In bacteria, which factors initiate replication at the origin and which region is melted to permit loading of the helicase?

• A. DnaA binds oriC, oligomerizes, and melts the AT-rich region to allow loading of the DnaB helicase by DnaC.
• B. DnaC binds oriC and melts an AT-rich region to load DnaB helicase without DnaA.
• C. DnaA binds oriC but does not melt DNA; helicase loads with DnaC later.
• D. DnaB helicase loads onto DNA first, then DnaA oligomerizes.

Answer: (A)

DNA replication starts in bacteria when the initiator DnaA binds the origin oriC, forms an oligomer, and destabilizes the adjacent AT-rich segment called the DNA unwinding element (DUE). That melting creates the single-stranded template needed for loading the helicase. The helicase DnaB cannot load on its own; it requires the loader DnaC, which escorts DnaB to the melted origin and facilitates its loading onto the DNA. This sequence—DnaA binding and oligomerizing, melting the AT-rich DUE, then DnaC delivering DnaB for loading—is why the statement that best fits is the one describing DnaA opening the origin by melting the AT-rich region to permit loading of DnaB helicase by DnaC.