Biochemical Techniques Practice Questions

Q: Concentration of DNA is measured by:

Absorption at 260 nm. **Explanation:** **1. Why Option A is Correct:** The concentration and purity of DNA are most commonly measured using **UV Spectrophotometry**. DNA contains nitrogenous bases (purines and pyrimidines) with conjugated double bonds that naturally absorb ultraviolet light. The maximum absorption peak for nucleic acids occurs at **260 nm**. According to the **Beer-Lambert Law**, the amount of light absorbed is directly proportional to the concentration of the DNA in the sample. * *High-yield fact:* An Optical Density (OD) of 1.0 at 260 nm corresponds to approximately 50 µg/mL of double-stranded DNA. **2. Why Other Options are Incorrect:** * **B. D-oxyribose estimation:** While DNA contains deoxyribose, measuring the sugar component (e.g., via the Dische diphenylamine test) is a colorimetric chemical assay. It is less precise, more destructive to the sample, and not the standard method for rapid quantification compared to spectrophotometry. * **C. Infrared examination:** IR spectroscopy is used to identify functional groups and molecular structures (vibrational modes) rather than quantifying the concentration of nucleic acids in a clinical or research setting. * **D. Ultrasound examination:** This is a diagnostic imaging modality used to visualize internal body structures; it has no application in measuring molecular concentrations of DNA. **Clinical Pearls for NEET-PG:** * **Purity Check (A260/A280 Ratio):** To check for protein contamination, the ratio of absorbance at 260 nm to 280 nm is calculated. * **Pure DNA:** Ratio of **~1.8**. * **Pure RNA:** Ratio of **~2.0**. * A lower ratio indicates protein or phenol contamination. * **Hyperchromicity:** When DNA is denatured (melting), its absorbance at 260 nm **increases**. This is known as the hyperchromic effect.

Q: Which of the following methods is used for the analysis of the C-terminal end of a polypeptide?

Akabori method. **Explanation:** Protein sequencing involves identifying the specific order of amino acids in a polypeptide chain. This is typically achieved by identifying the N-terminal (amino) and C-terminal (carboxyl) ends. **1. Why Akabori Method is Correct:** The **Akabori method (Hydrazinolysis)** is a chemical technique used for **C-terminal analysis**. In this process, the polypeptide is heated with hydrazine ($NH_2-NH_2$). Hydrazine cleaves all peptide bonds and converts every amino acid into an amino acid hydrazide, **except for the C-terminal amino acid**. Since the C-terminal residue has a free carboxyl group (not involved in a peptide bond), it remains as a free amino acid, which can then be isolated and identified via chromatography. **2. Analysis of Incorrect Options:** * **Sanger’s Method (1-fluoro-2,4-dinitrobenzene / FDNB):** This reagent reacts with the free $\alpha$-amino group of the **N-terminal** amino acid to form a yellow DNP-amino acid derivative. It is used for N-terminal analysis. * **Edman’s Degradation (Phenylisothiocyanate / PITC):** This is the most common method for **N-terminal** sequencing. It removes one amino acid at a time from the N-terminus without hydrolyzing the rest of the peptide chain, allowing for automated sequencing (Sequenator). **High-Yield Clinical Pearls for NEET-PG:** * **N-terminal Reagents:** Sanger’s (FDNB), Edman’s (PITC), and Dansyl Chloride. * **C-terminal Reagents:** Akabori method (Hydrazinolysis) and **Carboxypeptidases** (enzymatic method). * **Cyanogen Bromide (CNBr):** Specifically cleaves peptide bonds at the carboxyl side of **Methionine** residues. * **Trypsin:** Cleaves at the carboxyl side of **Lysine and Arginine** (basic amino acids).

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

Gel filtration chromatography. ### 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**.

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

Mass spectrometry. **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.

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

Western blot. **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).

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

Densitometry. **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.

Q: Molecular size of proteins is assessed by which of the following methods?

Absorption mass spectroscopy. ### Explanation **Correct Option: B. Absorption Mass Spectroscopy** Mass Spectrometry (MS) is the gold standard for determining the **molecular weight (size)** and structural composition of proteins. It works by ionizing chemical species and sorting the ions based on their **mass-to-charge (m/z) ratio**. Techniques like MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization - Time of Flight) are specifically designed to measure the mass of large biomolecules like proteins with extreme precision. **Analysis of Incorrect Options:** * **A. Sedimentation:** While ultracentrifugation can provide an estimate of molecular weight via the Svedberg unit (sedimentation coefficient), it primarily measures the rate at which particles settle. It is more commonly used for separating organelles or determining the density of lipoproteins rather than precise molecular sizing of individual proteins. * **C. Lyophilization:** Also known as freeze-drying, this is a **preservation technique**. It removes water from a protein sample to increase its shelf life and stability; it is not an analytical method for size assessment. * **D. Salting Out:** This is a **purification technique** based on solubility. By adding high concentrations of salts (like ammonium sulfate), proteins precipitate out of solution. It does not provide information regarding the molecular size. **High-Yield Clinical Pearls for NEET-PG:** * **SDS-PAGE:** Another common method to estimate protein molecular weight based on electrophoretic mobility. * **Gel Filtration Chromatography:** Also known as Size-Exclusion Chromatography; it separates proteins based on size (larger proteins elute first). * **Proteomics:** Mass Spectrometry is the cornerstone of proteomics, used in clinical settings for identifying bacterial strains (MALDI-TOF) and detecting metabolic disorders in newborns (Tandem MS).

Q: Flow cytometry analysis is performed for which of the following estimations?

CD4/CD8 estimation in AIDS. **Explanation:** **Flow Cytometry** is a sophisticated laser-based technology used to analyze the physical and chemical characteristics of particles (usually cells) as they flow in a fluid stream through a beam of light. **Why Option C is Correct:** The primary clinical application of flow cytometry is **Immunophenotyping**. In patients with HIV/AIDS, specific surface markers (Clusters of Differentiation) are identified using fluorescent-labeled monoclonal antibodies. Flow cytometry measures the fluorescence intensity to quantify the absolute counts and ratios of **CD4+ (T-helper cells)** and **CD8+ (T-cytotoxic cells)**. This is the gold standard for monitoring disease progression and the efficacy of Antiretroviral Therapy (ART). **Why Other Options are Incorrect:** * **A. Blood glucose estimation:** Typically performed using enzymatic methods like the **Glucose Oxidase-Peroxidase (GOD-POD)** method or hexokinase method. * **B. Separation of proteins:** Achieved through **Electrophoresis** (based on charge/size) or **Chromatography**. * **D. Estimation of LDL:** Calculated using the **Friedewald formula** [LDL = Total Cholesterol – (HDL + VLDL)] or measured via direct homogeneous assays. **High-Yield Clinical Pearls for NEET-PG:** * **Principle:** Flow cytometry works on the principles of **Light Scattering** (Forward scatter for size; Side scatter for granularity/complexity) and **Fluorescence**. * **Other Applications:** Diagnosis of leukemias/lymphomas, DNA ploidy analysis, and detecting **PNH (Paroxysmal Nocturnal Hemoglobinuria)** by looking for the absence of CD55 and CD59. * **FACS:** Fluorescence-Activated Cell Sorting is a specialized type of flow cytometry that not only analyzes but also physically sorts cells into different containers.

Q: What is the principle of the Benedict's test for urine reducing substances?

Cu2+ ions are reduced to Cu+ ions by enediols. ### Explanation **1. Why Option A is Correct:** Benedict’s test is a semi-quantitative test used to detect **reducing sugars** (like glucose, fructose, and galactose) in urine. In an alkaline medium, reducing sugars undergo tautomerization to form highly reactive **enediols**. These enediols act as reducing agents; they donate electrons to the **cupric ions (Cu²⁺)** present in the Benedict’s reagent (as copper sulfate), reducing them to **cuprous ions (Cu⁺)**. These cuprous ions then react with hydroxyl ions to form cuprous hydroxide, which upon heating yields a **cuprous oxide (Cu₂O) precipitate**. The color change (from blue to green, yellow, orange, or brick red) depends on the concentration of the reducing sugar. **2. Why Other Options are Incorrect:** * **Option B:** This is the reverse of the actual reaction. Oxidation involves losing electrons, whereas the copper ions must *gain* electrons (reduction) to form the precipitate. * **Options C & D:** Benedict’s reagent specifically utilizes **Copper (Cu)**, not Iron (Fe). Iron-based redox reactions are characteristic of other biochemical processes (like the Fenton reaction or Cytochrome chain), but not standard urine sugar testing. **3. Clinical Pearls for NEET-PG:** * **Specificity:** Benedict’s test is **not specific for glucose**. It gives positive results for other reducing sugars (lactose in pregnancy, fructose in Essential Fructosuria, galactose in Galactosemia) and non-sugar reducers (Vitamin C, Uric acid, Salicylates). * **Glucose Specificity:** To confirm if a positive Benedict’s test is specifically due to glucose, the **Glucose Oxidase method (Dipstick)** is used. * **Composition:** Benedict’s reagent contains **Copper sulfate** (provides Cu²⁺), **Sodium carbonate** (provides alkaline medium), and **Sodium citrate** (prevents spontaneous precipitation of Cu(OH)₂ by acting as a chelating agent).

Question 1

Concentration of DNA is measured by:

Accepted Answer

Absorption at 260 nm

Answer

D-oxyribose estimation

Answer

Infrared examination

Answer

Ultrasound examination

Question 2

Which of the following methods is used for the analysis of the C-terminal end of a polypeptide?

Accepted Answer

Akabori method

Answer

Sanger's method

Answer

Edman's degradation method

Answer

None of the above

Question 3

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

Accepted Answer

Gel filtration chromatography

Answer

Adsorption chromatography

Answer

Paper chromatography

Answer

None of these

Question 4

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

Accepted Answer

Mass spectrometry

Answer

X-ray crystallography

Answer

NMR spectrometry

Answer

All of the above

Question 5

Which of the following is NOT a nucleic acid test?

Accepted Answer

Western blot

Answer

Southern blot

Answer

Northern blot

Answer

Microarray

Question 6

Affinity chromatography is based on which principle?

Accepted Answer

The specific binding of a protein constituent to another molecule.

Answer

Protein-protein interactions.

Answer

Protein-carbohydrate interactions.

Answer

None of the above.

Question 7

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

Accepted Answer

Densitometry

Answer

Chromatography

Answer

Precipitation

Answer

Electrophoresis

Question 8

Molecular size of proteins is assessed by which of the following methods?

Accepted Answer

Absorption mass spectroscopy

Answer

Sedimentation

Answer

Lyophilization

Answer

Salting out

Question 9

Flow cytometry analysis is performed for which of the following estimations?

Accepted Answer

CD4/CD8 estimation in AIDS

Answer

Blood glucose estimation

Answer

Separation of proteins

Answer

Estimation of LDL

Question 10

What is the principle of the Benedict's test for urine reducing substances?

Accepted Answer

Cu2+ ions are reduced to Cu+ ions by enediols

Answer

Cu+ ions are oxidized to Cu2+ ions by enediols

Answer

Fe2+ ions are reduced to Fe+ ions by enediols

Answer

Fe+ ions are reoxidized to Fe2+ ions by enediols

Biochemical Techniques — MCQs

Biochemical Techniques — MCQs

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