Metabolism of Pyrimidine

Ans.B

Nucleosides are derivatives of purines and pyrimidines that have a sugar linked to ring nitrogen of a purine or pyrimidine

Numerals with a prime (eg, 2′ or 3′) distinguish atoms of the sugar from those of the heterocycle. The sugar in ribonucleosides is D-ribose, and in deoxyribonucleosides is 2-deoxy-D-ribose.

Both sugars are linked to the heterocycle by an a-N-glycosidic bond, almost always to the N-1 of a pyrimidine or to N-9 of a purine“.

Additional phosphoryl groups, ligated by acid anhydride bonds to the phosphoryl group of a mononucleotide, form nucleoside diphosphates and triphosphates

D i.e. Reye syndrome decrease cytosolic carbamoyl phosphate

Complex thiamine biosynthesis occurs in bacteria, some protozoans, plants, and fungi. The thiazole and pyrimidine moieties are biosynthesized separately and then combined to form thiamine monophosphate (ThMP) by the action of thiamine-phosphate synthase

Aspartate transcarbamylase (ATCase), the catalyst for the first reaction unique to pyrimidine biosynthesis, is a target of feedback regulation by two nucleotides triphosphates: cytidine triphosphate (CTP) and adenosine triphosphate. CTP, an end product of the pyrimidine biosynthetic pathway, inhibits ATCase, whereas the purine nucleotide ATP activates it (feed-forward regulation).

The major regulatory step in prokaryotes is the reaction catalyzed by aspartate transcarbamoylase (ATC) which is allosterically inhibited by CTP.

Ans. is Cytosine , Thymine And Uracil

• Purines: Two major purine bases found both in DNAs as well as RNAs are (l) Adenine (A) and (ii) Guanine (G).
• Pyrimidines: Three major pyrimidine bases are: (i) Cytosine (C), (ii) Uracil (U) and (iii) Thymine (T). Cytosine and
  uracil are found in RNAs and cytosine and Thymine are found in DNAs.
•  Uracil is not found in DNAs and thymine is not found in RNAs.