Figure 2.

Schematic illustration of an in vitro selection cycle. The selection cycle starts with the incubation of the library (ssDNA or RNA) with the target molecule. After separating unbound from bound sequences, the latter are recovered and amplified by RT-PCR in the case of RNA libraries or PCR when using DNA libraries. After PCR, the single-strand nucleic acids are generated, for example, by λ-exonuclease digestion (DNA) or by in vitro transcription (RNA). The obtained library is subsequently used for the next selection cycle. After several selection (5–16) cycles, the enriched library is cloned and sequenced to identify individual monoclonal aptamers contained in the library.