Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 181: What is the study of the multiplication of proteins in a disease process called?

A. Proteomics (Correct Answer)
B. Genomics
C. Glycomics
D. Nucleomics

Explanation: ### Explanation **1. Why Proteomics is Correct:** **Proteomics** is the large-scale study of the entire set of proteins (the proteome) expressed by a genome, cell, tissue, or organism. In a disease process, protein expression patterns change—certain proteins may be overexpressed (multiplied), modified, or downregulated. Studying these changes helps in identifying biomarkers for diagnosis and understanding the pathophysiology of diseases like cancer or neurodegenerative disorders. **2. Analysis of Incorrect Options:** * **Genomics (Option B):** This is the study of the complete set of DNA (genes) within an organism. While it provides the blueprint, it does not account for post-translational modifications or the actual functional state of the cell during disease. * **Glycomics (Option C):** This focuses on the study of **glycans** (sugars/carbohydrates) and their structures and functions, particularly how they attach to proteins and lipids. * **Nucleomics (Option D):** This is a less common term referring to the study of the **nucleome**—the complete set of components in the cell nucleus, including its 3D architecture and regulatory elements. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Central Dogma Flow:** Genomics (DNA) → Transcriptomics (mRNA) → Proteomics (Proteins) → Metabolomics (Metabolites). * **Techniques:** The "Gold Standard" for proteomics is **Mass Spectrometry (MS)**, often preceded by **2D-Gel Electrophoresis** to separate proteins. * **Dynamic Nature:** Unlike the genome, which is relatively constant, the **proteome is highly dynamic** and changes constantly in response to internal and external stimuli (stress, drugs, or disease). * **Clinical Application:** PSA (Prostate-Specific Antigen) and Troponin are classic examples of protein biomarkers studied via proteomic principles for clinical diagnosis.

Question 182: Which of the following is NOT a method used for fusing two cells in genetic recombination techniques?

A. Fusion mediated by Ethylene Glycol
B. Fusion mediated by electric current
C. Fusion mediated by viral transformation
D. Fusion mediated by altering membrane viscosity (Correct Answer)

Explanation: **Explanation:** Cell fusion (protoplast fusion) is a critical technique in genetic recombination and hybridoma technology. To achieve fusion, the electrostatic repulsion between cell membranes must be overcome to allow lipid bilayer merging. **Why Option D is Correct:** While **altering membrane viscosity** (fluidity) is a physiological consequence of temperature changes or chemical treatments, it is **not a standalone method** used to induce the fusion of two distinct cells. Membrane fluidity is a prerequisite for fusion, but simply changing the viscosity does not provide the necessary force or "bridge" required to merge two separate cell membranes into a single hybrid cell. **Analysis of Incorrect Options:** * **A. Ethylene Glycol (PEG):** Polyethylene Glycol (PEG) is the most common **chemical fusogen**. It acts as a dehydrating agent, bringing membranes into close proximity and inducing the formation of "protein-free" patches where lipid bilayers can merge. * **B. Electric Current (Electrofusion):** This physical method uses short, high-voltage electrical pulses to create temporary pores in the cell membrane (**electroporation**). When two cells are in contact, these pores allow the cytoplasm to mix, leading to fusion. * **C. Viral Transformation (Sendai Virus):** Certain viruses, most notably the **Sendai virus** (inactivated), possess fusion proteins on their envelope that naturally facilitate the merging of host cell membranes. This was the primary method used in early somatic cell genetics. **NEET-PG High-Yield Pearls:** * **Hybridoma Technology:** Cell fusion is the foundational step in creating hybridomas (B-cell + Myeloma cell) for **Monoclonal Antibody (mAb)** production. * **Fusogens:** PEG is the gold standard chemical fusogen; Sendai virus is the classic biological fusogen. * **Applications:** These techniques are essential for gene mapping, producing "monoclones," and studying nucleo-cytoplasmic interactions.

Question 183: Fluoride is used in which type of samples for biochemical estimations?

A. Blood glucose estimation samples (Correct Answer)
B. Urine glucose estimation samples
C. Both blood and urine glucose estimation samples
D. None of the above

Explanation: **Explanation:** **1. Why Option A is Correct:** Sodium fluoride (NaF) is the preferred additive for **blood glucose estimation** because it acts as a potent **antiglycolytic agent**. In a blood sample, RBCs and WBCs continue to metabolize glucose via the glycolytic pathway even after collection, leading to a decrease in glucose levels by approximately 5–7% per hour. Fluoride ions inhibit the enzyme **Enolase** (which converts 2-phosphoglycerate to phosphoenolpyruvate) by forming a complex with magnesium and phosphate. This "locks" the glucose concentration, ensuring an accurate measurement of the patient's glycemic status at the time of draw. It is typically used in combination with Potassium Oxalate (anticoagulant) in the **Grey-top Vacutainer**. **2. Why Other Options are Incorrect:** * **Option B & C:** Fluoride is not used for **urine glucose estimation**. In urine, glucose is typically detected using semi-quantitative methods like Benedict’s test or glucose oxidase strips. The primary concern in urine samples is bacterial growth, which is managed by preservatives like Thymol or Toluene, rather than antiglycolytic agents. * **Option D:** This is incorrect as the application of fluoride in blood chemistry is a standard laboratory practice. **3. NEET-PG High-Yield Pearls:** * **The "Enolase" Connection:** Always remember that Fluoride inhibits Enolase. This is a frequent "match the following" or "assertion-reason" question. * **Urease Inhibition:** Fluoride also inhibits the enzyme **Urease**. Therefore, fluoride-containing samples **cannot** be used for urea estimation if the laboratory uses the urease method. * **Timing:** While fluoride inhibits glycolysis, its effect is not instantaneous (it takes 1-2 hours to fully inhibit the enzyme). For immediate results, the sample should be centrifuged and plasma separated promptly.

Question 184: Movement of proteins from the nucleus to the cytoplasm can be observed using which technique?

A. Fluorescence in situ hybridization (FISH)
B. Fluorescence Recovery After Photobleaching (FRAP) (Correct Answer)
C. Confocal microscopy
D. Electron microscopy

Explanation: ### Explanation **Correct Option: B. Fluorescence Recovery After Photobleaching (FRAP)** **Why it is correct:** FRAP is a powerful technique used to study the **mobility and dynamics** of molecules (proteins or lipids) within living cells. The process involves: 1. Tagging the target protein with a fluorescent marker (e.g., GFP). 2. Using a high-intensity laser to "bleach" (extinguish fluorescence) in a specific area, such as the nucleus. 3. Monitoring the **rate of fluorescence recovery** in that bleached area. If fluorescence returns to the nucleus, it indicates that unbleached proteins have moved from the cytoplasm into the nucleus (or vice versa). Thus, it is the gold standard for measuring **nucleocytoplasmic shuttling** and lateral diffusion in membranes. **Why incorrect options are wrong:** * **A. FISH:** Used to detect and localize specific **DNA sequences** on chromosomes or specific RNA targets. It is a static technique used for genetic mapping and diagnosing numerical/structural chromosomal aberrations, not for observing real-time protein movement. * **C. Confocal Microscopy:** While this provides high-resolution 3D images of cells, it is a visualization tool. On its own, it doesn't measure the *kinetic movement* or diffusion rates as specifically as FRAP does. * **D. Electron Microscopy:** Provides ultra-high resolution of cellular ultrastructure but typically requires fixed (dead) samples, making it impossible to observe the dynamic movement of proteins in a living state. **High-Yield Facts for NEET-PG:** * **FRAP** = Measures **Diffusion/Mobility** (think "Recovery = Movement"). * **FRET (Fluorescence Resonance Energy Transfer)** = Measures **Protein-Protein Interaction** (the "Molecular Ruler"). * **FISH** = Clinical gold standard for **Prader-Willi/Angelman syndromes** and **HER2/neu** amplification in breast cancer. * **Flow Cytometry** = Used for cell counting, sorting, and identifying surface markers (e.g., CD4 counts in HIV).

Question 185: Which enzyme is used in Polymerase Chain Reaction (PCR)?

A. Reverse transcriptase
B. Taq polymerase (Correct Answer)
C. RNA polymerase
D. None of the above

Explanation: **Explanation:** **Taq polymerase** is the correct answer because it is a **thermostable DNA polymerase** derived from the bacterium *Thermus aquaticus*. The PCR process involves repeated cycles of high temperatures (94–96°C) for DNA denaturation. Unlike human DNA polymerase, which would denature and lose function at these temperatures, Taq polymerase remains stable and active, allowing it to synthesize new DNA strands by adding deoxynucleotides (dNTPs) to a primer. **Analysis of Incorrect Options:** * **Reverse transcriptase:** This enzyme synthesizes DNA from an RNA template. While used in **RT-PCR** (to study RNA viruses like HIV or SARS-CoV-2), it is not the standard enzyme for the classical PCR process which amplifies DNA. * **RNA polymerase:** This enzyme synthesizes RNA from a DNA template during transcription. It is not involved in DNA amplification or the PCR cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** 1. Denaturation (~95°C), 2. Annealing (~55°C), 3. Extension (~72°C). * **Components required:** Template DNA, Primers (forward and reverse), dNTPs, Taq polymerase, and $Mg^{2+}$ (cofactor). * **Applications:** Diagnosis of genetic mutations (e.g., Sickle cell anemia), detection of infectious agents (TB, Hepatitis), and forensic medicine (DNA fingerprinting). * **Pfu Polymerase:** Another thermostable enzyme used when higher "proofreading" accuracy is required, as Taq lacks 3' to 5' exonuclease activity.

Question 186: What is the most accurate technique for quantifying gene expression?

A. Northern blot
B. PCR
C. Real-Time Reverse Transcriptase PCR (Correct Answer)
D. Reverse Transcriptase PCR

Explanation: **Explanation:** **Why Real-Time Reverse Transcriptase PCR (qRT-PCR) is correct:** Gene expression is measured by the amount of mRNA produced by a specific gene. To quantify this, mRNA must first be converted into complementary DNA (cDNA) using the enzyme **Reverse Transcriptase**. While standard PCR only allows for "end-point" detection, **Real-Time PCR (qPCR)** monitors the amplification process as it occurs using fluorescent dyes or probes. This allows for the precise quantification of the initial amount of template RNA. Therefore, qRT-PCR is the "gold standard" for sensitive and accurate quantification of gene expression. **Analysis of Incorrect Options:** * **Northern Blot:** This is a classical technique used to detect specific RNA sequences. However, it is labor-intensive, requires large amounts of RNA, and is significantly less sensitive and less quantitative than PCR-based methods. * **PCR (Polymerase Chain Reaction):** Standard PCR amplifies DNA, not RNA. Without the reverse transcription step, it cannot measure gene expression. * **Reverse Transcriptase PCR (RT-PCR):** While this converts RNA to DNA for amplification, it is typically a **semi-quantitative** or qualitative "end-point" analysis. It tells you if a gene is expressed but does not provide the high-level accuracy of "Real-Time" monitoring for quantification. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for COVID-19:** qRT-PCR is the definitive diagnostic test for SARS-CoV-2, detecting viral RNA. * **Ct Value (Cycle Threshold):** In Real-Time PCR, a lower Ct value indicates a higher initial viral load or higher gene expression. * **Southern vs. Northern vs. Western:** Remember the mnemonic **SNOW DROP** (Southern-DNA, Northern-RNA, Western-Protein).

Question 187: What color does a positive Fouchet's test yield?

A. Red
B. Green (Correct Answer)
C. Violet
D. Yellow

Explanation: **Explanation:** **Fouchet’s test** is a qualitative biochemical method used to detect the presence of **bilirubin** in urine. This is a crucial diagnostic tool for identifying obstructive jaundice or hepatocellular jaundice, where conjugated bilirubin is excreted in the urine (bilirubinuria). 1. **Why Green is Correct:** The test relies on the oxidation of bilirubin. When Fouchet’s reagent (which contains trichloroacetic acid and ferric chloride) is added to urine pre-treated with barium chloride, the ferric chloride acts as an oxidizing agent. It oxidizes the yellow-colored **bilirubin** into **biliverdin**, which is **green** in color. A positive result is indicated by the appearance of a green or bluish-green precipitate. 2. **Analysis of Incorrect Options:** * **Red:** This is characteristic of a positive **Rothera’s test** (detecting ketones) or a positive **Benzidine test** (detecting blood/hemoglobin). * **Violet:** A violet or purple ring is seen in the **Hopkins-Cole test** (for tryptophan) or the **Biuret test** (for proteins). * **Yellow:** This is the baseline color of normal urine and the color of bilirubin itself before oxidation. **High-Yield Clinical Pearls for NEET-PG:** * **Principle:** Oxidation of bilirubin to biliverdin. * **Reagent Components:** Trichloroacetic acid (precipitates proteins) and Ferric chloride (oxidizing agent). * **Clinical Significance:** Bilirubin appears in urine only when it is **conjugated** (water-soluble). Therefore, Fouchet’s test is positive in obstructive and hepatic jaundice but **negative in hemolytic jaundice** (where unconjugated bilirubin predominates). * **Pre-step:** Barium chloride is added first to concentrate the bilirubin by adsorbing it onto the barium sulfate precipitate.

Question 188: Xanthoproteic reaction is due to the formation of?

A. Trinitrophenol (Correct Answer)
B. Mononitrophenol
C. Nitric acid
D. Pyrocathechol

Explanation: **Explanation:** The **Xanthoproteic reaction** is a qualitative test used to detect the presence of proteins containing **aromatic amino acids** (specifically Tyrosine and Tryptophan; Phenylalanine gives a weak or negative result). **Why Trinitrophenol is correct:** When concentrated Nitric acid ($HNO_3$) is added to a protein solution, the aromatic rings of the amino acids undergo a **nitration reaction**. For example, Tyrosine reacts with nitric acid to form nitro-derivatives. Upon heating and subsequent addition of an alkali (like NaOH or $NH_4OH$), the yellow color intensifies to orange due to the formation of nitrated salts. **Trinitrophenol** (also known as Picric acid) is a representative polynitrated aromatic compound formed during this process, giving the characteristic yellow precipitate. **Analysis of Incorrect Options:** * **B. Mononitrophenol:** While nitration begins with a single nitro group, the reaction typically proceeds to higher degrees of nitration (polynitration) under the test conditions to produce the characteristic color. * **C. Nitric acid:** This is the **reagent** used to perform the test, not the product formed. * **D. Pyrocatechol:** This is a dihydroxybenzene derivative and is not a product of the nitration of aromatic amino acids in this reaction. **High-Yield Clinical Pearls for NEET-PG:** * **The "Yellow Skin" Sign:** If concentrated nitric acid accidentally spills on the skin, it turns yellow. This is a real-life Xanthoproteic reaction occurring with the proteins (keratin) in the skin. * **Amino Acids involved:** Tyrosine and Tryptophan are the primary responders. Phenylalanine is highly stable and usually does not react easily under standard test conditions. * **Key Reagent:** Concentrated $HNO_3$. * **Color Change:** Yellow (with acid) $\rightarrow$ Orange (with alkali).

Question 189: The conversion of an optically pure isomer into a mixture of equal amounts of both dextro and levo forms is called as?

A. Polymerization
B. Stereoisomerism
C. Racemization (Correct Answer)
D. Fractionation

Explanation: ### Explanation **Correct Answer: C. Racemization** **1. Why Racemization is Correct:** Racemization is the process by which an optically active substance (a pure enantiomer) is converted into an optically inactive **racemic mixture**. A racemic mixture contains equal molar amounts (50:50) of both dextrorotatory ($d$ or $+$) and levorotatory ($l$ or $-$) isomers. Because the two forms rotate plane-polarized light in opposite directions with equal magnitude, the net optical rotation becomes zero. In biochemistry, this often occurs via a transient symmetric intermediate, such as the formation of a carbanion at the alpha-carbon of an amino acid. **2. Why Other Options are Incorrect:** * **A. Polymerization:** This is a chemical process where small monomeric units join together to form a large polymer (e.g., glucose units forming glycogen). It does not involve changes in optical activity. * **B. Stereoisomerism:** This is a broad category of isomerism where molecules have the same molecular formula but different spatial arrangements (includes enantiomers and diastereomers). It is a *classification*, not the *process* of conversion. * **C. Fractionation:** This is a separation technique used to isolate different components of a mixture based on physical properties like solubility or molecular weight (e.g., plasma protein fractionation). **3. High-Yield Clinical Pearls for NEET-PG:** * **Amino Acid Chirality:** All human proteins are composed of **L-amino acids**. However, **D-amino acids** are found in bacterial cell walls and certain antibiotics (e.g., Gramicidin). * **Racemases:** These are enzymes (classified under **Isomerases**) that catalyze the interconversion of enantiomers. A classic example is *Alanine racemase*, used by bacteria to produce D-alanine for peptidoglycan synthesis. * **Clinical Correlation:** The drug **Thalidomide** is a famous example where one isomer is therapeutic (sedative), but its spontaneous *in vivo* racemization produces the other isomer, which is highly teratogenic.

Question 190: Which of the following is NOT required for a standard polymerase chain reaction?

A. Taq polymerase
B. Deoxynucleotide triphosphates (dNTPs)
C. Dideoxynucleotides (Correct Answer)
D. Magnesium ions

Explanation: ### Explanation **Core Concept: Polymerase Chain Reaction (PCR)** PCR is an *in vitro* enzymatic method used to amplify specific DNA sequences. The process mimics natural DNA replication but requires specific components to function in a thermal cycler. **Why Dideoxynucleotides (ddNTPs) are NOT required:** Dideoxynucleotides (ddNTPs) lack a **3'-OH group**, which is essential for forming phosphodiester bonds. When a ddNTP is incorporated, DNA synthesis terminates immediately. Therefore, ddNTPs are used in **Sanger Sequencing (Chain Termination Method)**, not in standard PCR, where the goal is to synthesize full-length DNA strands. **Analysis of Other Options:** * **Taq Polymerase:** A heat-stable DNA polymerase (derived from *Thermus aquaticus*) required to extend the primers by adding nucleotides at high temperatures. * **dNTPs (dATP, dCTP, dGTP, dTTP):** These are the "building blocks" or substrates required to synthesize the new DNA strand. * **Magnesium ions (Mg²⁺):** These act as a mandatory **cofactor** for DNA polymerase activity. They stabilize the negative charges on the phosphate backbone and facilitate the catalysis of phosphodiester bonds. **High-Yield NEET-PG Pearls:** 1. **Steps of PCR:** Denaturation (94-96°C) → Annealing (50-65°C) → Extension (72°C). 2. **Primers:** PCR requires two synthetic, short oligonucleotide primers that are complementary to the 3' ends of the target DNA. 3. **RT-PCR:** Uses Reverse Transcriptase to convert RNA into cDNA before amplification (Gold standard for COVID-19 diagnosis). 4. **Real-Time PCR (qPCR):** Used to quantify the amount of DNA/RNA in a sample using fluorescent dyes.

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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