Centrifugation and Ultracentrifugation — MCQs
Which of the following methods is not used for protein purification?
Techniques used for protein expression proteomics study include:
Which of the following is not classified as a molecular motor?
Which of the following separates proteins solely on the basis of their molecular size?
Forceps may be preferred over vacuum for operative delivery due to the following reasons, EXCEPT:
Which of the following methods of protein separation is not dependent on molecular size?
Separation of proteins based on size is done by
Protein purification and separation can be done by all except:
The technique shown in the image is:
The shown pattern in electrophoresis is due to:
Centrifugation and Ultracentrifugation Indian Medical PG Practice Questions and MCQs
Question 1: 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 2: Techniques used for protein expression proteomics study include:
- A. PolyAcrylamide Gel Electrophoresis (PAGE)
- B. Gene Expression Analysis (indirectly related to proteomics)
- C. Mass Spectrometry
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - All listed techniques—**Polyacrylamide Gel Electrophoresis (PAGE)**, **Gene Expression Analysis**, and **Mass Spectrometry**—are used in protein expression proteomics studies, either directly or indirectly, to analyze and quantify proteins. - The integration of these various techniques provides a comprehensive approach to understanding protein expression profiles. *PolyAcrylamide Gel Electrophoresis (PAGE)* - **PAGE** (including 1D and 2D-PAGE) is a fundamental technique for separating proteins based on their **molecular weight** and **isoelectric point**, which is crucial for visualizing and quantifying expressed proteins. - It often serves as an initial separation step before more detailed analysis, such as **mass spectrometry**. *Gene Expression Analysis (indirectly related to proteomics)* - Although **gene expression analysis** (e.g., using **RT-PCR** or **microarrays**) measures mRNA levels, it is indirectly related to proteomics because mRNA levels often **correlate with protein levels**. - It provides insights into the **transcriptional regulation** that influences protein expression, complementing direct protein analysis. *Mass Spectrometry* - **Mass spectrometry** is a powerful and widely used technique in proteomics for **identifying, quantifying, and characterizing proteins** and peptides by measuring their **mass-to-charge ratio**. - It can be used for both **discovery proteomics** (identifying novel proteins) and **targeted proteomics** (quantifying specific proteins).
Question 3: Which of the following is not classified as a molecular motor?
- A. Kinesin
- B. Dynein
- C. Actin (Correct Answer)
- D. Myosin
Explanation: ***Actin*** - **Actin** is a globular multi-functional protein that forms **microfilaments**, which are a crucial component of the cytoskeleton in eukaryotic cells and play a vital role in cellular processes such as muscle contraction, cell motility, and cytokinesis. - While actin interacts with molecular motors like myosin to generate force and movement, actin itself is a **filamentous track** upon which motors move, not a motor protein. *Kinesin* - **Kinesin** is a motor protein that moves along **microtubules**, typically transporting cargo towards the **plus end** of the microtubule. - It uses **ATP hydrolysis** to power its movement, acting as a molecular motor in various cellular processes. *Dynein* - **Dynein** is another molecular motor protein that also moves along **microtubules**, but typically transports cargo towards the **minus end** of the microtubule. - It is crucial for processes like **ciliary and flagellar movement** and intracellular transport. *Myosin* - **Myosin** is a family of **motor proteins** best known for its role in muscle contraction, where it interacts with actin filaments. - It uses **ATP hydrolysis** to move along actin filaments, generating contractile force and movement.
Question 4: Which of the following separates proteins solely on the basis of their molecular size?
- A. Isoelectric focusing
- B. Chromatography on a diethylaminoethyl (DEAE) cellulose column
- C. Gel filtration chromatography (Correct Answer)
- D. Chromatography on a carboxymethyl (CM) cellulose column
Explanation: ***Gel filtration chromatography*** - Also known as **size-exclusion chromatography**, this method separates proteins based on their **hydrodynamic volume** (molecular size and shape). - Larger proteins pass through the column more quickly because they are excluded from the pores of the stationary phase, while smaller proteins enter the pores and have a longer, more tortuous path. *Isoelectric focusing* - This technique separates proteins based on their **isoelectric point (pI)**, which is the pH at which the protein has no net electrical charge. - Proteins migrate through a pH gradient until they reach the point where their net charge is zero. *Chromatography on a diethylaminoethyl (DEAE) cellulose column* - **DEAE cellulose** is an **anion-exchange resin**, meaning it binds **negatively charged** proteins. - Separation is based on the **net charge** of the protein at a given pH. *Chromatography on a carboxymethyl (CM) cellulose column* - **CM cellulose** is a **cation-exchange resin**, meaning it binds **positively charged** proteins. - Separation is based on the **net charge** of the protein at a given pH.
Question 5: Forceps may be preferred over vacuum for operative delivery due to the following reasons, EXCEPT:
- A. Vacuum requires more clinical skills than forceps (Correct Answer)
- B. Forceps are more commonly associated with fetal facial injury
- C. Vacuum has more chance of formation of cephalhematoma
- D. Vacuum is preferred in certain cases to minimize trauma and reduce transmission risks
Explanation: ***Vacuum requires more clinical skills than forceps*** - This statement is **incorrect** - vacuum extraction typically requires **less clinical skill** than forceps application - Forceps application demands precise knowledge of fetal head position, station, and careful maneuvering, requiring more training and expertise - Since vacuum actually requires less skill (not more), this is NOT a valid reason to prefer forceps over vacuum - **This is the correct answer to the EXCEPT question** *Forceps are more commonly associated with fetal facial injury* - This is **true** - forceps application involves direct compression and traction on the fetal head - This increases risk of **facial nerve palsies**, **bruising**, **lacerations**, and **skull fractures** - However, this is a **disadvantage** of forceps, not a reason to prefer them - Despite this, in certain clinical situations (e.g., need for rapid delivery, specific fetal positions), forceps may still be chosen when their advantages outweigh this risk *Vacuum has more chance of formation of cephalhematoma* - This is **true** - vacuum extraction creates suction on the fetal scalp, leading to blood accumulation under the periosteum - **Cephalhematoma** occurs more frequently with vacuum (10-20%) compared to forceps (1-2%) - This is a valid reason why forceps might be preferred when avoiding scalp trauma is important *Vacuum is preferred in certain cases to minimize trauma and reduce transmission risks* - This is **true** - vacuum causes less maternal perineal trauma compared to forceps - In cases of maternal infections (HIV, HSV), vacuum may reduce transmission risk due to fewer maternal lacerations - However, when rapid delivery is essential or specific fetal positions require rotation, forceps may still be chosen despite vacuum having these advantages
Question 6: Which of the following methods of protein separation is not dependent on molecular size?
- A. Gel filtration chromatography
- B. SDS-PAGE
- C. Ultracentrifugation
- D. Ion-exchange chromatography (Correct Answer)
Explanation: ***Ion-exchange chromatography*** - This method separates proteins based on their **net charge** at a specific pH. - Proteins bind to a charged resin based on their charge, independent of their size. *Gel filtration chromatography* - Separates proteins based on their **molecular size** and shape as they pass through a porous matrix. - **Larger molecules** elute first as they cannot enter the pores, while smaller molecules are retained. *SDS-PAGE* - **Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis** separates proteins primarily based on their **molecular weight**. - SDS denatures proteins and confers a uniform negative charge, allowing migration through a gel matrix based on size. *Ultracentrifugation* - Separates macromolecules and particles based on their **sedimentation rate**, which is influenced by **molecular mass**, density, and shape. - While molecular size is a factor, density and shape also play significant roles in the separation process.
Question 7: Separation of proteins based on size is done by
- A. Affinity chromatography
- B. High performance liquid chromatography
- C. SDS-Polyacrylamide gel electrophoresis (Correct Answer)
- D. Ion exchange chromatography
Explanation: ***SDS-Polyacrylamide gel electrophoresis*** - **SDS-PAGE** separates proteins primarily based on their **molecular weight** (size). - Proteins are denatured and coated with negatively charged **SDS**, causing them to migrate through a polyacrylamide gel based on size. *Affinity chromatography* - This technique separates proteins based on their **specific binding affinity** to a ligand. - It does not directly separate based on size, but rather on **molecular recognition**. *High performance liquid chromatography* - **HPLC** is a chromatographic technique that separates molecules in a complex mixture, but the primary basis of separation depends on the column type. - While some HPLC methods (**size-exclusion HPLC**) can separate by size, it is a broader technique and not the most specific answer for protein size separation in general context. *Ion exchange chromatography* - This method separates proteins based on their **net charge** at a particular pH. - Proteins bind to a charged resin and are eluted by increasing salt concentration or changing pH, not based on size.
Question 8: Protein purification and separation can be done by all except:
- A. Densitometry (Correct Answer)
- B. Electrophoresis
- C. Centrifugation
- D. Chromatography
Explanation: ***Correct: Densitometry*** - **Densitometry** is a quantification technique used to measure the optical density or darkness of a material - It quantifies the amount of protein in a gel or blot **after separation has already occurred** by other methods - It does **not separate or purify** proteins; it only measures and quantifies them - Used for analyzing results from electrophoresis, Western blots, and similar techniques *Incorrect: Electrophoresis* - Separates proteins based on **charge, size, or shape** when subjected to an electric field through a matrix - Common techniques include SDS-PAGE (by molecular weight), IEF (by isoelectric point), and native PAGE - Routinely used for both analytical separation and preparative purification of proteins *Incorrect: Centrifugation* - Separates proteins based on **molecular weight, shape, and density** using centrifugal forces - Used for initial clarification of cell lysates, separating organelles, and concentrating proteins - Differential and density gradient centrifugation are key purification techniques *Incorrect: Chromatography* - Encompasses multiple techniques: **ion-exchange** (charge), **size exclusion** (molecular weight), **affinity** (specific binding), and **hydrophobic interaction** chromatography - Highly effective for both analytical separation and purification of proteins to high purity - Gold standard for protein purification in biochemistry laboratories
Question 9: The technique shown in the image is:
- A. High performance liquid chromatography (Correct Answer)
- B. Haemoglobin electrophoresis
- C. Gel electrophoresis
- D. Tandem mass spectrometry
Explanation: ***High performance liquid chromatography*** - The image displays a **chromatogram** with distinct peaks labeled HbA1c, HbF, HbA0, and HbA2, separated based on their chemical properties. - This separation and detection method is characteristic of **High Performance Liquid Chromatography (HPLC)**, a technique used for quantifying different hemoglobin fractions. *Tandem mass spectrometry* - **Tandem mass spectrometry (MS/MS)** identifies compounds based on their mass-to-charge ratio and fragmentation patterns, which would look like mass spectra, not peaks on a time-based chromatogram. - While MS/MS is highly sensitive and specific, it doesn't produce the type of **elution profile** seen in the image. *Haemoglobin electrophoresis* - **Hemoglobin electrophoresis** separates hemoglobins based on their electrical charge, resulting in bands on a gel or a densitometric scan, not the **distinct chromatogram peaks** shown here. - While used for hemoglobin analysis, the visual representation is typically different, often displaying bands that reflect migration distance. *Gel electrophoresis* - **Gel electrophoresis** separates molecules, such as proteins or nucleic acids, by size and charge through a gel matrix, producing distinct **bands** that can be visualized. - This method would not produce the continuous **elution peaks over time** as observed in the provided graph, which indicates a liquid chromatography technique.
Question 10: The shown pattern in electrophoresis is due to:
- A. Charges (Correct Answer)
- B. Molecular weight
- C. Bound oxygen
- D. Hydrophobicity
Explanation: ***Charges*** - Electrophoresis separates molecules, such as **hemoglobin variants**, based on their **electrical charge** when placed in an electric field. - Different amino acid substitutions in hemoglobin lead to changes in net charge, causing them to migrate at different rates in the gel, as seen by the distinct bands for Hb A, Hb S/D, and Hb A2. *Molecular weight* - While molecular weight can influence migration in some electrophoretic techniques (e.g., SDS-PAGE), standard hemoglobin electrophoresis primarily separates based on **charge differences**, not molecular size. - All common hemoglobin variants have a very similar **molecular weight** (approximately 64,500 Da), so this factor would not effectively separate them into distinct bands. *Bound oxygen* - The amount of **bound oxygen** to hemoglobin does not significantly alter its overall electrical charge or molecular weight to cause distinct bands in electrophoresis. - Oxygen binding is a dynamic process and does not account for the **structural differences** in hemoglobin variants that lead to their electrophoretic separation. *Hydrophobicity* - Hydrophobicity is a characteristic of molecules, but it is not the primary principle by which standard **gel electrophoresis** separates hemoglobin variants. - Techniques like **hydrophobic interaction chromatography** would exploit hydrophobicity, not the electrophoretic gel shown.
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