specific proteins based on their unique physical and chemical properties. **1. Why Gel Filtration Chromatography is Correct:** Also known as **Size-Exclusion Chromatography (SEC)**, this technique separates proteins strictly based on their **molecular size (hydrodynamic radius)**. The column is packed with porous beads (e.g., dextran or agarose). * **Mechanism:** Large molecules cannot enter the pores and are "excluded," traveling around the beads and eluting **first**. Small molecules enter the pores, taking a longer, more tortuous path, and thus elute **later**. **2. Why the Other Options are Incorrect:** * **A. Ion Exchange Chromatography:** Separates proteins based on their **net surface charge**. It uses stationary phases (anion or cation exchangers) that bind proteins of the opposite charge. * **C. Affinity Chromatography:** Separates proteins based on **high-specificity biological interactions** (e.g., Enzyme-Substrate, Antigen-Antibody, or Hormone-Receptor). It is the most selective method. * **D. Hydrophobic Interaction Chromatography (HIC):** Separates proteins based on their **surface hydrophobicity**. Proteins bind to the hydrophobic stationary phase at high salt concentrations and are eluted by decreasing the salt gradient. **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Weight Determination:** Gel filtration is the primary method used to estimate the quaternary structure/molecular weight of a protein in its native state. * **Order of Elution:** In Gel Filtration, **Largest = First**; in SDS-PAGE (electrophoresis), **Smallest = Fastest/Bottom**. * **Desalting:** Gel filtration is commonly used in labs to "desalt" a protein solution (removing small salt ions from large protein molecules).

Question 63: Various enzymes are used in clinical laboratories for the estimation of various substances. Peroxidase enzyme is used in estimating which of the following?

A. Haemoglobin
B. Ammonia
C. Creatinine
D. Glucose (Correct Answer)

Explanation: **Explanation:** The estimation of blood glucose is most commonly performed using the **GOD-POD method** (Glucose Oxidase - Peroxidase method). This is a coupled enzymatic reaction: 1. **Glucose Oxidase (GOD)** catalyzes the oxidation of glucose to gluconic acid and **hydrogen peroxide ($H_2O_2$)**. 2. **Peroxidase (POD)** then catalyzes the reaction between $H_2O_2$ and a chromogen (e.g., 4-aminophenazone and phenol) to form a colored complex (Quinoneimine). The intensity of the color formed is directly proportional to the glucose concentration. **Analysis of Incorrect Options:** * **Ammonia:** Usually estimated using the **Glutamate Dehydrogenase (GLDH)** method, where ammonia reacts with $\alpha$-ketoglutarate to form glutamate, with a concomitant decrease in NADH absorbance. * **Creatinine:** Most commonly measured by the **Jaffe’s Reaction** (reaction with alkaline picrate) or enzymatically using **Creatininase/Creatinase**, but not typically peroxidase. * **Haemoglobin:** Estimated via the **Drabkin’s method**, which converts hemoglobin to cyanmethemoglobin using potassium ferricyanide and cyanide. **High-Yield Clinical Pearls for NEET-PG:** * **Hexokinase Method:** This is the **Reference Method** for glucose estimation because it is more specific than the GOD-POD method. * **Fluoride (Grey top tube):** Used for blood collection to inhibit **Enolase**, preventing glycolysis so that glucose levels remain stable for estimation. * **Trinder’s Reaction:** The specific name for the color-producing reaction involving Peroxidase and a chromogen.

Question 64: What is the focal length of a high power objective lens of a microscope?

A. 40mm
B. 16mm
C. 20mm
D. 4mm (Correct Answer)

Explanation: **Explanation:** In microscopy, the **focal length** of an objective lens is inversely proportional to its magnification power. As the magnification increases, the focal length and the working distance (the space between the lens and the slide) decrease. 1. **Why 4mm is correct:** A standard **high-power objective** typically has a magnification of **40x**. For such lenses, the focal length is approximately **4mm**. This short focal length is necessary to achieve high resolution and magnification, requiring the lens to be positioned very close to the specimen. 2. **Why the other options are incorrect:** * **40mm:** This is an extremely long focal length, not typically used in standard compound light microscopy objectives. * **16mm:** This is the characteristic focal length for a **low-power objective (10x)**. * **20mm:** This does not correspond to a standard objective lens used in medical laboratory microscopy (standard steps are usually 4x, 10x, 40x, and 100x). * *Note:* For an **oil immersion lens (100x)**, the focal length is even shorter, typically around **1.8mm to 2mm**. **High-Yield Facts for NEET-PG:** * **Total Magnification:** Calculated by multiplying the power of the objective lens by the power of the ocular lens (eyepiece). (e.g., 40x objective × 10x eyepiece = 400x magnification). * **Resolving Power:** The ability to distinguish two close points as separate. It is determined by the wavelength of light and the **Numerical Aperture (NA)**. * **Numerical Aperture:** High-power and oil immersion lenses have higher NA values to capture more light, which is essential as the focal length decreases. * **Oil Immersion:** Cedar wood oil or synthetic oil is used with the 100x lens because its refractive index is similar to glass, preventing light refraction and increasing resolution.

Question 65: Which substance is often confused with protein in certain biochemical tests?

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

Explanation: ### Explanation In biochemical analysis, particularly during urine testing for proteinuria, **Phosphates** are the most common substance to cause a **false-positive** result. **1. Why Phosphates are the Correct Answer:** The most frequent point of confusion occurs during the **Heat and Acetic Acid Test** (Boiling Test). When urine is heated, a white cloudiness or precipitate may form. This turbidity can be caused by either **precipitated proteins** (albumin) or **precipitated phosphates** (calcium and magnesium phosphates). * **The Differentiation:** To distinguish between the two, a few drops of 3% acetic acid are added. If the turbidity is due to phosphates, it will **dissolve** and the urine will become clear (as phosphates are soluble in acid). If the turbidity persists or increases, it confirms the presence of protein. **2. Analysis of Incorrect Options:** * **Nitrates:** These are typically tested in urine (Nitrite test) to screen for urinary tract infections (UTIs) caused by Gram-negative bacteria. They do not form precipitates that mimic proteins. * **Sulphates:** While present in urine, they do not precipitate upon heating in a manner that interferes with standard protein qualitative tests. * **Bile Salts:** These are detected using **Hay’s Sulphur Test** (based on surface tension). While they are a pathological finding in jaundice, they do not produce a white cloudiness that mimics protein precipitation. **3. Clinical Pearls for NEET-PG:** * **False Positives for Protein:** Highly concentrated urine, alkaline urine (which favors phosphate precipitation), and certain drugs (e.g., penicillins, radiocontrast media). * **Bence-Jones Proteins:** A unique protein found in Multiple Myeloma that precipitates at 40–60°C but **redissolves** on boiling (100°C), unlike albumin. * **Sulfosalicylic Acid (SSA) Test:** A more sensitive cold precipitation test for proteins; however, it can also give false positives with radiographic dyes and high doses of penicillin.

Question 66: Hemoglobin electrophoresis is based on which principle?

A. Molecular weight
B. Charge (Correct Answer)
C. Solubility
D. Calorimetric properties

Explanation: **Explanation:** **1. Why Charge is Correct:** Electrophoresis is the process of separating charged molecules in an electric field. Hemoglobin (Hb) molecules are composed of amino acids, many of which carry side-chain charges. At a specific pH (typically alkaline pH 8.6), hemoglobin molecules acquire a **net negative charge** and migrate toward the positive electrode (anode). The rate of migration depends primarily on the **net electrical charge** of the Hb variant. For example, in HbS, a neutral valine replaces a negatively charged glutamic acid, making HbS less negative than HbA, thus causing it to move slower toward the anode. **2. Why Other Options are Incorrect:** * **Molecular Weight:** While techniques like SDS-PAGE separate proteins by size, standard Hb electrophoresis does not. Most Hb variants (HbA, HbS, HbC) have nearly identical molecular weights, making size-based separation ineffective. * **Solubility:** This is the principle behind the **"Solubility Test"** (using sodium dithionite) used specifically to screen for Sickle Cell Hemoglobin (HbS), not for general electrophoresis. * **Calorimetric Properties:** This refers to heat measurement or color intensity (spectrophotometry) and is used for quantifying substances, not for separating protein variants. **3. Clinical Pearls & High-Yield Facts:** * **Alkaline Electrophoresis (pH 8.6):** The standard screening method. The order of migration from Anode (+) to Cathode (-) is: **A** (Fastest), **F**, **S**, **C** (Slowest). *Mnemonic: **A** fat **S**low **C**at.* * **Acid Electrophoresis (pH 6.2):** Used as a confirmatory test to differentiate variants that co-migrate at alkaline pH (e.g., HbS from HbD). * **HbA1c:** Measured via HPLC or Ion-exchange chromatography, which also utilizes charge differences but in a column format.

Question 67: Protein fragments separation is performed by which technique?

A. Western blot
B. Chromatography (Correct Answer)
C. Centrifugation
D. Ultrafiltration

Explanation: ### Explanation **Correct Option: B. Chromatography** Chromatography is the gold standard technique for the separation of protein fragments based on their physical and chemical properties. Depending on the specific method used, proteins are separated by **size** (Size-exclusion chromatography), **charge** (Ion-exchange chromatography), or **binding affinity** (Affinity chromatography). In biochemical research and clinical diagnostics, it is the primary method used to isolate specific protein fragments from a complex mixture. **Analysis of Incorrect Options:** * **A. Western Blot:** This is a technique used for the **detection and identification** of a specific protein using antibodies. While it involves separation via electrophoresis (SDS-PAGE) as an initial step, its primary purpose is qualitative or semi-quantitative identification, not the preparative separation of fragments. * **C. Centrifugation:** This technique separates particles based on **density and sedimentation rate**. While it can separate organelles or large protein aggregates (ultracentrifugation), it lacks the resolution required to separate individual protein fragments of similar densities. * **D. Ultrafiltration:** This is a membrane-based separation process used primarily for **concentrating** a protein solution or "desalting" it. It separates molecules based on a coarse molecular weight cutoff but cannot precisely fractionate different protein fragments. **NEET-PG High-Yield Pearls:** * **Affinity Chromatography:** The most specific method; utilizes the high affinity of proteins for specific ligands (e.g., Enzyme-Substrate, Antigen-Antibody). * **SDS-PAGE:** Separates proteins strictly based on **molecular weight** by masking their intrinsic charge with a negative charge. * **Isoelectric Focusing (IEF):** Separates proteins based on their **pI (Isoelectric point)**. * **ELISA vs. Western Blot:** ELISA is used for screening (high sensitivity), while Western Blot is used for confirmation (high specificity), notably in HIV testing.

Question 68: Which of the following statements is true regarding High-Performance Liquid Chromatography (HPLC) and Gas Chromatography?

A. In Gas Chromatography, the stationary phase is always solid.
B. In HPLC, the stationary phase is solid.
C. In HPLC, the mobile phase is a liquid or solid.
D. In Gas Chromatography, the mobile phase is always gas. (Correct Answer)

Explanation: **Explanation:** Chromatography is a fundamental biochemical technique used to separate components of a mixture based on their relative affinities for a **stationary phase** and a **mobile phase**. **1. Why Option D is Correct:** In **Gas Chromatography (GC)**, the mobile phase is strictly an inert carrier gas (such as Helium, Nitrogen, or Argon). Its primary role is to transport the volatile sample through the column. Since the separation depends on the sample being in a vapor state, the mobile phase must be gaseous. **2. Analysis of Incorrect Options:** * **Option A:** In GC, the stationary phase can be either a **solid** (Gas-Solid Chromatography) or a **liquid** coated on a solid support (Gas-Liquid Chromatography). Therefore, it is not "always" solid. * **Option B:** In **HPLC**, the stationary phase is typically a solid (silica-based) or a liquid film bonded to solid particles. However, the defining feature of HPLC is the high pressure applied to the liquid mobile phase, not just the state of the stationary phase. * **Option C:** In HPLC, the mobile phase is **always a liquid** (solvent). It can never be a solid, as it must flow through the packed column under high pressure. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **HPLC Use:** It is the "Gold Standard" for the estimation of **HbA1c** (Glycated Hemoglobin) and for screening hemoglobinopathies (e.g., Thalassemia). * **GC Use:** Primarily used for detecting **volatile compounds**, such as blood alcohol levels, fatty acids, and toxicology screens. * **Key Difference:** GC requires the sample to be volatile and thermally stable, whereas HPLC can analyze non-volatile, polar, and thermally unstable biological molecules (like proteins and drugs).

Question 69: Molecular separation of two proteins with the same charge can be achieved by which technique?

A. Ion exchange chromatography
B. Dialysis
C. Gel filtration chromatography (Correct Answer)
D. Electrophoresis

Explanation: ### Explanation The correct answer is **Gel filtration chromatography** (also known as Size-Exclusion Chromatography). **1. Why Gel Filtration Chromatography is Correct:** This technique separates proteins based on their **molecular size and shape**, not their charge. The stationary phase consists of porous beads. * **Large proteins** cannot enter the pores and travel around the beads, eluting **first**. * **Small proteins** enter the pores, taking a longer path, and elute **later**. Since the separation depends entirely on the "hydrodynamic volume," two proteins with the same charge but different sizes can be easily separated. **2. Why Other Options are Incorrect:** * **Ion exchange chromatography:** Separates proteins based on their **net charge**. If two proteins have the same charge, they will bind to the resin with similar affinity and will not be separated effectively. * **Dialysis:** This is a process used for **desalting** or changing buffers. It uses a semi-permeable membrane to separate small molecules (like salts) from large macromolecules (proteins). It is not a precise method for separating two different proteins from each other. * **Electrophoresis:** Standard native electrophoresis separates proteins based on their **charge-to-mass ratio**. If proteins have the same charge and similar mass, they will migrate together. (Note: SDS-PAGE separates by mass, but it denatures proteins). **3. High-Yield Clinical Pearls for NEET-PG:** * **Reverse Phase Chromatography:** Separates proteins based on **hydrophobicity**. * **Affinity Chromatography:** The most **specific** technique; uses ligand-receptor or antigen-antibody interactions (e.g., separating Insulin using anti-insulin antibodies). * **Isoelectric Focusing (IEF):** Separates proteins based on their **pI (Isoelectric point)**. * **Elution Order in Gel Filtration:** Remember: **"Big drops first."** The larger the molecular weight, the faster it elutes.

Question 70: What banding technique is used for studying translocations involving the centromere?

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

Explanation: **Explanation:** **C-banding (Constitutive Heterochromatin banding)** is the specific technique used to stain the **centromeres** and other regions containing constitutive heterochromatin (such as the distal portion of the Y chromosome). It involves pre-treating chromosomes with an alkali (like barium hydroxide) followed by Giemsa staining. This process denatures euchromatin while preserving the highly repetitive DNA at the centromere, making it the gold standard for identifying **dicentric chromosomes** and **centromeric translocations**. **Analysis of Incorrect Options:** * **Q-banding (Quinacrine):** Uses fluorescent dyes to produce bright and dull bands. It was the first banding method but requires a fluorescence microscope and the signal fades quickly. It is useful for identifying the Y chromosome and polymorphisms. * **G-banding (Giemsa):** The most common clinical technique. Chromosomes are treated with **Trypsin** followed by Giemsa. It produces dark (AT-rich, gene-poor) and light (GC-rich, gene-active) bands, used for general karyotyping. * **R-banding (Reverse):** The "reverse" of G-banding. It stains GC-rich regions (euchromatin) darkly. It is particularly useful for studying the **ends of chromosomes (telomeres)**, which often stain poorly in G-banding. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for G-banding:** **G**iemsa + **T**rypsin (**G**reat **T**eacher). * **Centromere = C-banding:** Always associate "C" with Centromere and Constitutive heterochromatin. * **Telomeres = R-banding:** Best for distal deletions or translocations. * **Karyotyping Phase:** All these techniques are performed on cells arrested in **Metaphase** using Colchicine (which inhibits spindle formation).

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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