Final answer:
3′ mRNA end processing ensures appropriate termination of transcription for pol I and pol III by specific sequences and structures, such as the 18-nucleotide sequence for pol I and the mRNA hairpin for pol III.
Step-by-step explanation:
3′ mRNA end processing is critical for termination of transcription by both RNA polymerase I (pol I) and RNA polymerase III (pol III). For genes transcribed by pol I, there is a specific 18-nucleotide sequence that is recognized by a termination protein. On the other hand, pol III utilizes an mRNA hairpin for termination, reminiscent of rho-independent termination seen in prokaryotes. In contrast, RNA polymerase II (pol II) elongates the transcript by 1,000 to 2,000 nucleotides past the gene, which is later trimmed during mRNA processing, including the addition of a 3' poly(A) tail by poly(A) polymerase recognizing a sequence near the 3' end of the mRNA.