26. Which is correct for termination of transcription in eukaryotes?
(1) Terminates prior to polyadenylation site
(2) Terminates during polyadenylation site
(3) Terminates after polyadenylation site
(4) Forms hair pin loop termination

Mechanism of Transcription Termination in Eukaryotes

Question Recap:

Which is correct for termination of transcription in eukaryotes?
(1) Terminates prior to polyadenylation site
(2) Terminates during polyadenylation site
(3) Terminates after polyadenylation site
(4) Forms hairpin loop termination

Correct Answer:

(3) Terminates after polyadenylation site

Detailed Explanation:

In eukaryotic transcription, particularly for RNA polymerase II, termination of transcription occurs downstream of the polyadenylation site, not before or exactly at it.

• The polyadenylation signal (PAS) in the pre-mRNA (commonly AAUAAA) is recognized by protein complexes such as CPSF and CstF, which cleave the nascent RNA about 10-30 nucleotides downstream of the PAS. This cleavage defines the mature 3′ end of the mRNA and allows polyadenylation to occur.

• Importantly, RNA polymerase II continues transcribing beyond this cleavage site. The actual termination of transcription happens after the polyadenylation site, at a more distal location on the DNA template.

• Two main models explain how termination occurs after cleavage:

  • The torpedo model, where a 5’→3′ exonuclease degrades the downstream RNA and catches up to RNA polymerase II, causing its release.

  • The allosteric model, where changes in the elongation complex after passing the polyadenylation site lead to termination.

• This mechanism ensures proper processing of the mRNA and recycling of the polymerase.

• In contrast, RNA polymerase III termination involves recognition of a poly-T sequence on the DNA and may involve RNA hairpin structures, but this is distinct from Pol II termination.

Why Other Options Are Incorrect:

1. Terminates prior to polyadenylation site
   Incorrect. The cleavage and polyadenylation occur before termination; polymerase continues beyond the polyA site.

2. Terminates during polyadenylation site
   Incorrect. Termination is not simultaneous with cleavage/polyadenylation but follows it.

3. Forms hairpin loop termination
   Incorrect for Pol II termination. Hairpin loop termination is characteristic of bacterial intrinsic termination and also relevant for Pol III termination, but not for Pol II.

Supporting References:

• PMC article on eukaryotic mRNA 3′ end formation and termination explains cleavage at the polyadenylation site precedes termination by RNA polymerase II.

• Wikipedia on eukaryotic transcription describes termination occurring downstream of the polyadenylation site, involving CPSF and CstF factors and models of termination.

• Recent reviews highlight the torpedo and allosteric models of termination occurring after the polyadenylation site.

• Polyadenylation Wikipedia page details cleavage and polyadenylation preceding termination and polymerase release downstream.

• RNA polymerase III termination can involve hairpin structures but differs from Pol II termination.

Summary Table

Conclusion

Eukaryotic transcription termination by RNA polymerase II occurs after the polyadenylation site, following cleavage of the pre-mRNA. This allows proper 3′ end formation and subsequent release of RNA polymerase II from the DNA template.