Now this is the primer. And the primer basically contains the sequence of nucleotides. And on the final nucleotide, we have the sugar with a base that contains this free three prime hydroxyl group. So if we are to label these carbons here, this is carbon number one, carbon number two, carbon number three, four and five. And so this three position carbon contains a free hydroxyl group that can nucleophilically attack this phosphorus atom of this triphosphate group. And this is the phosphodiastic bond that is formed that we spoke about earlier.