Final answer:
Termination of transcription in eukaryotes varies depending on the RNA polymerase involved. RNA polymerase II transcripts are processed to add a protective poly-A tail beyond the coding sequence. RNA polymerases I and III terminate transcription via recognition of specific sequences or structural signals.
Step-by-step explanation:
3′ mRNA End Processing and Termination of Transcription
The process of termination of transcription is distinct for different RNA polymerases in eukaryotes. RNA polymerase II continues transcription beyond the gene's coding sequence, producing a pre-mRNA tail that will be removed later.
This is followed by additional mRNA processing steps which involve the addition of a poly-A tail and cleavage of the pre-mRNA. In contrast, RNA polymerases I and III require termination signals. Specifically, RNA polymerase I transcription terminates following recognition of an 18-nucleotide sequence signal by a termination protein. RNA polymerase III relies on a hairpin structure formed by the internally complementary sequences near the 3' end of the transcript to terminate transcription, similar to rho-independent termination in prokaryotes.
The addition of a poly-A tail to the 3' end is critical, as it protects the mRNA from degradation and signals for the transcript to be exported out of the nucleus.
The sequence involved in this process includes an AAUAAA consensus sequence and a GU-rich sequence, where the pre-mRNA is cleaved before the poly-A tail is added. Subsequently, the pre-mRNA is stabilized with RNA-stabilizing proteins and a 5' cap is added. These modifications are essential for the mRNA's stability and its recognition by the translation machinery.