By way of review:

Fig. 5 - Folate needed for all purines plus thymidylate

Purines and pyrimidines each have common biosynthetic pathways, with the individual bases being formed by terminal modifications. In the purine pathway, N⁵, N¹⁰-methenyl THF are needed early on and are thus required for all purines (even ones like IMP that are not contained in DNA/RNA). After their utilization in purine biosynthesis, both N⁵, N¹⁰-methenyl THF and N¹⁰-formyl THF are recycled as THF (see figure above). Pyrimidines, on the other hand, do not require folate for the "core" biosynthetic reactions. It is only the very terminal step in the formation of dTMP that requires folate in the form of N⁵,N¹⁰-methylene THF. More about this reaction in the next question...