1. Reaching the Termination Sequence:
* RNA polymerase (the enzyme that transcribes DNA) moves along the DNA template, reading the sequence and adding complementary RNA nucleotides.
* When it encounters a termination sequence on the DNA, it signals the end of transcription. This sequence is typically a series of adenine (A) nucleotides, known as a polyadenylation signal.
2. Polyadenylation Signal Recognition:
* Proteins in the cell recognize the polyadenylation signal and bind to it.
3. Cleavage of the RNA Transcript:
* The RNA transcript is cut, or cleaved, at a specific site just downstream of the polyadenylation signal.
4. Addition of a Poly(A) Tail:
* An enzyme called poly(A) polymerase adds a string of adenine (A) nucleotides to the 3' end of the newly cleaved RNA transcript. This tail is called the poly(A) tail.
5. Dissociation of RNA Polymerase:
* After the poly(A) tail is added, RNA polymerase detaches from the DNA template, releasing the newly synthesized RNA transcript.
Termination Signals:
* There are different types of termination sequences in prokaryotes and eukaryotes.
* In prokaryotes, transcription can be terminated by a rho-dependent or rho-independent mechanism.
* Rho-dependent: A protein called rho binds to the RNA transcript and moves towards RNA polymerase, eventually causing it to detach.
* Rho-independent: A hairpin structure forms in the RNA transcript, which causes RNA polymerase to pause and eventually dissociate.
* In eukaryotes, the polyadenylation signal is the primary termination signal, but other factors may also be involved.
Importance of Termination:
* Proper termination ensures that:
* The correct length of RNA transcript is produced.
* The RNA transcript is protected from degradation.
* The DNA template is available for further transcription.
Overall, transcription ends with a series of precisely coordinated events that ensure the accurate and efficient production of RNA transcripts.
