Template-directed synthesis on the oligonucleotide d(C7-G-C7).

When the deoxynucleotide template d(C7-G-C7) is incubated with the activated nucleotides 2-MeImpG and 2-MeImpC, a series of oligomers of G up to the sevenmer and a series of copolymers of composition GnC with n = 3 to 13 are formed. Oligomers GnC with n greater than 7 are completely degraded by pancreatic ribonuclease, establishing that they contain a 3' to 5' internucleotide bond between 5'-C and 3'-G within a sequence of the form (pG)ipC(pG)j. As expected, (pG)7-Cp and (pG)6-Cp are major hydrolysis products. Detailed analysis of the product distribution shows that a substantial fraction of the oligomeric products are of the type (pG)ipC(pG)j with i less than 7. This shows that product synthesis does not necessarily begin at the 3' terminus of the template. The significance of this finding in terms of the origin of molecular replication is discussed.