<Li> Self - splicing group I introns that are removed by RNA catalysis </Li> <Li> Self - splicing group II introns that are removed by RNA catalysis </Li> <P> Group III introns are proposed to be a fifth family, but little is known about the biochemical apparatus that mediates their splicing . They appear to be related to group II introns, and possibly to spliceosomal introns . </P> <P> Nuclear pre-mRNA introns (spliceosomal introns) are characterized by specific intron sequences located at the boundaries between introns and exons . These sequences are recognized by spliceosomal RNA molecules when the splicing reactions are initiated . In addition, they contain a branch point, a particular nucleotide sequence near the 3' end of the intron that becomes covalently linked to the 5' end of the intron during the splicing process, generating a branched (lariat) intron . Apart from these three short conserved elements, nuclear pre-mRNA intron sequences are highly variable . Nuclear pre-mRNA introns are often much longer than their surrounding exons . </P>

Do introns contain coding information for a gene