Differences between group I self-splicing introns and group II self splicing introns:

GROUP I SELF SPLICING INTRONS:

1. Group-I is found in bacteriophage; mRNA; tRNA, rRNA of lower eukaryotes prevently in higher plants and sporadically in tRNA, rRNA of bacteria.
2. Group I of ORF codes for a homing endonuclease, that is responsible for its mobility.
3. In the splicing mechanism group I requires GTP.
4. Guanine nucleoside cofactor is used whose 3-OH makes a nucleophilic attack on the phospho and forms 3,5, phosphodiester bond. The released 3-exon end now attacks the 3- intron end in a similar nucleophilic attack and the intron is excised and exons ligated.
5. Group I has the core structure with a paired region (P1-P9) that forms two domains,  P4-P6 domain (P5, P4, P6, P6a) and P3-P9 domain (P3, P8, P7, P9 helices) with tertiary structure stabilizing it.

GROUP II SELF SPLICING INTRONS:

1. Group II is found in mobile genetic elements in bacteria, organellar elements of archaea and eukaryotes.
2. Group II ORF has multifunctional: Reverse transcriptase, mutases activity.
3. Group II is independent of GTP.
4. 2-OH group within the intron makes a nucleophilic attack on the 5- splice site and forms a lariat like structure. This lariat like structure acts as an intermediate. 3-end of the exon makes a nucleophilic attack on the 3-end of the intron and lariat shaped intron is excised and the exons are ligated.
5. Group II forms 6 typical stem- loop structure D1- linked by tertiary interaction.