Group I introns are a type of self-splicing intron, a non-coding sequence of RNA that can remove itself from a transcript without the need for additional proteins or enzymes. These introns require a guanosine nucleotide as a cofactor to initiate the splicing process, which makes them unique compared to other introns. The splicing mechanism involves two transesterification reactions that result in the excision of the intron and the ligation of the exons.