Studies on Polynucleotides. XXVII.1 The Stepwise Synthesis of Specific Deoxyribopolynucleotides (7).1 The Synthesis of Polynucleotides Containing Deoxycytidine and Deoxyguanosine in Specific Sequences and of Homologous Deoxycytidine Polynucleotides Terminating in Thymidine3