A flow chart shows the reactants that are required for the 10 steps in the biosyntiesis of lMP, the first purine nucleotide formed de novo (Figure 14-7). Note that PRPP participates in the initial reaction and that the purine ring is constructed on phosphoribose. The 5-membered imidazole ring (5-aminoimidazole ribotide) is first synthesized. Successive reactions result in the formation of IMP, which is a complete purine nucleotide. IMP occurs at a crossroad in the pathway of purine biosynthesis. Separate pathways lead to the formation of AMP and GMP.