Regulation of Gene Expression — MCQs
Which of the following bacteria produces a toxin that inhibits protein synthesis?
Which is the correct sequence of steps in isolating desirable protein using recombinant DNA technology? 1. Expression of protein and lysis of the bacterial cell 2. Incorporation of genes into bacteria 3. SDS PAGE 4. Protein elution 5. Column chromatography
What is the primary function of the sigma subunit of prokaryotic RNA polymerase?
Which statement is false regarding the LAC OPERON MODEL?
Which of the following bacteria is microaerophilic?
Which bacterium is particularly notorious for producing extended-spectrum beta-lactamases (ESBLs) and carbapenemases, contributing to significant antibiotic resistance in hospital settings?
The process of attenuation can be achieved by all except:
Which of the following statements is true regarding the sigma factor?
Bacteria can transfer genetic information through all of the following methods except:
Which type of RNA is primarily involved in gene silencing?
Regulation of Gene Expression Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following bacteria produces a toxin that inhibits protein synthesis?
- A. Corynebacterium diphtheriae (Correct Answer)
- B. Bacillus anthracis
- C. Staphylococcus aureus
- D. Vibrio cholerae
Explanation: ***Corynebacterium diphtheriae*** - This bacterium produces the **diphtheria toxin**, which is an **A-B toxin**. - The **A subunit** of the toxin inhibits **protein synthesis** by inactivating elongation factor-2 (EF-2), leading to cell death. *Bacillus anthracis* - Produces **anthrax toxin**, which consists of three components: protective antigen, edema factor, and lethal factor. - The **lethal factor** cleaves specific protein kinases, while the **edema factor** acts as an adenylate cyclase, disturbing cell water balance, but neither directly inhibits protein synthesis in the same manner as diphtheria toxin. *Staphylococcus aureus* - Produces numerous toxins, including **superantigens** (like toxic shock syndrome toxin-1) and **exfoliative toxins**, which cause scaly skin, and **alpha-toxin**, which forms pores in cell membranes. - These toxins have diverse mechanisms of action, but none primarily inhibit **protein synthesis**. *Vibrio cholerae* - Produces **cholera toxin**, an **A-B toxin**, which acts on intestinal cells. - The **A subunit** irreversibly activates **adenylate cyclase**, leading to increased cyclic AMP (cAMP) levels and massive efflux of water and electrolytes, causing severe diarrhea, but it does not inhibit protein synthesis.
Question 2: Which is the correct sequence of steps in isolating desirable protein using recombinant DNA technology? 1. Expression of protein and lysis of the bacterial cell 2. Incorporation of genes into bacteria 3. SDS PAGE 4. Protein elution 5. Column chromatography
- A. 2,1,3,5,4 (Correct Answer)
- B. 2,4,5,3,1
- C. 1,2,4,3,5
- D. 1,5,2,4,3
Explanation: ***2,1,3,5,4*** - This sequence accurately reflects the typical order of operations in **recombinant protein isolation**: first, the gene is introduced into bacteria, then protein is expressed and cells lysed, followed by **SDS-PAGE as an intermediate quality check** to confirm protein expression before proceeding to purification steps (column chromatography and elution). - The process starts with gene incorporation, includes an analytical checkpoint after lysis, and ends with purified protein elution. *2,4,5,3,1* - This sequence is incorrect because **protein elution (4)** and **column chromatography (5)** are purification steps that occur *after* protein expression and cell lysis. - **Lysis (1)** cannot happen after elution, as cells must be lysed first to release the protein for purification. *1,2,4,3,5* - This sequence is incorrect because **expression and lysis (1)** must occur *after* the gene has been **incorporated into bacteria (2)** - the gene must be present before it can be expressed. - Additionally, **protein elution (4)** should follow **column chromatography (5)**, as elution is the step where protein is collected from the chromatography column. *1,5,2,4,3* - This sequence is incorrect because **incorporation of genes (2)** must be the first step - the gene needs to be in the bacteria before any expression, lysis, or purification can occur. - Starting with **expression and lysis (1)** before gene incorporation is impossible.
Question 3: What is the primary function of the sigma subunit of prokaryotic RNA polymerase?
- A. Is inhibited by α-amanitin
- B. Specifically recognizes the promoter site (Correct Answer)
- C. Is part of the core enzyme
- D. Inhibits the activity of RNA polymerase
Explanation: ***Specifically recognizes the promoter site*** - The **sigma subunit** is crucial for **transcription initiation** in prokaryotes, enabling the RNA polymerase holoenzyme to specifically bind to **promoter sequences** on the DNA. - This specific recognition ensures that transcription begins at the correct start site, making it a key component for accurate gene expression. *Inhibits the activity of RNA polymerase* - The sigma subunit does not inhibit RNA polymerase; rather, it **facilitates** its activity by guiding it to the correct transcription start sites. - After initiation, the sigma subunit often **dissociates** from the core enzyme, allowing the core polymerase to proceed with elongation. *Is inhibited by α-amanitin* - **α-amanitin** is a toxin that primarily inhibits **eukaryotic RNA polymerases**, particularly RNA polymerase II, and is not known to inhibit prokaryotic RNA polymerase or its sigma subunit. - Prokaryotic RNA polymerase has a different structure and mechanism, rendering it **insensitive** to α-amanitin. *Is part of the core enzyme* - The sigma subunit is **not considered part of the core enzyme**; the core enzyme consists of the α, β, β', and ω subunits. - Together with the core enzyme, the sigma subunit forms the **RNA polymerase holoenzyme**, which is responsible for initiating transcription.
Question 4: Which statement is false regarding the LAC OPERON MODEL?
- A. A gene= GALACTOSIDE ACETYLASE
- B. PROMOTER IS A TRANS ACTING FACTOR (Correct Answer)
- C. Y gene = GALACTOSIDE PERMEASE
- D. CAMP-CAP COMPLEX IS A POSITIVE REGULATOR
Explanation: ***PROMOTER IS A TRANS ACTING FACTOR*** - A **promoter** is a region of DNA that initiates transcription of a particular gene. It is a **cis-acting element**, meaning it influences the expression of genes on the same DNA molecule. - **Trans-acting factors** are molecules (like proteins) that can diffuse through the cell and act on any DNA molecule, not just the one they were transcribed from. *CAMP-CAP COMPLEX IS A POSITIVE REGULATOR* - The **cAMP-CAP complex** (also known as the **cAMP-CRP complex**) binds to the promoter region of the *lac* operon and **activates transcription** when glucose levels are low and lactose is present. - This binding enhances the affinity of RNA polymerase for the promoter, thus acting as a **positive regulator** of gene expression. *Y gene = GALACTOSIDE PERMEASE* - The ***lacY* gene** in the *lac* operon codes for **galactoside permease**. - Galactoside permease is responsible for transporting **lactose into the bacterial cell**. *A gene= GALACTOSIDE ACETYLASE* - The ***lacA* gene** in the *lac* operon codes for **galactoside acetylase** (**thiogalactoside transacetylase**). - This enzyme's precise physiological role in lactose metabolism is **not fully understood**, but it may help detoxify cells by acetylating non-metabolizable galactosides.
Question 5: Which of the following bacteria is microaerophilic?
- A. Campylobacter (Correct Answer)
- B. Pseudomonas
- C. Salmonella
- D. Vibrio cholerae
Explanation: ***Campylobacter*** - **Campylobacter jejuni** is a classic example of a microaerophilic bacterium, thriving in environments with **reduced oxygen (5-10% O2)** and **increased CO2 (5-10%)**. - This specific atmospheric requirement is crucial for its **growth** and **virulence**, often leading to gastroenteritis. *Vibrio cholerae* - **Vibrio cholerae** is a **facultative anaerobe**, meaning it can grow in both the presence and absence of oxygen. - It does not require low oxygen environments; optimal growth occurs **aerobically**. *Pseudomonas* - **Pseudomonas aeruginosa** is an **obligate aerobe**; it requires oxygen for respiration and growth. - It uses oxygen as the **final electron acceptor** in its electron transport chain. *Salmonella* - **Salmonella enterica** is a **facultative anaerobe**, capable of switching between aerobic and anaerobic respiration. - It can grow in the presence of oxygen, as well as in **anaerobic conditions** by fermenting sugars.
Question 6: Which bacterium is particularly notorious for producing extended-spectrum beta-lactamases (ESBLs) and carbapenemases, contributing to significant antibiotic resistance in hospital settings?
- A. Pseudomonas
- B. Staphylococcus
- C. Streptococcus
- D. Klebsiella (Correct Answer)
Explanation: ***Klebsiella*** - **_Klebsiella pneumoniae_** is particularly well-known for producing a wide range of beta-lactamases, including both **Extended-Spectrum Beta-Lactamases (ESBLs)** and **carbapenemases**, making it a significant cause of **hospital-acquired infections** that are difficult to treat. - The presence of these enzymes allows it to hydrolyze and inactivate many commonly used antibiotics, leading to **multidrug resistance**. *Pseudomonas* - While *Pseudomonas aeruginosa* can produce various resistance mechanisms, including **carbapenemases (e.g., VIM, IMP)** and **AmpC beta-lactamases**, it is not as frequently associated with ESBL production as *Klebsiella*. - *Pseudomonas* is notorious for its intrinsic resistance to many antibiotics and its ability to form **biofilms**. *Staphylococcus* - **_Staphylococcus aureus_** is well-known for **Methicillin-Resistant *Staphylococcus aureus* (MRSA)**, which is resistant to beta-lactam antibiotics due to the acquisition of the **_mecA_ gene**, encoding altered penicillin-binding proteins. - However, *Staphylococcus* species are not commonly associated with the production of ESBLs or carbapenemases in the same way Gram-negative bacteria like *Klebsiella* are. *Streptococcus* - While some streptococcal species can develop resistance to antibiotics like **penicillin and macrolides**, they are not typically associated with the production of ESBLs or carbapenemases. - Resistance in *Streptococcus pneumoniae*, for example, often involves **alterations in penicillin-binding proteins**, similar to MRSA.
Question 7: The process of attenuation can be achieved by all except:
- A. Repeated cultures in artificial media
- B. Serial passage in an experimental host (Correct Answer)
- C. Growing bacteria in unconventional host
- D. Growing bacteria in adverse environment
Explanation: ***Serial passage in an experimental host*** - This option is **too vague and non-specific** to reliably achieve attenuation. The term "experimental host" could refer to *any host used in laboratory settings*, including the **natural host** of the pathogen. - Serial passage in the **natural/susceptible host** may actually **maintain or increase virulence** rather than attenuate, as the pathogen continues to adapt to its preferred environment. - **Attenuation requires passage in *unnatural* or *unfavorable* conditions** - not just "an experimental host." *Repeated cultures in artificial media* - This method attenuates pathogens by forcing adaptation to an **in vitro environment**, leading to loss of virulence factors unnecessary for artificial growth but crucial for host infection. - **Examples:** *BCG vaccine* (attenuated *M. bovis* after 230 passages), *Sabin polio vaccine* (passage in monkey kidney cells). - Virulence genes become non-functional over many passages in cell-free media. *Growing bacteria in unconventional host* - Passage in a **non-natural host** (different species) causes loss of virulence factors specific to the original host. - **Example:** *Yellow fever 17D vaccine* (passage in chicken embryos), *measles vaccine* (passage in chicken embryo fibroblasts). - This is a **specific application** of serial passage that reliably achieves attenuation. *Growing bacteria in adverse environment* - Exposure to **suboptimal conditions** (unfavorable temperatures, pH, oxygen tension) causes genetic mutations or loss of virulence plasmids. - **Example:** *Live attenuated influenza vaccine* (cold-adapted strains grown at 25°C instead of 37°C). - Selects for strains prioritizing survival over pathogenesis.
Question 8: Which of the following statements is true regarding the sigma factor?
- A. It is a subunit of DNA polymerase.
- B. It is a subunit of RNA polymerase. (Correct Answer)
- C. It initiates DNA replication.
- D. It is a subunit of the 50s ribosome.
Explanation: ***It is a subunit of RNA polymerase.*** - The **sigma factor** is a crucial component of **bacterial RNA polymerase**, guiding it to specific promoter regions on the DNA. - It plays a vital role in **initiation of transcription** by recognizing and binding to the **-10 and -35 boxes** of the promoter. *It is a subunit of DNA polymerase.* - **DNA polymerase** is primarily involved in **DNA replication and repair**, not transcription. - Its subunits, such as the **beta clamp** or **alpha subunit**, are distinct from the sigma factor. *It initiates DNA replication.* - **DNA replication** is initiated by **DNA helicases** unwinding the double helix and **primase** synthesizing RNA primers. - The sigma factor's role is in **transcription**, the synthesis of RNA from a DNA template. *It is a subunit of the 50s ribosome.* - The **50S ribosomal subunit** is a component of the **ribosome**, responsible for **peptide bond formation** during translation. - Its subunits are ribosomal proteins and ribosomal RNA molecules, not the sigma factor.
Question 9: Bacteria can transfer genetic information through all of the following methods except:
- A. Through conjugation
- B. Through bacteriophages
- C. Through meiotic recombination (Correct Answer)
- D. By taking up soluble DNA fragments across their cell wall from other species
Explanation: ***Through meiotic recombination*** - **Meiotic recombination** (crossing over) is a process that occurs during **meiosis**, which is cell division specific to sexually reproducing eukaryotes. - Bacteria reproduce asexually through **binary fission** and do not undergo meiosis, thus meiotic recombination is not a mechanism for genetic transfer in bacteria. *Through conjugation* - **Conjugation** is a direct transfer of genetic material (usually a **plasmid**) from one bacterium to another through a **pilus**. - This process allows for the spread of genes, such as those conferring **antibiotic resistance**. *Through bacteriophages* - Genetic transfer via **bacteriophages** is known as **transduction**. - Bacteriophages are **viruses that infect bacteria**, occasionally transferring bacterial DNA from one host to another during their reproductive cycle. *By taking up soluble DNA fragments across their cell wall from other species* - This process is called **transformation**, where bacteria directly take up **naked DNA fragments** from their environment. - Transformation can occur naturally in some bacterial species or can be induced in laboratory settings, allowing for the acquisition of new genetic traits.
Question 10: Which type of RNA is primarily involved in gene silencing?
- A. rRNA
- B. tRNA
- C. miRNA (Correct Answer)
- D. mRNA
Explanation: ***miRNA*** - **miRNA** (microRNA) is a small non-coding RNA molecule that plays a crucial role in **post-transcriptional regulation of gene expression**. - It functions by binding to complementary messenger RNA (mRNA) molecules, leading to **mRNA degradation** or **inhibition of translation**, thereby silencing genes. - miRNA is the primary RNA type involved in **gene silencing** through the RNA interference (RNAi) pathway. *rRNA* - **rRNA** (ribosomal RNA) is a primary component of **ribosomes**, the cellular machinery responsible for protein synthesis. - Its main function is to **catalyze peptide bond formation** and provide structural integrity to the ribosome, not gene silencing. *tRNA* - **tRNA** (transfer RNA) is responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - It acts as an adapter molecule, translating the **genetic code** in mRNA into an amino acid sequence. *mRNA* - **mRNA** (messenger RNA) carries genetic information from **DNA to ribosomes** for protein synthesis. - While mRNA can be targeted by gene silencing mechanisms (like miRNA), it is not the RNA type that performs the silencing function itself.
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