![](https://cos-cdn.shuashuati.com/pipixue-web/2020-1231-2005-12/ti_inject-812ce.png)
There are at least ___ structural classes of ribozymes that engage in self-cleavage; all use general acid and base catalysis to promote the attack of a 2′-hydroxyl group on an adjacent phosphodiester bond. The splicing reaction that occurs in a spliceosome relies on a catalytic center formed by the U2, U5, and U6 snRNAs.