Biochemical Techniques — MCQs

Biochemical Techniques Indian Medical PG Practice Questions and MCQs

Question 261: Which of the following results is provided by Northern blot analysis?

A. Detects specific nucleic acid sequences
B. Detects DNA sequences
C. Detects RNA molecules (Correct Answer)
D. Determines RNA structure

Explanation: ***Detects RNA molecules*** - Northern blot analysis is a molecular biology technique used specifically to study **RNA molecules**. - It allows for the detection and quantification of **specific RNA sequences** within a sample. *Detects specific nucleic acid sequences* - While correct in a broad sense, this option is too general; blot analysis techniques are specified by the type of nucleic acid they detect. - **Northern blot** specifically detects RNA, whereas **Southern blot** detects DNA. *Detects DNA sequences* - This is incorrect; the detection of **DNA sequences** is performed by **Southern blot analysis**, not Northern blot. - Northern blot involves the separation and detection of **RNA fragments**. *Determines RNA structure* - Northern blot analysis primarily focuses on the **presence, size, and amount** of specific RNA molecules, not their complex three-dimensional structure. - Techniques like **NMR spectroscopy** or **X-ray crystallography** are used to determine RNA structure.

Question 262: Which of the following is the least suitable source for DNA extraction?

A. CSF (Correct Answer)
B. Hair roots
C. Semen
D. Buccal mucosa

Explanation: ***CSF*** - **Cerebrospinal fluid (CSF)** contains a relatively **low number of cells**, making it a poor source for DNA extraction compared to other bodily fluids due to the scarcity of nuclear DNA. - While DNA can be extracted from CSF for specific diagnostic purposes (e.g., detection of pathogens), it is generally **not the preferred source** for DNA profiling or genetic studies due to the limited yield and potential for degradation. *Hair roots* - **Hair roots** (specifically the follicular tag) contain a significant number of **nucleated cells**, making them an excellent source for DNA extraction. - The DNA extracted from hair roots is often robust and sufficient for **forensic analysis** and genetic testing. *Semen* - **Semen** contains a high concentration of **sperm cells**, which are rich in nuclear DNA, making it a very good source for DNA extraction. - It is frequently used in **forensic investigations** and paternity testing due to its high DNA content. *Buccal mucosa* - **Buccal cells** scraped from the inside of the cheek provide a non-invasive and **abundant source of nucleated cells** for DNA extraction. - This method is widely used for genetic testing, **ancestry tracing**, and clinical diagnostics because of its ease of collection and high DNA yield.

Question 263: The probe used in Western blot is:

A. Antibody (Correct Answer)
B. mRNA
C. DNA
D. tRNA

Explanation: ***Antibody*** - In a **Western blot**, the primary probe used to detect specific proteins is typically an **antibody** that specifically binds to the target protein. - This antibody is often labeled (e.g., fluorescently or enzymatically) or recognized by a secondary, labeled antibody, allowing for visualization of the target. *mRNA* - **mRNA** (messenger RNA) is used as a probe in techniques like **Northern blotting** to detect specific RNA sequences, not proteins. - It carries genetic information from DNA to synthesize proteins but does not directly probe for protein presence in a Western blot. *DNA* - **DNA** is used as a probe in techniques such as **Southern blotting** to detect specific DNA sequences. - It is not used as a probe to identify proteins in a Western blot. *tRNA* - **tRNA** (transfer RNA) molecules are involved in **protein synthesis** by carrying specific amino acids to the ribosome. - They are not used as probes in any standard blotting technique, including Western blotting.

Question 264: Which method of protein detection does not alter its function?

A. Detecting with heat coagulation
B. Detecting with 2-D electrophoresis
C. Detecting with UV light at 280 nm (Correct Answer)
D. Detecting with Coomassie blue dye after electrophoresis by SDS-PAGE

Explanation: ***Detecting with UV light at 280 nm*** - Proteins absorb UV light at 280 nm due to the presence of **aromatic amino acids** like **tryptophan, tyrosine, and phenylalanine** within their structure. - This method is **non-destructive** and allows for the quantification of protein concentration without altering the protein's native structure or function. *Detecting with heat coagulation* - Heating a protein solution above its denaturation temperature causes the protein to **unfold and aggregate**, irreversibly altering its secondary, tertiary, and quaternary structures. - This process leads to **loss of protein function** as the denatured protein can no longer perform its biological role. *Detecting with 2-D electrophoresis* - While a powerful separation technique, 2-D electrophoresis involves exposing proteins to **denaturing conditions** (e.g., SDS, reducing agents, pH extremes) during separation. - These conditions can **irreversibly alter** the protein's native conformation, making it unsuitable for subsequent functional assays. *Detecting with Coomassie blue dye after electrophoresis by SDS-PAGE* - **SDS-PAGE** (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis) involves denaturing proteins with **SDS and heat**, which unfolds proteins and masks their intrinsic charge. - Subsequent staining with **Coomassie blue dye** binds to the denatured protein, but the original function of the protein is already lost due to the harsh denaturing conditions.

Question 265: What is the technique known as 'nick translation' used for in molecular biology?

A. Translation of RNA into protein
B. Controlling gene expression
C. Labeling DNA using nucleotides (Correct Answer)
D. Breaking down DNA into fragments

Explanation: ***Labeling DNA using nucleotides*** - **Nick translation** is a technique used to **incorporate labeled nucleotides** into a DNA probe. - This process involves creating single-stranded breaks (nicks) in the DNA, followed by the progressive addition of labeled nucleotides by **DNA polymerase I**, while simultaneously degrading the existing DNA strand ahead of it. *Translation of RNA into protein* - This process is known as **protein synthesis** or simply **translation**, where **ribosomes** decode mRNA sequences into polypeptide chains. - It does not involve the labeling of DNA using nicks or DNA polymerase I. *Controlling gene expression* - **Gene expression control** refers to the mechanisms that regulate the transcription and translation of genes into functional products. - This involves a variety of molecular mechanisms such as **transcription factors**, epigenetics, and RNA interference, which are unrelated to nick translation. *Breaking down DNA into fragments* - The process of breaking down DNA into fragments is typically achieved using **restriction enzymes** or **mechanical shearing**. - While nick translation involves nicks, its primary purpose is labeling, not fragmentation for size reduction.

Question 266: Which of the following methods is not used for protein purification?

A. PCR amplification (Correct Answer)
B. Electrophoresis
C. Chromatography
D. Centrifugation

Explanation: ***PCR amplification*** - **PCR (Polymerase Chain Reaction)** is a technique used to **amplify specific DNA sequences**, not proteins. - PCR works exclusively with nucleic acids and has **no role in protein purification**. - This is the correct answer as it is completely unrelated to protein work. *Chromatography* - **Chromatography** (ion-exchange, size-exclusion, affinity chromatography) is the **gold standard method** for protein purification [2]. - It separates proteins based on charge, size, hydrophobicity, or specific binding properties [3]. - Essential technique in all protein purification workflows. *Centrifugation* - **Centrifugation** separates components based on **density and sedimentation rate** [1]. - Used in protein purification for **cell lysis, debris removal, and subcellular fractionation** [1]. - Important initial step in most protein purification protocols. *Electrophoresis* - **Electrophoresis** (SDS-PAGE, native PAGE) is primarily an **analytical technique** for protein characterization [4]. - Used to **assess purity, determine molecular weight, and analyze protein samples**. - While preparative electrophoresis exists, it is **rarely used** compared to chromatography for routine purification.

Question 267: Which of the following statements about Taq DNA polymerase is correct?

A. Optimum temperature for chain elongation is 75°C (Correct Answer)
B. Denatures at high temperatures
C. Provides high fidelity during DNA synthesis
D. Exhibits 3' to 5' exonuclease activity

Explanation: ***Optimum temperature for chain elongation is 75°C*** - **Taq polymerase** is a **thermostable enzyme** isolated from *Thermus aquaticus*, functioning optimally at high temperatures. - The optimal temperature for the **elongation step** in PCR, where Taq polymerase synthesizes new DNA strands, is typically around **72-78°C**, with 75°C falling within this optimal range. *Denatures at high temperatures* - While all proteins will eventually denature at extremely high temperatures, Taq polymerase is specifically known for its **thermostability** and **resistance to denaturation** at temperatures required for DNA strand separation in PCR (typically 94-98°C). - Its ability to withstand these high temperatures without significant loss of activity is its key advantage for use in **Polymerase Chain Reaction (PCR)**. *Provides high fidelity during DNA synthesis* - **Taq polymerase** is known for its relatively **low fidelity** due to the lack of 3' to 5' exonuclease activity (proofreading). - This low fidelity results in a higher error rate during DNA synthesis compared to other polymerases with proofreading capabilities, leading to more **mutations** during PCR. *Exhibits 3' to 5' exonuclease activity* - **Taq polymerase** typically **lacks 3' to 5' exonuclease activity**, meaning it does not have the ability to proofread and remove incorrectly incorporated nucleotides. - This absence of proofreading contributes to its relatively **lower fidelity** during DNA replication compared to other polymerases that possess this activity.

Question 268: What is Northern blot used to detect?

A. Protein
B. Immunoglobulin
C. RNA (Correct Answer)
D. DNA

Explanation: ***RNA*** - **Northern blot** is a laboratory technique used to detect specific **RNA** molecules among a mixture of RNA. - It involves separating RNA fragments by **gel electrophoresis**, transferring them to a membrane, and then probing with a labeled complementary sequence. *Protein* - **Proteins** are typically detected using a **Western blot**, which involves similar separation and transfer techniques but uses **antibodies** as probes. - While RNA codes for proteins, Northern blot *directly* detects RNA transcripts, not the resulting protein products. *Immunoglobulin* - **Immunoglobulins** (antibodies) are a type of protein, and their detection usually falls under **Western blot** or specific immunological assays like **ELISA**. - Northern blot is specifically designed for nucleic acid analysis, not protein detection. *DNA* - **DNA** is detected using a **Southern blot** technique, which also involves electrophoresis, transfer to a membrane, and hybridization with a complementary probe. - The name "Northern blot" was coined as a play on "Southern blot" because it uses similar methodology but for RNA instead of DNA.

Question 269: Which test is commonly used to assess the mutagenic potential of carcinogens?

A. Ames test (Correct Answer)
B. Redox test
C. Bacteriophage
D. Gene splicing

Explanation: ***Ames test*** - The **Ames test** uses specific strains of bacteria (e.g., *Salmonella typhimurium* and *E. coli*) to detect **mutagenic compounds** by reverse mutation. - It measures the ability of a chemical to induce **mutations** that restore the bacteria's ability to grow in a histidine-deficient medium. *Redox test* - A **redox test** measures the **oxidation-reduction potential** of a solution or system, indicating the balance between oxidizing and reducing agents. - While it can reflect cellular stress, it does not directly assess a substance's **mutagenic potential** or ability to cause DNA damage. *Bacteriophage* - A **bacteriophage** is a **virus that infects bacteria** and uses the bacterial cell's machinery to replicate itself. - While bacteriophages are used in genetic research as **vectors** or for studying gene expression, their primary role is not to directly assess the mutagenic potential of carcinogens. *Gene splicing* - **Gene splicing** is a molecular biology technique involving the **cutting and recombining of DNA segments**, often used in genetic engineering. - It is a method for creating new genetic combinations, not a test for directly evaluating the mutagenic potential of a compound.

Question 270: Which of the following statements accurately describes the Western blotting process?

A. Western blotting involves SDS-PAGE separation of proteins, transfer to a membrane, and detection using specific antibodies (Correct Answer)
B. SDS-PAGE is a technique used for the initial separation of proteins
C. Enzyme-linked antibodies are utilized for detection of proteins
D. Proteins are separated based on their size using SDS-PAGE

Explanation: ***Western blotting involves SDS-PAGE separation of proteins, transfer to a membrane, and detection using specific antibodies*** - This statement accurately summarizes the entire process of **Western blotting**, from initial separation to final detection. - The technique specifically uses **SDS-PAGE** to separate proteins by size, followed by **transfer to a membrane** for antibody-based detection. *SDS-PAGE is a technique used for the initial separation of proteins.* - While SDS-PAGE is used for the initial **separation of proteins** in Western blotting, this statement alone does not describe the complete Western blotting process. - It omits two crucial steps: the **transfer to a membrane** and the **antibody-based detection**. *Enzyme-linked antibodies are utilized for detection of proteins.* - **Enzyme-linked antibodies** are indeed used for protein detection in Western blotting, but this statement only describes one specific part of the detection phase. - It does not cover the initial and crucial steps of **protein separation** or **membrane transfer**. *Proteins are separated based on their size using SDS-PAGE.* - Proteins are indeed **separated based on their size** using SDS-PAGE in Western blotting, due to the denaturing effect of SDS. - This statement accurately describes one component but fails to encompass the subsequent steps of **membrane transfer and antibody detection**, which are integral to the complete Western blot.

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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