Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 111: Restriction fragment length polymorphism is used for what purpose?

A. Analysis of chromosome structures (Correct Answer)
B. DNA estimation
C. Synthesis of nucleic acids
D. Detecting proteins in a cell

Explanation: **Explanation:** **Restriction Fragment Length Polymorphism (RFLP)** is a molecular biology technique used to detect variations in homologous DNA sequences. It relies on the principle that specific enzymes (Restriction Endonucleases) cut DNA at specific recognition sites. If a mutation or variation exists at these sites, the resulting DNA fragments will differ in length. **Why Option A is Correct:** RFLP is used for the **analysis of chromosome structures** because it identifies variations in the physical structure and sequence of DNA. By comparing fragment patterns, clinicians can map genes, identify genetic polymorphisms, and detect structural changes or mutations associated with hereditary diseases. It serves as a "genetic fingerprint" for specific chromosomal regions. **Why Other Options are Incorrect:** * **B. DNA estimation:** This is typically done using **Spectrophotometry** (measuring absorbance at 260 nm) or Fluorometry, not RFLP. * **C. Synthesis of nucleic acids:** This refers to processes like **PCR (Polymerase Chain Reaction)** or automated DNA synthesis, whereas RFLP is an analytical/diagnostic tool. * **D. Detecting proteins in a cell:** Protein detection is achieved via **Western Blotting** or ELISA. RFLP is strictly a DNA-based technique. **NEET-PG High-Yield Pearls:** * **Mechanism:** Uses Restriction Endonucleases (e.g., EcoRI) followed by Gel Electrophoresis and Southern Blotting. * **Clinical Applications:** Paternity testing, forensic "DNA fingerprinting," and prenatal diagnosis of disorders like **Sickle Cell Anemia** (where a mutation abolishes a restriction site). * **Key Limitation:** It requires a large amount of high-quality DNA compared to PCR-based methods. * **Related Technique:** **VNTRs** (Variable Number Tandem Repeats) are the molecular basis for the polymorphisms observed in RFLP.

Question 112: What is a molecule used to detect the presence of a specific fragment of DNA or RNA?

A. Primosome
B. Probe (Correct Answer)
C. Pseudogene
D. Signal

Explanation: **Explanation:** **Correct Answer: B. Probe** A **probe** is a single-stranded sequence of DNA or RNA (typically 10–1000 nucleotides long) that is complementary to a specific target nucleic acid sequence. It is labeled with a radioisotope (e.g., $^{32}P$) or a fluorescent marker. Through the process of **hybridization**, the probe binds to its complementary sequence, allowing for the detection, identification, and quantification of specific genes or fragments in a sample. **Analysis of Incorrect Options:** * **A. Primosome:** This is a protein complex responsible for creating RNA primers during DNA replication. It consists of DNA primase and DNA helicase. It is a functional unit of replication, not a detection tool. * **C. Pseudogene:** These are genomic DNA sequences that are similar to functional genes but have lost their protein-coding ability due to accumulated mutations (e.g., premature stop codons). They are "genomic fossils." * **D. Signal:** In biochemical assays, a "signal" is the output (light, color, or radioactivity) generated after a probe has bound to its target, but the molecule itself used for detection is the probe. **High-Yield Clinical Pearls for NEET-PG:** * **Southern Blotting:** Uses a DNA probe to detect specific **DNA** sequences. * **Northern Blotting:** Uses a DNA/RNA probe to detect specific **RNA** sequences (measures gene expression). * **FISH (Fluorescence In Situ Hybridization):** Uses fluorescent probes to detect chromosomal abnormalities (e.g., trisomies, translocations like BCR-ABL) directly in tissues or cells. * **Stringency:** The conditions (temperature and salt concentration) under which hybridization occurs. High stringency ensures the probe binds only to a perfectly matched target.

Question 113: Which test is used for cholesterol?

A. Salkowski test (Correct Answer)
B. Schilling test
C. Zellwenger’s test
D. Brodie’s test

Explanation: **Explanation:** **Correct Option: A. Salkowski test** The Salkowski test is a classic colorimetric reaction used to detect the presence of cholesterol. In this test, cholesterol is dissolved in chloroform and treated with concentrated sulfuric acid ($H_2SO_4$). The acid acts as a dehydrating agent, leading to the formation of **bicholestadiene**. This results in a characteristic color change: the upper chloroform layer turns **red**, while the lower acid layer exhibits a **yellow fluorescence**. Another common test for cholesterol is the **Libermann-Burchard test**, which yields a green color. **Analysis of Incorrect Options:** * **B. Schilling test:** This is a nuclear medicine test used to determine the cause of **Vitamin B12 (cobalamin) deficiency** by evaluating its absorption. It helps differentiate between Pernicious Anemia (intrinsic factor deficiency) and malabsorption syndromes. * **C. Zellweger’s test:** This is not a standard biochemical test. However, **Zellweger Syndrome** is a high-yield clinical condition involving a defect in **peroxisome biogenesis**, leading to the accumulation of very-long-chain fatty acids (VLCFAs). * **D. Brodie’s test:** This is a clinical physical examination maneuver used in surgery to assess **venous valvular incompetency** in patients with varicose veins (Trendelenburg-Brodie test). **High-Yield Clinical Pearls for NEET-PG:** * **Cholesterol Precursor:** All 27 carbon atoms of cholesterol are derived from **Acetyl-CoA**. * **Rate-limiting enzyme:** HMG-CoA Reductase (inhibited by Statins). * **Libermann-Burchard Reaction:** Most sensitive test for cholesterol; uses acetic anhydride and $H_2SO_4$ to produce a **deep green** color. * **Zak’s Method:** A common laboratory method for the quantitative estimation of total serum cholesterol.

Question 114: Protein fragments separation is performed by which method?

A. Ultrafiltration
B. Chromatography
C. Centrifugation
D. All of the above (Correct Answer)

Explanation: **Explanation:** Protein separation and purification are fundamental processes in biochemistry, utilizing various physical and chemical properties of protein fragments such as size, charge, solubility, and binding affinity. 1. **Ultrafiltration:** This method separates protein fragments based on **molecular size and shape**. By using semi-permeable membranes with specific pore sizes (molecular weight cut-offs), smaller fragments pass through while larger proteins are retained. It is commonly used for concentrating protein samples. 2. **Chromatography:** This is the most versatile technique for separation. It includes **Size Exclusion Chromatography** (separates by size/molecular weight), **Ion-Exchange Chromatography** (separates by net charge), and **Affinity Chromatography** (separates based on specific biological interactions). 3. **Centrifugation:** This technique utilizes **sedimentation velocity** and density. Differential centrifugation or density gradient ultracentrifugation (using sucrose or CsCl) allows for the separation of protein complexes and fragments based on their Svedberg units (S). Since all three methods are standard laboratory techniques used to isolate or fractionate proteins, **Option D** is the correct answer. **High-Yield NEET-PG Pearls:** * **Salting Out:** A common initial step in protein purification using **Ammonium Sulfate** to decrease protein solubility. * **SDS-PAGE:** Separates proteins strictly by **molecular weight** by denaturing them and providing a uniform negative charge. * **Isoelectric Focusing (IEF):** Separates proteins based on their **pI (isoelectric point)**, where their net charge is zero. * **Dialysis:** Uses a semi-permeable membrane primarily to remove salts and small molecules from a protein solution, rather than separating protein fragments from each other.

Question 115: Which of the following procedures is used to separate and detect DNA fragments?

A. Southern blotting (Correct Answer)
B. Northern blotting
C. Western blotting
D. DNA electrophoresis

Explanation: **Explanation:** The correct answer is **Southern blotting**. This technique is the gold standard for identifying specific DNA sequences within a complex mixture. **1. Why Southern Blotting is Correct:** Southern blotting (named after Edwin Southern) involves a multi-step process: DNA digestion by restriction endonucleases, separation by size via gel electrophoresis, transfer (blotting) to a nitrocellulose membrane, and finally, **hybridization with a labeled DNA probe**. This allows for both the separation and the specific detection of target DNA fragments. **2. Analysis of Incorrect Options:** * **Northern blotting:** Used for the detection and separation of **RNA** molecules. It is primarily used to study gene expression (mRNA levels). * **Western blotting:** Used for the detection of specific **proteins** using antibodies. It is the confirmatory test for HIV (detecting p24 or gp120/160). * **DNA electrophoresis:** While this technique **separates** DNA fragments based on size and charge, it does not inherently **detect** a specific sequence unless followed by a blotting or staining procedure (like Ethidium Bromide). Southern blotting is the complete procedure for both separation and specific identification. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O**oo = **O**oo (Placeholder) * **W**estern = **P**rotein * **Southwestern Blotting:** A hybrid technique used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Clinical Application:** Southern blotting is used in DNA fingerprinting, detecting gene mutations (e.g., sickle cell anemia), and identifying viral integration into host DNA.

Question 116: What is the unit of mass equal to one twelfth the mass of an atom of carbon-12?

A. Mole
B. Dalton (Correct Answer)
C. Equivalent
D. None of the above

Explanation: ### Explanation **Correct Answer: B. Dalton** The **Dalton (Da)**, also known as the **unified atomic mass unit (u)**, is defined as exactly **1/12th the mass of a single carbon-12 atom**. In biochemistry and molecular biology, it is the standard unit used to express the molecular weight of proteins, nucleic acids, and other macromolecules. * One Dalton is approximately equal to the mass of one nucleon (a proton or a neutron), which is $1.66 \times 10^{-24}$ grams. * In medical biochemistry, large molecules are often expressed in **kilodaltons (kDa)**. For example, Albumin is approximately 66 kDa. **Why other options are incorrect:** * **A. Mole:** This is the SI unit for the **amount of substance**. One mole contains Avogadro’s number ($6.022 \times 10^{23}$) of entities (atoms or molecules). While related to mass, it is a measure of quantity, not a unit of individual atomic mass. * **C. Equivalent:** This unit measures the **chemical combining power** of a substance. It is calculated by dividing the molar mass by the valence (n-factor). It is commonly used in clinical settings to express electrolyte concentrations (e.g., mEq/L). **High-Yield Clinical Pearls for NEET-PG:** * **Molecular Weight vs. Mass:** While "molecular weight" is technically dimensionless, in clinical practice, it is synonymous with molecular mass expressed in Daltons. * **Glomerular Filtration Barrier:** The kidney's basement membrane acts as a size-selective filter. Molecules larger than **60–70 kDa** (like Albumin) are generally not filtered, which is why their presence in urine (Albuminuria) indicates glomerular damage. * **SDS-PAGE:** This common biochemical technique separates proteins primarily based on their molecular mass in **Daltons**.

Question 117: Optical transmission through a solution depends on what factor?

A. Time
B. Concentration (Correct Answer)
C. Scale
D. Path length

Explanation: **Explanation:** The optical transmission of light through a solution is governed by the **Beer-Lambert Law**, which is the fundamental principle behind colorimetry and spectrophotometry used in clinical biochemistry. 1. **Why Concentration is Correct:** According to **Beer’s Law**, the amount of light absorbed by a solution is directly proportional to the **concentration** of the solute. As concentration increases, more molecules are available to interact with and absorb the incident light, thereby decreasing the optical transmission. In the laboratory, we measure absorbance ($A$) to calculate the unknown concentration ($C$) of substances like blood glucose, urea, or creatinine using the formula: $A = \varepsilon cl$ (where $\varepsilon$ is the molar absorptivity, $c$ is concentration, and $l$ is path length). 2. **Why Other Options are Incorrect:** * **Path length:** While transmission also depends on path length (**Lambert’s Law**), in standard clinical laboratory practice, the path length (cuvette width) is kept **constant** (usually 1 cm). Therefore, the variable factor determining transmission in a diagnostic sample is the concentration. * **Time:** Optical transmission is an instantaneous physical property and does not depend on time, unless a kinetic enzyme assay is being performed where concentration changes over time. * **Scale:** This is a distractor and has no physical relevance to the interaction between light and matter. **High-Yield Clinical Pearls for NEET-PG:** * **Beer-Lambert Law:** $A = \log_{10}(1/T)$, where $T$ is Transmittance. Absorbance is inversely and logarithmically related to transmittance. * **Wavelength Selection:** The wavelength used ($\lambda_{max}$) is where the substance shows maximum absorbance to ensure highest sensitivity. * **Blanking:** A "reagent blank" is used to subtract the absorbance of the reagents themselves, ensuring the final reading reflects only the analyte concentration.

Question 118: Which of the following methods cannot be used to detect gene expression?

A. RT-PCR
B. cDNA microarray
C. Northern blot
D. Southern blot and Immunohistochemistry (Correct Answer)

Explanation: **Explanation:** **Gene expression** refers to the process by which information from a gene is used to synthesize functional gene products—primarily **mRNA** (transcription) and **proteins** (translation). To detect gene expression, a technique must identify either the specific mRNA or the protein product. **Why Option D is Correct:** * **Southern Blot:** This technique is used to detect specific **DNA** sequences. Since DNA is present in almost every cell regardless of whether a gene is "turned on" or "off," Southern blotting identifies the presence or structure of a gene (e.g., deletions, insertions), but **not** its expression. * **Immunohistochemistry (IHC):** While IHC *does* detect protein expression in tissues, the combination in Option D makes it the best choice because Southern Blotting is strictly for DNA analysis, not expression. (Note: In many competitive exams, if a pair contains one definitely incorrect technique for the context, the entire option is considered the answer). **Why Other Options are Incorrect:** * **RT-PCR (Reverse Transcriptase PCR):** Converts mRNA into cDNA to quantify gene expression levels. It is the gold standard for detecting low-abundance mRNA. * **cDNA Microarray:** Allows for the simultaneous analysis of the expression of thousands of genes by measuring mRNA levels. * **Northern Blot:** Specifically used to detect and quantify **RNA** (mRNA), making it a classic method for studying gene expression. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** (ignore) = **O** (ignore) * **W**estern = **P**rotein * **Southwestern Blot:** Used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **ELISA & Western Blot:** Both detect protein expression; Western blot is more specific (confirmatory test for HIV).

Question 119: What is the anticoagulant of choice in electrolyte estimation?

A. Lithium heparin (Correct Answer)
B. Trisodium citrate
C. EDTA
D. All of the above

Explanation: **Explanation:** The estimation of electrolytes (Sodium, Potassium, Chloride) requires a sample that closely reflects the physiological state of the blood. **Lithium heparin** is the anticoagulant of choice for this purpose. **Why Lithium Heparin is Correct:** Heparin works by activating antithrombin III, which neutralizes thrombin and prevents fibrin formation. It is preferred for electrolyte analysis because it does not significantly alter the concentration of the ions being measured. Lithium heparin is specifically used because lithium is not routinely measured in a standard electrolyte panel; therefore, any minute amount added by the anticoagulant does not interfere with the results of Na⁺ or K⁺. **Why Other Options are Incorrect:** * **Trisodium Citrate:** This anticoagulant works by chelating calcium. It contains a high concentration of sodium, which would falsely and significantly elevate the measured sodium levels. * **EDTA (Ethylenediaminetetraacetic acid):** EDTA is a potent chelator of divalent cations (Calcium and Magnesium). Most importantly, the most common form used is **K₂EDTA** or **K₃EDTA**, which would cause a massive, clinically impossible elevation in **Potassium (K⁺)** levels and a false decrease in Calcium. * **Sodium Fluoride (Gray top):** Though not an option here, it is worth noting it contains sodium and would also interfere with electrolyte estimation. **High-Yield Clinical Pearls for NEET-PG:** * **The "Order of Draw":** To prevent cross-contamination of additives (like EDTA into a heparin tube), a specific sequence must be followed during blood collection. * **Pseudohyperkalemia:** Using EDTA instead of Heparin for electrolytes is a common cause of "spurious" or false hyperkalemia. * **Heparin Types:** While Lithium Heparin is for electrolytes, **Sodium Heparin** should be avoided for electrolyte panels but can be used for other specialized tests. * **Gold Standard:** For the most accurate results, **Serum** (plain red/yellow top) is often preferred over plasma, but if plasma is required for a faster turnaround time, Lithium Heparin is the standard.

Question 120: Which color in Benedict's test indicates that no reducing sugar is present?

A. Blue (Correct Answer)
B. Green
C. Orange
D. Brick red

Explanation: **Explanation:** **Benedict’s test** is a semi-quantitative chemical assay used to detect the presence of **reducing sugars** (such as glucose, fructose, galactose, lactose, and maltose). The reagent contains cupric ions ($Cu^{2+}$) in the form of copper sulfate, which gives the solution its characteristic **deep blue color**. 1. **Why Blue is Correct:** When the reagent is heated with a sample, reducing sugars (which have a free aldehyde or ketone group) reduce the blue cupric ions ($Cu^{2+}$) to insoluble red cuprous oxide ($Cu_2O$). If **no reducing sugar** is present, no reduction occurs, the copper remains in its cupric state, and the solution stays **Blue**. This represents a negative result. 2. **Why Other Options are Incorrect:** The color change in Benedict's test follows a gradient based on the concentration of reducing sugar: * **Green (Option B):** Indicates a trace amount of reducing sugar (approx. 0.5–1.0 g%). * **Orange (Option C):** Indicates a moderate amount of reducing sugar (approx. 1.5–2.0 g%). * **Brick Red (Option D):** Indicates a high concentration of reducing sugar (>2.0 g%). **High-Yield Clinical Pearls for NEET-PG:** * **Non-reducing sugars:** **Sucrose** is the most common non-reducing sugar and will give a negative (Blue) Benedict’s test because its glycosidic bond involves both anomeric carbons, leaving no free aldehyde or ketone group. * **Clinical Use:** Historically used to detect glucose in urine (**Glucosuria**), a hallmark of Diabetes Mellitus. * **False Positives:** Can occur with high levels of ascorbic acid (Vitamin C), urates, or certain drugs like salicylates and cephalosporins. * **Benedict’s vs. Fehling’s:** Benedict’s reagent is more stable and more sensitive than Fehling’s solution.

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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