Question

Given below are two statements
Statement I: Self-replicating, circular, extrachromosomal DNA present in bacteria is known as Plasmid
Statement II: Copy number of Plasmids is fixed in a bacterial cell
In light of the above statements, choose the correct answer from the options given below:

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Plasmid copy number is a dynamic equilibrium. High-copy plasmids replicate in a relaxed fashion and can be artificially boosted (amplified) in the laboratory using protein synthesis inhibitors like chloramphenicol.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Plasmids are common extrachromosomal genetic elements in bacteria. They replicate independently of the host chromosome and often carry accessory genes that confer selective advantages, such as antibiotic resistance.

Step 2: Detailed Explanation:
Statement I Analysis: A plasmid is classically defined as a double-stranded, circular DNA molecule that exists and replicates independently of the host bacterial chromosome. This definition is accurate, so Statement I is true.
Statement II Analysis: The term "copy number" refers to the average number of molecules of a specific plasmid maintained per host cell.
While plasmids are generally categorized as low-copy-number (stringent replication control, 1–5 copies) or high-copy-number (relaxed replication control, 10–100+ copies), this value is not absolutely fixed.
The copy number can vary depending on several factors, including:
1. The physiological state of the host cell.
2. Environmental growth conditions, such as temperature and nutrient availability.
3. The presence of specific replication inhibitors or selective agents.
4. Mutations in the plasmid-encoded replication control genes.
Because copy number fluctuates based on biological and physical parameters, Statement II is false.

Step 3: Final Answer:
Hence, Statement I is true but Statement II is false.