Which plasmid replication mechanism is characterized by unidirectional replication around the circle, often generating a single-stranded DNA intermediate?

• A. Rolling-circle replication.
• B. Bidirectional theta replication.
• C. Linear replication.
• D. Conservative replication.

Answer: (A)

Rolling-circle replication starts when a nick is made in one strand of the circular plasmid, creating a free 3’ end that serves as the primer. DNA polymerase extends from that end, synthesizing a new strand while simultaneously displacing the original complementary strand as it goes around the circle. Because synthesis proceeds in one direction around the circle, replication is unidirectional. The displaced strand remains single-stranded for a while, forming a single-stranded DNA intermediate that can later serve as a template for synthesizing the complementary strand, yielding double-stranded plasmids. This combination of a nick-initiated, unidirectional copy around the circle and an ssDNA intermediate is the hallmark of rolling-circle replication, distinguishing it from theta (bidirectional) replication, linear replication, or other models.