Pyrimidine synthesis involves ?

Question

Pyrimidine synthesis involves all of the following, except:

a) Synthesis of the base while ribose is attached.

b) Stimulation of synthesis by purines.

c) An orotic acid intermediate.

d) Inhibition by pyrimidines.

e) All of these are features of pyrimidine synthesis.

Answer 1

a) Synthesis of the base while ribose is attached.

Answer 2

a). Is wrong  - ribose is attached to the orotate by PRPP forming OMP which is then converted to UMP.

The others are correct - The regulation of pyrimidine synthesis occurs mainly at the first step which is catalyzed by aspartate transcarbamoylase, ATCase. Inhibited by CTP and activated by ATP, ATCase is a multifunctional protein in mammalian cells.