DOUBLE CHECK ANSWER What is the major mechanistic difference

DOUBLE CHECK ANSWER

What is the major mechanistic difference in purine and pyrimidine biosynthesis?

A. Purines are much simpler than pyrimidines

B. Pyrimidines utilize amino acids during synthesis

C. Purines are synthesized on the ring of ribose

D. IMP is formed in both pathways

E. Pyrimidine synthesis is performed before purine

Solution

Answer:

C. Purines are synthesized on the ring of ribose.

Pyrimidine ring is synthesized as orotate and attached to ribose phosphate and later is converted to common pyrimidine nucleotides.

Purine Synthesis: PRPP is the precursor on which the ring is added

Pyrimidine Synthesis: The ring is made first, then PRPP is added.

D. * IMP is formed in the purine synthesis pathway only.

E. Pyrimidine synthesis is performed before purine

Note: the contrast with purine synthesis in which a nucleotide is formed first while pyrimidines are first synthesized as the free base.