Step 1: Initiating replication at the Ori

Helicase enzymes bind to the ori sequences (“ori” means origin of replication) and start to unwind the double stranded DNA. This establishes two replication forks (Fig. 13a), or spots where the DNA is unwound and replication takes place. These forks move away from each other. The unwinding exposes single strands of DNA (black lines on top and bottom in Fig. 13a) that are the template for building a new strand. Topoisomerase can be seen working to relieve tension ahead of the replication fork in Figure 13a. Note that in all images, the shapes of the enzymes are randomly chosen and not reflective of the real shape of the enzymes.

Figure 13a. Helicase (triangle) and Topoisomerase (oval) work together to expose single strand DNA templates. Notice that the top template DNA is 3’ to 5’ when looking left to right and the bottom strand is 5’ to 3’ from left to right. Each strand of DNA (the top and the bottom) is called a “template” strand. Credit: M. Sutter, 2024.