Pool sizes required to find the isoleucine aptamer (black) and the hammerhead ribozyme (gray) (Fig. 1), making different assumptions about sequence requirements. The horizontal gray line represents 1000 zeptomoles, the limit of the Zeptomole World. The isoleucine aptamer is almost certainly a Zeptomole World molecule; the hammerhead may or may not be, depending on how much helix is added to its required sequences. However, its essential sequence components should certainly appear in zeptomole-scale pools. Thin lines show the minimal sites (fixed sequence only); dark lines show the maximal sites (counting paired bases as fixed in one state); and medium lines show the average (counting half the paired bases as fixed; details in how the paired bases are assigned are not visible on this scale). The graph shows pools required for 50% probability of occurrence; for 99% occurrence, multiply all pool sizes by a factor of 6. The maximal sequence for the hammerhead is clearly not a realistic case, or it would not be possible to reproducibly recover this motif from SELEX.
