Question

what are the discrete steps traversed through in a round of transcription? What distinguishes one step from another? What must occur for the polymerase to make the transition from one step to another?

Answer 1

Answers-

1. Discrete steps in transcription- Transcription involves the conversion of the genetic information stored in DNA to RNA. This conversion is catalysed by the involvement of the enzyme called RNA polymerase. The discrete steps of transcription are- a. initiation of transcription b. Elongation phase of transcription, and c. Termination of transcription.

a. Initiation of transcription can be further divided into the following sub stapes- i. formation of closed complex, ii. formation of open complex iii. formation of the initial transcribing complex iv. Promoter escape and v. formation of the ternary complex.

2. The features/distinguishing characters of each step:

a. Initiation- it is the step when the RNA pol enzyme is conditioned for initiating the process of the transcription. The RNA polymerase with the help of the sigma factor recognises and binds to the promoter of a gene and this forms the closed complex. Then there occurs a conformational change in the structure of the RNAP which causes the melting of the DNA around the promoter, resulting in the formation of the open complex. Once formed, ribonucleotides are brought into the RNAP active site and RNA synthesis occurs for the 1st time. This is called the formation of the initial transcribing complex. Then the RNAP escapes the promoter and starts transcribing RNA post the start site. This step is called the formation of the ternary complex(a complex of RNAP, DNA, RNA).

b. Elongation- This is the step when the RNAP has started to synthesize full-fledgedly. The RNA chain begins to grow in the 5'to 3' direction. The RNAP in the elongation phase is no more attached to the sigma factor. Also, the RNAP moves in a stepwise manner i.e. one nucleotide after the other.

c.Termination- once the transcription of the gene is complete, its time to terminate the process of transcription. This occurs either with the help of a protein called Rho or occurs independently by the RNA product. The Rho protein is like a helicase, that pushes the RNAP, pulls out the RNA and thus stops the process. On the other hand, when occurring independently, there are sequences called inverted repeats within the RNA itself, which forms hair-pin-loop. This hairpin loop causes the enzyme to become unstable and fall off from the DNA.

3. For the transition from one step to another, there occurs tremendous conformational change within the polymerase itself.For example, for the transition from closed complex to open complex, the crab-like-pincer structures of the RNAP moves forward and grabs the DNA tight. Again,for the promoter escape, the sigma factor is removed from the RNA polymerase.