Biochemical Techniques Practice Questions

Q: Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?

Phenylisothiocyanate. ***Phenylisothiocyanate*** - **Phenylisothiocyanate** (PITC), also known as Edman's reagent, is used in the **Edman degradation** method to identify the N-terminal amino acid. - It sequentially cleaves the **N-terminal amino acid** without hydrolyzing the rest of the peptide chain, allowing for identification by chromatography. *Trypsin* - **Trypsin** is a protease that cleaves peptide bonds at the carboxyl side of **lysine** and **arginine** residues. - It is used for peptide fragmentation, not for determining the N-terminal amino acid. *Carboxypeptidase* - **Carboxypeptidases** are exopeptidases that cleave amino acids from the **C-terminal end** of a polypeptide chain. - They are used to identify the C-terminal amino acid, not the N-terminal. *Cyanogen bromide* - **Cyanogen bromide (CNBr)** is a chemical reagent that specifically cleaves peptide bonds on the C-terminal side of **methionine** residues. - It is used for specific peptide fragmentation and not for N-terminal sequencing.

Q: Which of the following methods is least suitable for determining the structure of proteins?

High pressure liquid chromatography (HPLC). ***High pressure liquid chromatography (HPLC)*** ✓ - **HPLC** is a technique primarily used for **separation, purification, identification, and quantification** of components in a mixture based on their differing affinities for a stationary phase and a mobile phase. - While HPLC can provide valuable information about a protein's **purity, quantity, and molecular characteristics**, it does **not directly determine the three-dimensional structure** of proteins. - HPLC is fundamentally a **chromatographic separation technique**, not a structural determination method, making it the **least suitable** option for elucidating protein structure. *SDS-PAGE* - **SDS-PAGE** (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis) separates proteins based on **molecular weight** under denaturing conditions. - It provides information about protein **size and purity** but does not reveal the protein's **native three-dimensional structure**. - While useful for characterization, it is not a structure determination technique. *NMR spectrometry* - **NMR spectroscopy** (Nuclear Magnetic Resonance) is a powerful technique that can determine the **detailed three-dimensional structure of proteins** in solution. - Particularly effective for smaller proteins (typically <30-40 kDa), it detects magnetic properties of atomic nuclei, providing **atomic-level structural and dynamic information**. - This is a **standard method for protein structure determination**. *X-ray Crystallography* - **X-ray crystallography** is the **gold standard** for determining the **atomic and molecular structure** of proteins at high resolution. - By analyzing diffraction patterns from protein crystals, researchers can reconstruct **detailed 3D structures** with atomic precision. - This technique has solved the majority of known protein structures and is **highly suitable for structure determination**.

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

Detects RNA molecules. ***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.

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

CSF. ***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.

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

Detecting with UV light at 280 nm. ***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.

Q: The probe used in Western blot is:

Antibody. ***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.

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

Labeling DNA using nucleotides. ***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.

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

PCR amplification. ***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.

Q: What is Northern blot used to detect?

RNA. ***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 1

Which of the following reagents would be most useful in determining the N-terminal amino acid of a polypeptide?

Accepted Answer

Phenylisothiocyanate

Answer

Trypsin

Answer

Carboxypeptidase

Answer

Cyanogen bromide

Question 2

Which of the following methods is least suitable for determining the structure of proteins?

Accepted Answer

High pressure liquid chromatography (HPLC)

Answer

SDS Page

Answer

Crystallography

Answer

NMR spectrometry

Question 3

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

Accepted Answer

Detects RNA molecules

Answer

Detects specific nucleic acid sequences

Answer

Detects DNA sequences

Answer

Determines RNA structure

Question 4

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

Accepted Answer

CSF

Answer

Hair roots

Answer

Semen

Answer

Buccal mucosa

Question 5

Which method of protein detection does not alter its function?

Accepted Answer

Detecting with UV light at 280 nm

Answer

Detecting with heat coagulation

Answer

Detecting with 2-D electrophoresis

Answer

Detecting with Coomassie blue dye after electrophoresis by SDS-PAGE

Question 6

The probe used in Western blot is:

Accepted Answer

Antibody

Answer

mRNA

Answer

DNA

Answer

tRNA

Question 7

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

Accepted Answer

Labeling DNA using nucleotides

Answer

Translation of RNA into protein

Answer

Controlling gene expression

Answer

Breaking down DNA into fragments

Question 8

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

Accepted Answer

PCR amplification

Answer

Electrophoresis

Answer

Chromatography

Answer

Centrifugation

Question 9

What is Northern blot used to detect?

Accepted Answer

RNA

Answer

Protein

Answer

Immunoglobulin

Answer

DNA

Question 10

Which of the following statements accurately describes the Western blotting process?

Accepted Answer

Western blotting involves SDS-PAGE separation of proteins, transfer to a membrane, and detection using specific antibodies

Answer

SDS-PAGE is a technique used for the initial separation of proteins

Answer

Enzyme-linked antibodies are utilized for detection of proteins

Answer

Proteins are separated based on their size using SDS-PAGE

Biochemical Techniques — MCQs

Biochemical Techniques — MCQs

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