The SELEX Method | Key Terms | The Fundamentals of Biochemistry: Interactive Tutorials

Selection of Aptamers Using The SELEX Method

Basic steps in the SELEX selection process are presented in Fig. 1. Over 10¹⁵ different DNA oligonucleotides can be created by programming a DNA synthesizer to add equal amounts of G, A, T, and C phosphoramidites at each step of the synthesis. For the selection of DNA aptamers, this library can be used directly in the first round of a DNA SELEX process. For the selection of RNA aptamers, the random DNA oligonucleotide library has to be transformed into a RNA library before starting the first round of an RNA SELEX process. In essence, these repetitive cycles of in vitro selection and enzymatic amplification mimic Darwinian evolution, only that "survival-of-the-fittest" is carried out at the level of individual molecules as opposed to individual organisms. SELEX drives the selection towards a few structures optimized for binding a specific ligand.

Figure 1: The SELEX procedure. Aptamers are selected from the mainly nonfunctional pool of RNA by affinity chromatography. A large pool of random oligonucleotides is poured onto a column which is filled with a resin to which the target molecule of interest is covalently bound. Oligonucleotides that bind only weakly or not at all flow through the column and are discarded. Those that remain bound to the target are eluted by a solution containing a high concentration of the free ligand. The collected RNAs are transformed into a new pool by reverse transcriptase. This new pool of RNA molecules is then subjected to the same selection procedure.