Final answer:
For Rho-dependent termination, c) an inverted repeat upstream of a stretch of Us on the mRNA is necessary. The RNA polymerase stalls at a C-G rich site, forming a hairpin that causes a pause, followed by the release of the mRNA when weak U-A interactions occur.
Step-by-step explanation:
The correct answer to the question about what is required for a Rho-dependent terminator is C. an inverted repeat upstream of a stretch of Us on the mRNA. In Rho-dependent termination, the RNA polymerase stalls at the end of a gene when it encounters a site rich in C-G nucleotides on the DNA template. This region allows the mRNA to fold back onto itself, forming an inverted repeat that results in a stable hairpin structure. This hairpin then causes the polymerase to pause.
When the polymerase begins to transcribe a region full of A-T nucleotides following the C-G rich site, the resulting U-A-rich sequence in the mRNA forms only weak interactions with the DNA template. This instability, along with the stalled RNA polymerase, contributes to the release of the mRNA transcript, finalizing the process of termination.
It is important to note the difference between Rho-dependent and Rho-independent termination mechanisms. Rho-dependent termination requires the rho protein to terminate transcription, whereas Rho-independent termination does not involve the rho protein and relies solely on the intrinsic termination signals within the mRNA like the inverted repeat and the stretch of Us.