Question

Assertion (A): M13 bacteriophage uses rolling circle mechanism for DNA replication.
Reason (R): During rolling circle mechanism, double stranded replicative form of DNA is formed which replicates until there are about its hundred copies in the host bacterial cell.

• A. Both Assertion (A) and Reason (R) are true and the Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but the Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

M13 is highly popular as a cloning vector because its replication cycle allows researchers to easily recover either single-stranded DNA (which is packaged into budding phages) or double-stranded RF DNA (which behaves like a plasmid).

Answer 1

The Correct Option is A

Step 1: Evaluating the Assertion (A)
M13 is a filamentous bacteriophage containing a single-stranded circular DNA genome ($+$ strand). Upon entry into the E. coli host cell, the single-stranded DNA is converted into a double-stranded replicative form (RF) DNA by host enzymes. This dsRF DNA replicates using the rolling circle replication mechanism to amplify the viral genome. Thus, Assertion (A) is true.

Step 2: Evaluating the Reason (R)
During rolling circle replication, the host cell synthesizes the complementary strand, forming the dsRF. This replicative form replicates continuously to yield approximately $100 - 200$ copies of dsRF per host cell. Thus, Reason (R) is true.

Step 3: Logical Connection
The rolling circle replication mechanism is the specific biological process that allows the virus to produce its double-stranded replicative forms and amplify its genome copies. Hence, R is the correct biological explanation for how M13 replicates its circular DNA inside host cells.

Step 4: Conclusion
Both are true, and R is the correct explanation of A, matching option (A). Final Answer: (A)