Final answer:
DnaA-ATP is the protein that opens dsDNA at the oriC during the initiation of DNA replication, not DnaB helicase, which requires the DNA to be initially unwound by DnaA-ATP.
Step-by-step explanation:
The protein that opens double-stranded DNA (dsDNA) at the oriC is DnaA-ATP. Unlike DnaB helicase, which cannot initiate the opening of dsDNA, DnaA in its ATP-bound form binds to certain sequences within the oriC and causes a local unwinding of the DNA. This process is essential for the initiation of DNA replication. Once DnaA has bound and the DNA is locally unwound, DnaB helicase can then bind and continue to unwind the DNA strands, allowing replication to proceed. Subsequent to unwinding by DnaB helicase, single-stranded binding proteins stabilize the unwound DNA strands to prevent them from reannealing, and topoisomerase works to relieve the tension caused by unwinding. The primase synthesizes an RNA primer for DNA polymerase to begin synthesizing new DNA strands.