Compositions, methods and systems are provided for the loading of extended polymerase-nucleic acid complexes onto substrates. Primed polymerase-template complex comprising a polymerase enzyme and a circular nucleic acid template comprising a double stranded region connected at each end by a single-stranded hairpin region are provides in which the circular nucleic acid template comprises at least one reversible pause point. Nucleic acid synthesis is carried out such that a nascent strand is synthesized up to the reversible stop point producing an extended polymerase-template complex. The extended polymerase-template complex is then attached to a substrate. Such attached complexes can be used for single molecule sequencing in which the nucleic acid synthesis is re-initiated such that nucleic acid synthesis continues past the reversible stop point.