Answer:
The purine ring is built onto ribose-5-phosphate of PRPP for its de-novo nucleotide biosynthesis, while the ring structure of the pyrimidine bases are synthesized separately and then coupled to ribose-5-phosphate via the C-N glycosidic bond.
Step-by-step explanation:
Purines are produced as bases attached to the ribose 5-phosphate (pentose sugar). The adenine and guanine nucleotides derive from inosine monophosphate (IMP), which is synthesized on an existing ribose-phosphate. Thus, purine bases are built on a ribose sugar that is directly attached to the pyrimidine ring. On the other hand, pyrimidines are produced from the carbamoyl phosphate precursor. In this case, the ribose-5-phosphate pentose sugar is attached after the pyrimidine ring is made.