Final answer:
The DNA double helix starts to come apart at the origin of replication, which is a specific nucleotide sequence where replication begins. Helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs, resulting in the formation of replication forks. Two replication forks are formed at the origin of replication for bidirectional replication.
Step-by-step explanation:
The DNA double helix starts to come apart at the origin of replication. This is the specific nucleotide sequence where replication begins. In E. coli, the origin of replication is approximately 245 base pairs long and rich in AT sequences. The origin of replication is recognized by certain proteins, and an enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs. As the DNA opens up, Y-shaped structures called replication forks are formed, and two replication forks are formed at the origin of replication, allowing for bidirectional replication.