A target polynucleotide is expanded. In respect of each nucleotide in the target polynucleotide, the target polynucleotide comprises clock nucleotides and at least one signal nucleotide in a predetermined order. The clock nucleotides have a predetermined sequence common to each nucleotide in the target polynucleotide. The at least one signal nucleotide is characteristic of the identity of the respective nucleotide in the target polynucleotide. During translocation of the expanded polynucleotide through a nanopore, electrical measurements dependent on the polynucleotide within the pore are made, to derive an analysis signal. Clock signals derived from the clock nucleotides are identified. Relative to the positions of the identified clock signals, nucleotide signals derived from the least one signal nucleotide are derived to analyse the target polynucleotide. The predetermined sequence of the clock nucleotides comprises a restriction site for a restriction enzyme and at least one further nucleotide that extends the predetermined sequence.