 enzym synthesi deoxyribonucl acid xviii repair partial singl-strand dna templat dna polymeras partial singl-strand dna prepar limit digest strand exonucleas iii restor nativ fulli doubl-strand structur escherichia coli dna polymeras rate synthesi observ repair partial degrad primer polymeras system faster nativ dna newli synthes dna coval attach primer fulli repair dna resembl origin nativ dna judg appear electron micrograph cscl densiti-gradient analysi denatur genet activ dna synthesi repair phase produc structur coval link primer resembl nondenatur branch appear lack genet activ product nativ dna primer schildkraut richardson + kornberg 1964 