In biosynthesis the base is assembled first and then attached to ribose.

In pyrimidine biosynthesis, the base is assembled first and then attached to ribose. Pyrimidine is a heterocyclic aromatic organic compound which contains two nitrogen atoms at positions 1 and 3 of the six-member ring.Pyrimidine biosynthesis is a biological process which results in the formation of a pyrimidine nucleotide. In pyrimidine biosynthesis, pyrimidines (the base) are synthesized from carbamoyl phosphate and aspartate. Ribose-5-phosphate is then attached to yield pyrimidine nucleotides. Pyrimidine biosynthesis takes place both in the body and through organic synthesis.