The free bases(a, G, C U, tare not intermediates during the de novo synthesis: the purine ring is assembled on ribose phosphate to make amp and gmp, the pyrimidine ring is first synthesized as orotate(乳清酸), Which is then attached to ribose phosphate before being converted to UTP and ctP(dTmp is made from dUMP) The deoxyribonucleotides(dNDPs)are synthesized by reduction of ribonucleotides (NDPS)• The free bases (A, G, C, U, T) are not intermediates during the de novo synthesis: the purine ring is assembled on ribose phosphate to make AMP and GMP; the pyrimidine ring is first synthesized as orotate（乳清酸）, which is then attached to ribose phosphate before being converted to UTP and CTP (dTMP is made from dUMP). • The deoxyribonucleotides (dNDPs) are synthesized by reduction of ribonucleotides (NDPs)