In bacteria such as E. coli, what is the specific chromosomal site where bidirectional DNA replication initiates?

Introduction / Context:
Bacterial chromosomes begin replication at a defined locus to ensure complete and timely duplication before cell division. In E. coli, this locus is well characterized and called oriC.

Given Data / Assumptions:

• Single circular chromosome in typical bacteria.
• Initiator protein DnaA binds specific repeats to start replication.
• Replication proceeds bidirectionally from the origin.

Concept / Approach:
At oriC, DnaA boxes and AT-rich DNA unwinding elements facilitate local strand separation. Helicase loading and priming establish two forks that move in opposite directions until they converge near ter sites.

Step-by-Step Solution:

Verification / Alternative check:
Genetic and biochemical mapping shows that deletion or mutation of oriC blocks initiation, whereas ectopic insertion can create new initiation sites under controlled contexts.

Why Other Options Are Wrong:

• Promoter: Site for RNA polymerase initiation of transcription, not DNA replication.
• Start codon: Translation initiation on mRNA, not chromosomal replication.
• Terminator (ter): Replication termination, not initiation.
• Ribosome binding site: A translation feature, unrelated to DNA replication origin.

Common Pitfalls:
Confusing regulatory sites for transcription or translation with those for DNA replication, or using the generic term origin (ori) without the specific bacterial designation oriC.

Final Answer:
oriC (origin of replication).