Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 11: The Octant rule is related to which technique?

A. Spectroscopy (Correct Answer)
B. Chromatography
C. Electroscopy
D. None of the above

Explanation: The **Octant Rule** is a fundamental empirical rule used in **Circular Dichroism (CD) Spectroscopy**, which is a specialized branch of spectroscopy. ### 1. Why Spectroscopy is Correct The Octant Rule specifically applies to **Optical Rotatory Dispersion (ORD)** and **Circular Dichroism**. It is used to predict the sign (positive or negative) of the **Cotton Effect** in the CD spectrum of chiral ketones (like cyclohexanone derivatives). * **Mechanism:** By dividing the space around the carbonyl group into eight sections (octants), scientists can predict how substituents at different positions will contribute to the molecule's optical activity. * **Application:** It is a vital tool for determining the **absolute configuration** and conformation of organic molecules and steroids. ### 2. Why Other Options are Incorrect * **Chromatography:** This technique is used for the *separation* of mixtures based on differential distribution between a mobile and stationary phase (e.g., HPLC, GC). It does not involve the measurement of light rotation or octant spatial rules. * **Electroscopy:** This is an outdated term or refers to an electroscope (used to detect electric charge). It has no application in determining molecular configuration or structural biochemistry. ### 3. High-Yield Clinical Pearls for NEET-PG * **Circular Dichroism (CD):** Most commonly used in biochemistry to determine the **secondary structure of proteins** (alpha-helices and beta-sheets). * **Cotton Effect:** The characteristic change in optical rotation near an absorption band of a substance. * **X-ray Crystallography:** While the Octant rule helps with configuration, X-ray crystallography remains the "gold standard" for determining the 3D structure of proteins. * **Beer-Lambert Law:** The fundamental law governing absorption spectroscopy ($A = \epsilon cl$).

Question 12: Which of the following blotting techniques is used to detect DNA?

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

Explanation: **Explanation:** Blotting techniques are fundamental molecular biology tools used to identify specific macromolecules (DNA, RNA, or proteins) within a complex mixture. **1. Why Southern Blot is Correct:** The **Southern blot** is the gold standard technique for detecting specific **DNA** sequences. It was developed by Edwin Southern in 1975. The process involves digesting DNA with restriction endonucleases, separating the fragments by gel electrophoresis, transferring (blotting) them onto a nitrocellulose membrane, and finally hybridizing them with a labeled DNA probe complementary to the target sequence. **2. Analysis of Incorrect Options:** * **Western Blot:** Used to detect specific **Proteins**. It utilizes antibodies (primary and secondary) to identify target proteins after they have been separated by SDS-PAGE. * **Northern Blot:** Used to detect **RNA** (specifically mRNA) to study gene expression. It follows a similar principle to Southern blotting but does not require restriction digestion as RNA molecules are already short. * **Eastern Blot:** A specialized technique used to detect **post-translational modifications** of proteins, such as lipids, phosphates, or glycoconjugates. **3. NEET-PG High-Yield Pearls:** To remember these easily, use the mnemonic **SNOW DROP**: * **S**outhern — **D**NA * **N**orthern — **R**NA * **O** — **O** (No match) * **W**estern — **P**rotein * **Southwestern Blot:** A hybrid technique used to detect **DNA-binding proteins** (e.g., transcription factors). * **Clinical Application:** Southern blotting is clinically used in DNA fingerprinting, detecting gene mutations (like deletions or insertions), and diagnosing certain genetic disorders or viral infections.

Question 13: Which of the following techniques for purification of proteins can be made specific for a given protein?

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

Explanation: ### Explanation **Affinity Chromatography** is the correct answer because it is the only technique listed that relies on the **highly specific biological interaction** between a protein and a ligand. 1. **Why Affinity Chromatography is Specific:** This technique exploits the unique binding properties of a protein. A specific ligand (such as an antibody, substrate, or hormone) is covalently attached to an inert matrix. When a mixture is passed through the column, only the target protein binds to the ligand, while all other proteins are washed away. The protein is then eluted by adding a high concentration of free ligand or changing the pH. This provides the highest level of purification, often achieving a thousand-fold increase in purity in a single step. 2. **Why Other Options are Incorrect:** * **Dialysis (A):** This is a separation technique based purely on **molecular size** and concentration gradients across a semi-permeable membrane. It is used for desalting or buffer exchange, not for specific protein isolation. * **Gel Filtration Chromatography (C):** Also known as Size-Exclusion Chromatography, it separates proteins based on their **hydrodynamic volume (size and shape)**. It is non-specific as any proteins of similar size will elute together. * **Ion Exchange Chromatography (D):** This separates proteins based on their **net surface charge** at a given pH. Multiple proteins can share the same charge profile, making it less specific than affinity-based methods. ### High-Yield Clinical Pearls for NEET-PG: * **Gold Standard for Specificity:** Affinity chromatography is the "gold standard" for purifying recombinant proteins (e.g., using His-tags that bind to Nickel columns). * **Immunoaffinity:** A subtype using antibodies to isolate specific antigens/proteins. * **Molecular Sieve:** Another name for Gel Filtration; remember that **larger molecules elute first** because they are excluded from the pores of the beads. * **Isoelectric Point (pI):** In Ion Exchange, if pH > pI, the protein is negatively charged (anion) and binds to an **Anion Exchanger** (e.g., DEAE-cellulose).

Question 14: Reducing sugar in urine can be detected by which test?

A. Benedict's test
B. Fehling solution
C. Glucose-oxidase test
D. All of the above (Correct Answer)

Explanation: **Explanation:** The detection of sugar in urine (glycosuria) is a fundamental clinical biochemistry task. This question tests the distinction between **non-specific chemical tests** and **specific enzymatic tests**. 1. **Benedict’s Test (Option A):** This is a semi-quantitative, non-specific test for **reducing sugars** (glucose, fructose, galactose, lactose, pentoses). It relies on the reduction of cupric ions ($Cu^{2+}$) to cuprous oxide ($Cu_2O$) in an alkaline medium, resulting in a color change from blue to green, yellow, or brick red. 2. **Fehling’s Solution (Option B):** Similar to Benedict’s, this uses copper reduction. While less stable than Benedict’s reagent, it is a classic laboratory method used to detect reducing substances. 3. **Glucose-Oxidase Test (Option C):** This is a **highly specific** enzymatic method used in modern urine dipsticks. Glucose oxidase converts glucose to gluconic acid and hydrogen peroxide ($H_2O_2$). The $H_2O_2$ then reacts with a chromogen to produce a color change. Although it specifically targets glucose, it is the primary clinical method for detecting the most common reducing sugar in urine. Since all three methods are valid techniques for detecting sugar (specifically glucose) in a clinical or laboratory setting, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Benedict’s Test False Positives:** Can occur with other reducing substances like Vitamin C (ascorbic acid), salicylates, and certain antibiotics (cephalosporins). * **Inborn Errors of Metabolism:** Benedict’s test is positive in **Galactosemia** (galactose) and **Essential Fructosuria** (fructose), whereas the Glucose-oxidase test will be **negative** in these conditions. * **Sucrose:** It is a non-reducing sugar and will give a **negative** Benedict’s test unless it is first hydrolyzed.

Question 15: Which molecular technique is used for mapping large DNA segments, approximately 50-100 base pairs in length?

A. Restriction Fragment Length Polymorphism (RFLP) (Correct Answer)
B. Chromosome walking
C. Polymerase Chain Reaction (PCR)
D. None of the above

Explanation: ### Explanation **1. Why RFLP is the Correct Answer:** **Restriction Fragment Length Polymorphism (RFLP)** is a technique used to detect variations in homologous DNA sequences. It relies on the use of **restriction endonucleases**, which cut DNA at specific recognition sites. If a mutation or variation exists at a recognition site, the length of the resulting fragments will change. These fragments are typically in the range of **50–100 base pairs (or larger)** and are separated via gel electrophoresis. RFLP is a classic tool for mapping genes, detecting mutations (like Sickle Cell Anemia), and forensic analysis. **2. Why Other Options are Incorrect:** * **Chromosome Walking:** This is a method used to "map" or sequence very large regions of a chromosome (often hundreds of kilobases) by using overlapping clones. It is used to find a specific gene starting from a nearby known marker, rather than analyzing small 50–100 bp fragments. * **Polymerase Chain Reaction (PCR):** While PCR can amplify DNA segments of various sizes, it is primarily an **amplification technique**, not a mapping technique. While it can be used in conjunction with RFLP (PCR-RFLP), the specific mapping of fragment length variations described in the question is the hallmark of RFLP. **3. High-Yield Clinical Pearls for NEET-PG:** * **Sickle Cell Anemia:** RFLP can diagnose Sickle Cell Anemia because the mutation ($Glu \to Val$) destroys a recognition site for the restriction enzyme **MstII**. * **VNTRs:** RFLP often utilizes Variable Number Tandem Repeats (VNTRs) as markers for DNA fingerprinting. * **Southern Blotting:** RFLP analysis typically requires Southern Blotting to visualize the specific DNA fragments after electrophoresis. * **Requirement:** Unlike PCR, traditional RFLP requires a large amount of high-quality, non-degraded DNA.

Question 16: For the measurement of long DNA molecules (50-100 KB), which of the following techniques is used?

A. Chromosome walking
B. NICH
C. RFLP (Correct Answer)
D. SSLP

Explanation: **Explanation:** **Correct Option: C (RFLP – Restriction Fragment Length Polymorphism)** RFLP is a technique used to detect variations in homologous DNA sequences. It relies on the use of **Restriction Endonucleases** (molecular scissors) that cut DNA at specific recognition sites. When long genomic DNA molecules (ranging from 50 to 100 KB or more) are digested with these enzymes, they produce fragments of varying lengths. These fragments are then separated by gel electrophoresis and visualized via Southern Blotting. RFLP is a gold-standard classical technique for mapping large genomic regions, detecting mutations, and performing linkage analysis. **Why other options are incorrect:** * **A. Chromosome Walking:** This is a method used to clone or sequence progressively overlapping genomic clones to "walk" down a chromosome to find a specific gene. It is a sequencing strategy, not a primary measurement technique for DNA length. * **B. NICH (Non-Isotopic Colony Hybridization):** This is a screening method used to identify specific bacterial colonies containing a DNA of interest using non-radioactive probes. It does not measure the size of long DNA molecules. * **D. SSLP (Simple Sequence Length Polymorphism):** These are repetitive DNA sequences (like microsatellites) used as genetic markers. While they involve length variation, they typically involve much smaller fragments (PCR-based) rather than the 50-100 KB range associated with genomic RFLP analysis. **High-Yield Facts for NEET-PG:** * **Southern Blotting:** Used for DNA (Mnemonic: **S**outhern-**D**NA, **N**orthern-**R**NA, **W**estern-**P**rotein → **SNOW DROP**). * **RFLP Applications:** Historically used in forensic "DNA fingerprinting" and prenatal diagnosis of diseases like Sickle Cell Anemia (loss of *MstII* restriction site). * **Pulsed Field Gel Electrophoresis (PFGE):** If the question mentions extremely large DNA (up to several megabases), PFGE is the specific electrophoretic technique required.

Question 17: How is supercoiled DNA separated from relaxed DNA?

A. ELISA
B. Gel electrophoresis (Correct Answer)
C. DNA footprinting
D. DNA fingerprinting

Explanation: **Explanation:** The separation of DNA molecules based on their physical properties is a fundamental technique in molecular biology. **Why Gel Electrophoresis is correct:** Gel electrophoresis (typically using agarose) separates DNA fragments based on two primary factors: **size** and **conformation (shape)**. Even if two DNA molecules have the same number of base pairs, their shape dictates how easily they move through the gel matrix. * **Supercoiled DNA** is highly compact and "wound up," allowing it to snake through the pores of the gel very quickly. * **Relaxed (circular or nicked) DNA** is bulky and floppy, encountering more resistance. Consequently, supercoiled DNA migrates **faster** and further toward the anode than relaxed DNA of the same molecular weight. **Why other options are incorrect:** * **ELISA:** A serological technique used to detect and quantify proteins, antibodies, or hormones; it is not used for nucleic acid separation. * **DNA Footprinting:** A method used to identify the specific site where a protein (like a transcription factor) binds to a DNA sequence. * **DNA Fingerprinting:** A technique used for forensic identification or paternity testing by analyzing Variable Number Tandem Repeats (VNTRs) or Short Tandem Repeats (STRs). **High-Yield Clinical Pearls for NEET-PG:** * **Ethidium Bromide (EtBr):** The most common intercalating agent used to visualize DNA in gels; it fluoresces under UV light. * **Topoisomerases:** These are the enzymes responsible for converting supercoiled DNA into relaxed DNA (and vice versa) in vivo. Fluoroquinolones (e.g., Ciprofloxacin) act by inhibiting DNA Gyrase (Topoisomerase II). * **Migration Rule:** In standard electrophoresis, the rate of migration is: **Supercoiled > Linear > Relaxed Circular.**

Question 18: What is the specific test for ketohexoses?

A. Selivanoff's test (Correct Answer)
B. Osazone test
C. Molisch test
D. None of these

Explanation: **Explanation:** **1. Why Selivanoff’s Test is Correct:** Selivanoff’s test is a colorimetric reaction used specifically to distinguish **ketohexoses** (like fructose) from aldohexoses (like glucose). The principle relies on the fact that when heated with concentrated Hydrochloric Acid (HCl), ketoses undergo dehydration more rapidly than aldoses to form **5-hydroxymethylfurfural**. This intermediate then reacts with **resorcinol** to produce a characteristic **cherry-red (fiery red) complex**. While aldoses may eventually react, they do so much more slowly and produce a faint pink color. **2. Why Other Options are Incorrect:** * **Osazone Test:** This is used for the general identification of sugars based on the shape and melting point of crystals formed with phenylhydrazine. It cannot distinguish between glucose, fructose, and mannose because they all form the same needle-shaped glucosazone crystals (as they differ only at C1 and C2). * **Molisch Test:** This is a **general screening test for all carbohydrates**. It uses $\alpha$-naphthol and sulfuric acid to produce a purple/violet ring. It does not differentiate between types of sugars (aldose vs. ketose). **3. High-Yield Clinical Pearls for NEET-PG:** * **Fructose Metabolism:** Fructose enters glycolysis via Fructose-1-phosphate (in the liver) or Fructose-6-phosphate (in muscles). * **Essential Fructosuria:** A deficiency of **fructokinase**; it is a benign condition where fructose is found in the urine (positive Selivanoff’s and Benedict’s tests). * **Hereditary Fructose Intolerance (HFI):** A deficiency of **Aldolase B**, leading to intracellular trapping of Fructose-1-P, causing severe hypoglycemia and liver damage. * **Bial’s Test:** Used specifically for **Pentoses** (e.g., ribose), yielding a blue-green color.

Question 19: SDS-PAGE separates proteins based on which property?

A. Charge
B. Solubility
C. Size (Correct Answer)
D. Polarity

Explanation: **Explanation:** **SDS-PAGE (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis)** is a technique used to separate proteins primarily based on their **molecular weight (size)**. 1. **Why "Size" is correct:** Proteins have different intrinsic charges and 3D shapes. SDS is an anionic detergent that denatures proteins and coats them with a uniform negative charge. This masks the protein's native charge, giving all proteins a similar **charge-to-mass ratio**. When an electric field is applied, the proteins migrate toward the anode. The polyacrylamide gel acts as a molecular sieve; smaller proteins move faster through the pores, while larger proteins are retarded. Thus, separation is strictly a function of size. 2. **Why other options are incorrect:** * **Charge:** Native-PAGE or Ion-Exchange Chromatography separates proteins based on charge. In SDS-PAGE, charge is neutralized by SDS. * **Solubility:** This is the principle behind "Salting out" (using Ammonium Sulfate) to precipitate proteins. * **Polarity:** This is the basis for Reverse-Phase Chromatography or Partition Chromatography. **High-Yield Clinical Pearls for NEET-PG:** * **Beta-mercaptoethanol:** Often added to SDS-PAGE to break disulfide bonds, ensuring complete denaturation into individual polypeptide subunits. * **Isoelectric Focusing (IEF):** Separates proteins based on their **pI (Isoelectric point)**. * **2D-Electrophoresis:** Combines IEF (1st dimension) and SDS-PAGE (2nd dimension) to separate proteins by both **charge and size**. * **Western Blot:** Uses SDS-PAGE as the initial step before transferring proteins to a membrane for antibody detection.

Question 20: Which anticoagulant used in blood glucose estimation prevents glycolysis?

A. EDTA
B. Heparin
C. Sodium fluoride (Correct Answer)
D. Sodium citrate

Explanation: **Explanation:** The correct answer is **Sodium fluoride**. In blood glucose estimation, it is crucial to prevent the consumption of glucose by red blood cells (glycolysis) after the sample is drawn. **Mechanism of Action:** Sodium fluoride acts as a **glycolytic inhibitor**. It specifically inhibits the enzyme **Enolase** in the glycolytic pathway by forming a complex with magnesium and phosphate. Since Enolase is required to convert 2-phosphoglycerate to phosphoenolpyruvate, its inhibition halts the breakdown of glucose. Sodium fluoride is typically used in combination with **Potassium oxalate** (which acts as the anticoagulant by chelating calcium). **Analysis of Incorrect Options:** * **EDTA (Ethylenediaminetetraacetic acid):** Primarily used for Hematology (CBC) and HbA1c. It chelates calcium to prevent clotting but does not inhibit glycolytic enzymes. * **Heparin:** An indirect thrombin inhibitor used for arterial blood gases (ABG) and electrolyte analysis. It does not prevent glucose degradation. * **Sodium citrate:** Used for coagulation studies (PT/aPTT) and ESR (Westergren method). It chelates calcium but has no effect on glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Gray-top vacutainer:** Contains Sodium fluoride and Potassium oxalate; used specifically for glucose and lactate estimation. * **Rate of Glycolysis:** At room temperature, glucose levels in a blood sample decrease by approximately **5–7% per hour** if a preservative is not used. * **HbA1c Exception:** For Glycated Hemoglobin (HbA1c) estimation, **EDTA** is the preferred anticoagulant, not fluoride, as the test measures glucose attached to hemoglobin over time rather than free plasma glucose.

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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