In bidirectional DNA replication, how many replication forks are present in a single replication bubble?

Introduction / Context:
Replication often initiates at an origin, creating a bubble from which synthesis proceeds in both directions. This tests knowledge of fork number in such a structure.

Given Data / Assumptions:

• Replication is bidirectional in most bacteria and eukaryotes.
• A bubble forms as DNA unwinds locally around the origin.

Concept / Approach:
A replication fork is the Y-shaped junction where unwinding and synthesis occur. Bidirectional replication implies two forks moving away from the origin in opposite directions.

Step-by-Step Solution:
1) Origin fires and DNA melts locally.2) Two forks assemble on either side of the origin.3) Forks move outward, enlarging the bubble until they meet neighboring forks.

Verification / Alternative check:
Autoradiography and modern sequencing-based assays detect symmetric progression from origins, consistent with two forks per bubble.

Why Other Options Are Wrong:
One fork would imply unidirectional replication; three or four forks do not describe a single bubble from one origin.

Common Pitfalls:
Confusing multiple nearby bubbles (from multiple origins) with the number of forks within one bubble.

Final Answer:
Two replication forks