Step 2: Priming DNA with RNA

The unwinding by helicase creates two template strands at each fork, so primase enzymes must work to prime both template strands for copying (Fig. 13b). The primers are made of RNA nucleotides and these nucleotides assist the next enzyme (DNA pol III) in attaching to the template DNA strands at various locations to read them. You can see that one primase enzyme at each replication fork is working in the same direction as the helicase and one primase is working away from the helicase. 

Figure 13b. Primase primes the template strands with RNA primers. In this figure, the primase enzymes placed primers soon after helicase opened up the DNA to allow for reading of the template strands. Credit: M. Sutter, 2024.