Question

How is transcription initiated and terminated? What is the importance of the promoter consensus sequence? How and why do consensus sequences differ from one another?

Answer 1

Transcription initiation starts with RNA polymerase binding to DNA of the gene at the region called promoter. The promoter sequence in the DNA signals the polymerase to bind at that particular sequence and start transcribing. Each gene has its own promoter where RNA polymerase binds, transcription bubble forms and trancription is initiated. The promoter sequence in eukaryotes have a consensus sequence called TATA box. This sequence will allow the RNA polymerase and other transcription factors to bind for initiation of transcription.

Transcription termination happens when a stop signal is seen on the DNA. It happens once the polymerase transcribes a sequence of DNA known as a terminator. In Rho dependent termination, the RNA contains a binding for a protein called Rho factor. Rho factor binds to this sequence and starts climbing up the transcript towards RNA polymerase. Rho pulls the RNA transcript and the template DNA strand apart releasing the RNA molecule and ending transcription. Rho independent termination depends on specific sequences in the DNA template strand. As the RNA polymerase approached the end of the gene being transcribed, it hits a region rich in G and C nucleotides. The RNA transcribed from this region folds back on itself. The hairpin that causes polymerase to stall.

A typical promoter contains two important DNA sequences, the -10 and -35 elements. RNA polymerase recognizes the polymerase in the right spot to start transcibing a target gene.

Many eukaryotic promoters have a sequence called a TATA box which plays a role much like that of the -10 element in bacteria.