Conditional primer extension for single-molecule detection

A composition for producing a nucleic acid, the composition comprising:

• (a) a strand displacing polymerase and a mixture of dNTPs;
• (b) a nucleic acid probe linked to a first target-binding molecule, wherein the nucleic acid probe comprises, in the following order, a first toehold domain, a first double-stranded domain, a first stopper, a second double-stranded domain, a second stopper, a third double-stranded domain, and a second toehold domain, wherein the first stopper and the second stopper are molecules or modifications that terminate polymerization by the strand displacing polymerase;
• and
• (c) a nucleic acid primer strand linked to a second target-binding molecule, wherein the nucleic acid primer strand comprises a first single-stranded primer domain that can bind to the first toehold domain of the nucleic acid probe.