Final answer:
Group I and Group II splicing are self-splicing mechanisms with Group II forming a lariat akin to spliceosomal splicing. Critical sequences for splicing are intron/exon boundaries and a branch point adenine. mRNA modifications before leaving the nucleus include 5' capping and poly-A tail addition.
Step-by-step explanation:
Similarities and Differences Between Group I, II, and III Splicing
Splicing is the process by which introns, non-coding sequences, are removed from a pre-mRNA transcript and the remaining exons, coding sequences, are reconnected to form a mature mRNA. Group I and Group II splicing are both forms of self-splicing mechanisms, where introns can excise themselves without the help of additional proteins such as the small nuclear ribonucleoproteins (snRNPs) required in spliceosomal intron splicing.
Group I introns are ribozymes, capable of self-splicing by folding into secondary stem-loop structures, which do not require any protein machinery. Group II introns also self-splice and form a lariat structure at an adenine (A) residue branch site, similar to the spliceosomal mechanism. However, Group II introns encode proteins required for their own splicing in vivo. Group III splicing is less well characterized and shares features with the spliceosomal pathway, including similar critical sequences.
The critical sequences that mark introns for splicing are the intron-exon boundaries, with specific consensus sequences at the 5' and 3' ends, and a branch point adenine in the intron for lariat formation in spliceosomal and Group II introns. For example, in spliceosomal splicing, the 5' end consensus is GU and the 3' end is AG.
mRNA may also be modified through 5' capping and the addition of a poly-A tail before it leaves the nucleus, ensuring proper export to the cytoplasm and stability.