"A free nucleotide is like a single base floating around without a partner. So there would be A's, T's, C's, and G's with noone to hold hands with. Can you tell me which bases always stay together? Super! A's and T's and C's and G's. So if the DNA has unzipped and there are bases hanging out by itself, what do you think those free nucleotides might do? I hope you said that a free nucleotide would come in and attach itself to the correct base. "