Question

Which of the following is not a feature of the “torpedo” model for transcription termination in eukaryotic cells?

	A.	Conformational changes in the RNA trigger the termination of transcription.
	B.	RNA is cleaved downstream from the poly(A) site.
	C.	Exonuclease degradation of RNA promotes Pol II release.
	D.	Pol II still remains associated with the DNA template immediately after RNA synthesis.

Answer 1

Answer: Option A.

Explanation:

Torpedo model:

when synthesis of mRNA is completed, it is cleaved off at the poly-A site. (Hence, option B is right statement)

The left-over RNA strand and RNA polymerase II remain bound to the DNA template and transcription continues till now. (Hence, option D is right statement)

After this, an exonuclease protein binds to the left-over RNA strand. It removes the freshly transcribed nucleotides. This is called 'degrading' of the RNA. While degrading RNA, exonuclease moves towards the bound RNA polymerase II. Due to this, RNA polymerase II is pushed off by the exonuclease, as exonuclease continuously moves crossing RNA polymerase II. In this way, RNA polymerase II is removed off from the DNA tempelate and transcription gets terminated. (Hence, option C is right statement)

To protect the transcribed mRNA from degradation by the exonuclease, a modified guanne at 5' end is added. This called capping. A poly(A) tail is also added at the 3' end of a mRNA strand to protect it from exonucleases.