A replication bubble is made up of two growing forks, each one served by a single DNA polymerase complex, and so we need only look at a single fork to get an idea what’s going on. Again, DNA polymerase faces several problems here. First of all, it has to know where to start. As it turns out, there are certain DNA sequences that serve as signals to polymerase and other proteins, which bind to the DNA in a complicated way so as to create an open complex that begins the bubble. The details of this needn’t concern us right now. The second problem is that DNA polymerase has a minor, embarrassing design flaw—it can’t initiate nucleic acid strands! In other words, this enzyme, responsible for the replication of our entire genome, can’t lay down even the first nucleotide unless it has a primer, a chain of nucleotides already started for it. So the DNA polymerase complex contains an enzyme activity called the primase, which lays down a short run of RNA base-paired to the DNA. Once primase gives it a boost, DNA polymerase can proceed to form a daughter strand by matching nucleotides to the parent strand and linking them together with ligase.