Biochemical Techniques Practice Questions

Q: Million-Nasse's reaction is specific for which amino acid?

Tyrosine. ### Explanation **Correct Option: B (Tyrosine)** Millon’s reaction (or Millon-Nasse reaction) is a specific biochemical test used to detect the presence of **Tyrosine**. The reagent consists of mercuric nitrate and mercurous nitrate dissolved in concentrated nitric acid. * **Mechanism:** The reaction is specific to the **phenolic group** (a benzene ring with an attached hydroxyl group). Tyrosine is the only proteogenic amino acid containing a phenolic group. * **Result:** When the reagent is added to a solution containing Tyrosine and heated, a white precipitate forms, which subsequently turns **brick red** due to the formation of a mercury complex of nitrated tyrosine. **Analysis of Incorrect Options:** * **A. Tryptophan:** Detected by the **Hopkins-Cole test** (Glyoxylic acid reaction), which identifies the indole ring, producing a violet/purple ring. * **C. Phenylalanine:** Although it contains a benzene ring, it lacks the hydroxyl group (phenol) required for Millon’s test. It is typically identified via the **Xanthoproteic test** (though less reactive than Tyrosine/Tryptophan). * **D. Arginine:** Detected by the **Sakaguchi test**, which is specific for the guanidinium group, yielding a bright red color. **High-Yield Clinical Pearls for NEET-PG:** * **Xanthoproteic Test:** Detects aromatic amino acids (Tyrosine, Tryptophan, Phenylalanine) using concentrated nitric acid, resulting in a yellow color. * **Pauly’s Test:** Specific for Histidine and Tyrosine (detects imidazole and phenolic rings). * **Ninhydrin Test:** A general test for all alpha-amino acids (gives a Ruhemann's purple color), except Proline and Hydroxyproline, which give a yellow color. * **Sulfur Test (Lead Acetate):** Specific for sulfur-containing amino acids like Cysteine (Cystine), but **not** Methionine (as its sulfur is in a thioether bond).

Q: Which of the following labels corresponds to the condenser of the microscope?

C. ***C*** - The **condenser** is located beneath the **microscope stage** and above the **light source**, which matches the position of label C in the diagram. - Its primary function is to **focus and concentrate light** onto the specimen, improving illumination and image contrast. *B* - Label B typically corresponds to the **stage** where the specimen slide is placed, not the condenser. - The stage is positioned above the condenser and serves as the **specimen platform**. *D* - Label D usually represents the **objective lenses** or **eyepiece**, which are involved in **magnification** rather than light concentration. - These components are located above the stage, opposite to the condenser's position. *A* - Label A commonly indicates the **light source** or **diaphragm**, which are positioned below the condenser. - These components provide **illumination** or control **light intensity**, but do not focus light like the condenser.

Q: Real-time PCR is primarily used for what purpose?

Quantifying the extent of DNA amplification. **Explanation:** Real-time PCR, also known as **Quantitative PCR (qPCR)**, is a modification of the standard PCR technique. While traditional PCR only allows for "end-point" analysis (checking results after all cycles are finished), qPCR monitors the amplification process **as it occurs** (in real-time). **Why Option D is Correct:** The core principle of qPCR is the use of fluorescent dyes (like SYBR Green) or sequence-specific probes (like TaqMan). These markers emit fluorescence proportional to the amount of DNA synthesized. By measuring this fluorescence at each cycle, clinicians can determine the initial concentration of the target DNA template. This makes it the gold standard for **quantification**. **Analysis of Incorrect Options:** * **Option A:** Amplification of RNA requires **Reverse Transcriptase PCR (RT-PCR)**. While qPCR is often combined with RT-PCR to measure RNA levels (RT-qPCR), the "Real-time" component specifically refers to the quantification, not the RNA-to-DNA conversion. * **Option B:** This describes **Standard PCR**. While qPCR does amplify DNA, its *primary* and distinguishing purpose is quantification. * **Option C:** PCR techniques are exclusive to nucleic acids. Protein amplification is not a function of PCR (Proteins are analyzed via Western Blot or ELISA). **High-Yield Clinical Pearls for NEET-PG:** * **Cycle Threshold (Ct) Value:** The number of cycles required for the fluorescent signal to cross the background threshold. **Lower Ct = Higher initial viral/DNA load.** * **Clinical Use:** It is the diagnostic mainstay for measuring **Viral Load** (e.g., HIV, Hepatitis B/C) and monitoring response to antiviral therapy. * **Distinction:** Do not confuse **RT-PCR** (Reverse Transcriptase) with **Real-time PCR**. RT-PCR handles RNA; Real-time PCR handles quantification. (Note: COVID-19 testing uses both: RT-qPCR).

Q: What is essential for the Polymerase Chain Reaction (PCR)?

A thermostable enzyme is needed.. **Explanation:** The **Polymerase Chain Reaction (PCR)** is an *in vitro* method for the enzymatic amplification of specific DNA sequences. **Why Option A is correct:** The PCR process involves repeated cycles of high-temperature heating (denaturation at ~95°C) to separate DNA strands. A **thermostable DNA polymerase**, most commonly **Taq Polymerase** (derived from the bacterium *Thermus aquaticus*), is essential because it remains functional at these high temperatures. Unlike human DNA polymerase, it does not denature, allowing the reaction to proceed through multiple cycles without the need to add fresh enzyme at every step. **Analysis of Incorrect Options:** * **Option B:** While 2ⁿ represents the theoretical maximum yield, it is not an "essential requirement" for the reaction to occur; it is a mathematical outcome of exponential amplification. * **Option C:** Non-specific primer binding is a **technical error** or limitation (leading to artifacts), not an essential requirement for the technique. * **Option D:** A thermolabile enzyme (like the Klenow fragment used in early PCR) would be destroyed during the denaturation phase, making the modern automated PCR process impossible. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (95°C) → Annealing (50-65°C) → Extension (72°C). * **Components required:** Template DNA, Primers (forward and reverse), dNTPs, Mg²⁺ (cofactor), and Taq Polymerase. * **RT-PCR:** Uses Reverse Transcriptase to amplify RNA (e.g., Gold standard for COVID-19/SARS-CoV-2 diagnosis). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA as the reaction progresses using fluorescent dyes.

Q: Protein is purified using ammonium sulfate by which method?

Salting out. ### Explanation **1. Why "Salting Out" is Correct:** Salting out is a technique used to purify proteins based on their **solubility**. When high concentrations of neutral salts like **Ammonium Sulfate** $(NH_4)_2SO_4$ are added to a protein solution, the salt ions compete with the protein molecules for water molecules (solvation). As the salt concentration increases, the water molecules are "stripped" away from the protein's surface to hydrate the salt ions. This exposes the hydrophobic patches of the proteins, causing them to aggregate and precipitate out of the solution. Ammonium sulfate is the preferred reagent because of its high solubility and low toxicity to protein structure. **2. Why Other Options are Incorrect:** * **Ion Exchange Chromatography:** This technique separates proteins based on their **net surface charge** at a specific pH using stationary phases like DEAE-cellulose (anion exchange) or CM-cellulose (cation exchange). * **Mass Chromatography (Mass Spectrometry):** This is used to determine the **molecular mass** and chemical structure of a protein by measuring the mass-to-charge ratio of ions, rather than for bulk purification. * **Molecular Size Exclusion (Gel Filtration):** This separates proteins based on their **molecular weight/size** using porous beads. Smaller molecules get trapped in the beads and move slowly, while larger molecules elute first. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dialysis:** After salting out, dialysis is the mandatory next step to remove the excess salt from the protein sample. * **Hofmeister Series:** This ranks ions based on their ability to salt out proteins; Sulfate $(SO_4^{2-})$ is a powerful "kosmotrope" (stabilizer) used in this process. * **Specific Activity:** During purification, as you remove unwanted proteins, the *specific activity* (units of enzyme per mg of total protein) should **increase**, indicating successful purification.

Question 1

Million-Nasse's reaction is specific for which amino acid?

Accepted Answer

Tyrosine

Answer

Tryptophan

Answer

Phenylalanine

Answer

Arginine

Question 2

In a centrifugal distribution, which type of protein is precipitated first?

Accepted Answer

Fibrous

Answer

Globular

Answer

Completely disorganized

Answer

None of the above

Question 3

L-J chart is used for monitoring which of the following parameters?

Accepted Answer

Accuracy

Answer

Precision

Answer

Sensitivity

Answer

Specificity

Question 4

Which of the following labels corresponds to the condenser of the microscope?

Accepted Answer

C

Answer

B

Answer

D

Answer

A

Question 5

Real-time PCR is primarily used for what purpose?

Accepted Answer

Quantifying the extent of DNA amplification

Answer

The amplification of RNA molecules

Answer

The amplification of specific DNA segments

Answer

The amplification of proteins

Question 6

What specialized type of microscope enables quantitative measurement of the chemical constituents of cells?

Accepted Answer

Interference microscope

Answer

Optical microscope

Answer

Phase contrast microscope

Answer

Polarized microscope

Question 7

What is essential for the Polymerase Chain Reaction (PCR)?

Accepted Answer

A thermostable enzyme is needed.

Answer

2^n copies are formed after 'n' cycles.

Answer

Non-specific primer binding can occur.

Answer

A thermolabile enzyme is used.

Question 8

Which of the following statements regarding the microarray technique is false?

Accepted Answer

It can detect Robertsonian translocations.

Answer

Its core principle is DNA hybridization.

Answer

It is also known as a DNA chip or biochip.

Answer

It can detect deletions and insertions.

Question 9

Which of the following methods most accurately estimates blood creatinine level?

Accepted Answer

Enzyme assay

Answer

Jaffe method

Answer

Kinetic Jaffe method

Answer

Technicon method

Question 10

Protein is purified using ammonium sulfate by which method?

Accepted Answer

Salting out

Answer

Ion exchange chromatography

Answer

Mass chromatography

Answer

Molecular size exclusion

Biochemical Techniques — MCQs

Biochemical Techniques — MCQs

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