The ability of RNA molecules to form complex structures allows them to bind substrates and catalyze reactions through proximity and orientation effects as well as by transition state stabilization, even though RNA lacks a wide variety of functional groups. In addition, the ions that RNA normally binds may also have catalytic functions. DNA lacks the $2\text{'}$ functional group that is present in RNA. Moreover, in its double helix form, DNA is conformationally rigid and is therefore unable to form the required complex tertiary structures.