Bacterial Gene Expression — MCQs

10 questions

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

R-factor in bacteria is transferred by:

The Central Dogma of molecular biology includes all except?

Which one of the following antibiotics inhibits transpeptidation in bacterial cell wall synthesis?

False statements are:

The Shine-Dalgarno sequence is primarily associated with which biological process?

Which of the following statements is true regarding the sigma factor?

Bacteria can transfer genetic information through all of the following methods except:

False regarding bacterial plasmids is:

Q10

All the following terms are used to describe bacterial chromosome Except

Bacterial Gene Expression Indian Medical PG Practice Questions and MCQs

Question 1: 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 2: R-factor in bacteria is transferred by:

A. Transduction
B. Conjugation (Correct Answer)
C. Uptake of naked DNA
D. Genetic transfer from parent to offspring

Explanation: ***Conjugation*** - **R-factors** (resistance factors) are plasmids carrying genes for antibiotic resistance, and their primary mode of transfer between bacteria is through **conjugation**. - **Conjugation** involves direct cell-to-cell contact through a pilus, allowing the transfer of the plasmid DNA from a donor bacterium to a recipient bacterium. *Transduction* - **Transduction** involves the transfer of bacterial DNA by a **bacteriophage (virus)**, which can accidentally package bacterial genes instead of viral ones. - While it can transfer resistance genes, it is not the primary mechanism for R-factor spread across diverse bacterial species in the way conjugation is. *Uptake of naked DNA* - The **uptake of naked DNA** from the environment is known as **transformation**, which primarily occurs when bacteria are in a specific physiological state (competence). - This mechanism is less efficient for widespread R-factor dissemination compared to conjugation, which involves active transfer. *Genetic transfer from parent to offspring* - **Genetic transfer from parent to offspring** (vertical gene transfer) refers to the transmission of genetic material during cell division. - While R-factors are replicated and passed on to daughter cells, this does not explain their horizontal spread to new, unrelated bacteria.

Question 3: The Central Dogma of molecular biology includes all except?

A. Reverse transcription (Correct Answer)
B. Transcription
C. RNA replication
D. Translation

Explanation: ***Reverse transcription*** - The **original Central Dogma** as proposed by Francis Crick described the unidirectional flow of genetic information: **DNA → RNA → Protein**. - **Reverse transcription** (RNA → DNA) was **not part of the original Central Dogma** and was only discovered later with the identification of **reverse transcriptase enzyme** in retroviruses by Baltimore and Temin (1970). - While reverse transcription is now recognized as an important biological process, it represents an **exception to the classical Central Dogma** rather than a core component. *Transcription* - **Transcription** (DNA → RNA) is a **fundamental process** within the Central Dogma. - It involves synthesis of RNA from a DNA template and is the first step in gene expression in all living organisms. *Translation* - **Translation** (RNA → Protein) is the **second major step** of the Central Dogma. - This process occurs on ribosomes where mRNA is decoded into a specific sequence of amino acids to form proteins. *RNA replication* - **RNA replication** (RNA → RNA), while primarily seen in RNA viruses, is considered part of the **expanded Central Dogma**. - It represents one of the possible flows of genetic information in biological systems, though not universal to all organisms.

Question 4: Which one of the following antibiotics inhibits transpeptidation in bacterial cell wall synthesis?

A. Penicillin (Correct Answer)
B. Chloramphenicol
C. Amphotericin
D. Vancomycin

Explanation: ***Penicillin*** - Penicillin is a **beta-lactam antibiotic** that targets bacterial cell wall synthesis by inhibiting the enzyme **transpeptidase** (penicillin-binding proteins). - This inhibition prevents the cross-linking of **peptidoglycan strands**, leading to a weakened cell wall and subsequent bacterial lysis. *Chloramphenicol* - **Chloramphenicol** inhibits bacterial protein synthesis by binding to the **50S ribosomal subunit**, thereby preventing peptide bond formation (peptidyl transferase activity). - It does not act on the bacterial cell wall. *Vancomycin* - **Vancomycin** is a **glycopeptide antibiotic** that inhibits bacterial cell wall synthesis by binding to the **D-Ala-D-Ala terminus** of peptidoglycan precursors. - It prevents the **transglycosylation and transpeptidation steps** by blocking substrate access, but it does not directly inhibit the transpeptidase enzyme itself like beta-lactams do. - Its mechanism is distinct from penicillin's direct enzyme inhibition. *Amphotericin* - **Amphotericin B** is an **antifungal agent** that targets the fungal cell membrane by binding to **ergosterol**, forming pores that disrupt membrane integrity. - It has no activity against bacterial cell wall synthesis.

Question 5: False statements are:

A. DNA replication proceeds in one direction
B. Lagging strand is synthesized by RNA primase
C. All of the options (Correct Answer)
D. Bacteria have multiple origins of replication

Explanation: ***All of the options*** - All statements are **false**. DNA replication proceeds **bidirectionally**, bacteria typically have a **single origin of replication**, and the lagging strand is synthesized by **DNA polymerase** after an RNA primer is laid down by **RNA primase**. *DNA replication proceeds in one direction* - This statement is **false** because **DNA replication** is a **bidirectional process**, meaning it proceeds in both directions from the origin of replication. - Replication forks move away from the **origin** on both sides, unraveling the DNA and synthesizing new strands. *Bacteria have multiple origins of replication* - This statement is **false**. Most **bacteria** (prokaryotes) have a **single origin of replication** (oriC) on their circular chromosome. - In contrast, **eukaryotes** have **multiple origins of replication** on their linear chromosomes to replicate their much larger genomes efficiently. - While rare exceptions exist in some bacterial species, the general rule for bacterial DNA replication is a single origin. *Lagging strand is synthesized by RNA primase* - This statement is **false**. The **lagging strand** is primarily synthesized by **DNA polymerase III** (in prokaryotes) or **DNA polymerase δ** (in eukaryotes). - **RNA primase** is responsible for synthesizing short **RNA primers** that provide a starting point for DNA polymerase, but it does not synthesize the entire lagging strand itself.

Question 6: The Shine-Dalgarno sequence is primarily associated with which biological process?

A. Transcription
B. Translation (Correct Answer)
C. DNA replication
D. RNA splicing

Explanation: ***Translation*** - The **Shine-Dalgarno sequence** is a **ribosome-binding site** in prokaryotic messenger RNA (mRNA) that helps recruit the ribosome to the mRNA to initiate protein synthesis. - Its interaction with the **16S rRNA** of the small ribosomal subunit positions the start codon (AUG) correctly for **translation initiation**. - This sequence is located approximately **8 base pairs upstream** of the start codon in bacterial mRNA. *Transcription* - **Transcription** is the process of synthesizing RNA from a DNA template. - It involves elements like **promoters** and **enhancers**, not the Shine-Dalgarno sequence. *DNA replication* - **DNA replication** is the process by which DNA makes a copy of itself. - This process involves origins of replication, helicases, and DNA polymerases, with no role for the Shine-Dalgarno sequence. *RNA splicing* - **RNA splicing** is a eukaryotic process that removes introns from pre-mRNA. - The Shine-Dalgarno sequence is found in **prokaryotes**, which lack splicing machinery and introns.

Question 7: 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 8: 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 9: False regarding bacterial plasmids is:

A. Extrachromosomal
B. Transmission to different species
C. Eliminated by treating with radiation
D. Can cause lysogenic conversion (Correct Answer)

Explanation: ***Can cause lysogenic conversion*** - **Lysogenic conversion** is a phenomenon caused by **bacteriophages**, which are viruses that infect bacteria, not by plasmids directly. - It involves the integration of a **phage genome** into the bacterial chromosome, altering the bacterium's phenotype (e.g., toxin production). *Extrachromosomal* - Bacterial plasmids are indeed **extrachromosomal DNA molecules**, meaning they exist independently of the bacterial chromosome. - This characteristic allows them to be easily transferred between bacteria. *Transmission of different species* - Plasmids can be transmitted horizontally between bacteria, even across **different species**, through mechanisms like **conjugation**, **transformation**, or **transduction**. - This interspecies transmission is a major factor in the spread of **antibiotic resistance**. *Eliminated by treating with radiation* - Plasmids, like all DNA, can be eliminated or degraded by treatments such as **radiation** (e.g., UV) or certain chemicals. - Such treatments disrupt the plasmid DNA structure, preventing its replication or function.

Question 10: All the following terms are used to describe bacterial chromosome Except

A. Diploid (Correct Answer)
B. Circular
C. Nucleoid
D. Haploid

Explanation: ***Diploid*** - Bacterial chromosomes are typically **haploid**, meaning they contain only one copy of each gene, not two sets as in diploid organisms. - While bacteria can sometimes have multiple copies of their chromosome, these copies are usually identical replicas and do not represent distinct sets of genetic information. *Circular* - The bacterial chromosome is characteristically a **single, closed circular DNA molecule**. - This circular structure distinguishes bacterial chromosomes from the linear chromosomes found in eukaryotes. *Nucleoid* - The bacterial chromosome is located in a region of the cytoplasm called the **nucleoid**, as bacteria lack a membrane-bound nucleus. - The nucleoid is where the genetic material is condensed and organized within the bacterial cell. *Haploid* - Bacteria are essentially **haploid organisms**, possessing a single set of chromosomes. - This means they have only one allele for each gene, which can lead to rapid expression of mutations.

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