Final answer:
Group 2 and pre-mRNA splicosome introns are characterized by their requirement for spliceosomes for splicing in vivo, although Group 2 introns can self-splice in vitro. Group I introns are independently capable of self-splicing without spliceosomes.
Step-by-step explanation:
The characteristic of Group 2 introns and pre-mRNA splicosome introns is that they require spliceosomes for splicing. Group 2 introns can self-splice in vitro forming a lariat structure similarly to spliceosomal introns, suggesting that spliceosomal splicing may have evolved from Group II mechanisms. However, in vivo splicing of Group 2 introns typically depends on additional proteins. In contrast, Group I introns possess the capability to self-splice without assistance from spliceosomes or proteins. Eukaryotic pre-mRNA also undergoes modifications before exiting the nucleus, where introns are removed via splicing with the aid of spliceosomes composed of proteins and small nuclear RNAs (snRNAs), and the mRNA is modified with a 5' cap and a poly-A tail.