What are the types of introns?

There are at least four distinct types of introns:

• Group I introns: They are large self-splicing ribozymes that can catalyze their own excision from mRNA, tRNA and rRNA precursors. The secondary structure of group I introns has a nine-looped stem, which is required for splicing.
• Group II Introns: They are also self-splicing ribozymes that are found in rRNA, tRNA and mRNA, but they cut themselves differently than group I introns. Besides, the splicing of group II Introns involves the formation of a lariat structure.
• Nuclear pre-mRNA Introns: They are found in the nucleus in protein-coding genes that are removed by spliceosomes.
• Transfer RNA introns: They are found in tRNA genes and need proteins (enzymes) to be removed.