PURINE AND PYRIMIDINE NUCLEOTIDES INCORPORATED INTO MATERNAL AND FETAL RIBO NUCLEIC ACID DERIVE ALMOST EXCLUSIVELY FROM SYNTHESIS DE NOVO IN THE PREGNANT MOUSE是什么意思 PURINE AND PYRIMIDINE NUCLEOTIDES INCORPORATED INTO MATERNAL AND FETAL RIBO NUCLEIC ACID DERIVE ALMOST EXCLUSIVELY FROM SYNTHESIS DE NOVO IN THE PREGNANT MOUSE怎么读