Question

One way in which the anabolism of pyrimidine nucleotides differs from that of purine nucleotides is that

a)there is no feedback inhibition in the anabolism of pyrimidine nucleotides, while feedback inhibition is important in the synthesis of purine nucleotides
b)glycine appears in the pathway for formation of pyrimidine nucleotides but not in that for the formation of purine nucleotides
c)argininosuccinate is a key intermediate in pyrimidine biosynthesis
d)the pyrimidine ring is assembled before being bonded to the ribose phosphate, whereas the purine ring is bonded to the ribose phosphate as it is formed

Answer 1

The anabolism of pyrimidine nucleotides differs from that of purine nucleotides in that the pyrimidine ring is assembled first before attaching to ribose phosphate, whereas the purine ring is built step-by-step directly on the ribose sugar. Therefore, the correct option is D.

Step-by-step explanation:

One way in which the anabolism of pyrimidine nucleotides differs from that of purine nucleotides is that the pyrimidine ring is assembled before being bonded to the ribose phosphate, whereas the purine ring is bonded to the ribose phosphate as it is formed. In pyrimidine nucleotide biosynthesis, the six-membered pyrimidine ring is made first from carbon dioxide and ammonia to form carbamoyl phosphate.

After several steps, including combination with aspartate and ring closure, the orotic acid is attached to ribose to yield orotidylic acid, which is eventually converted into the pyrimidine nucleotides UMP, CMP, and TMP. Conversely, in purine nucleotide biosynthesis, the purine ring is assembled step-by-step directly on the ribose sugar, with contributions from amino acids like aspartate and glycine, and other molecules like glutamine, CO2, and formate.