#Id: 2446

#Unit 3. Fundamental Processes

One consequence of the rotation of DNA is illustrated In the two-domain model for transcription, as RNA polymerase moves with respect to the double helix, it generates positive supercoils (more tightly wound DNA) ahead of it and leaves negative supercoils (partially unwound DNA) behind it. For each helical turn traversed by RNA polymerase, + 1 turn is generated ahead and -1 turn behind.

#Id: 2447

#Unit 3. Fundamental Processes

The enzymes DNA gyrase, which introduces negative supercoils into DNA, and DNA topoisomerase I, which removes negative supercoils in DNA, are required to prevent topological stresses from building up in the course of transcription and replication. 

#Id: 2448

#Unit 3. Fundamental Processes

Blocking the activities of gyrase and topoisomerase therefore result in major changes in DNA supercoiling, which in turn affects transcription and replication.

#Id: 2449

#Unit 3. Fundamental Processes

In the absence of both activator and repressor, RNA polymerase occasionally binds the promoter spontaneously and initiates a low level (basal level) of transcription. 

#Id: 2450

#Unit 3. Fundamental Processes

The lac operator overlaps the promoter, and so the repressor bound to the operator physically prevents RNA polymerase from binding to the promoter and thus initiating RNA synthesis.

#Id: 2451

#Unit 3. Fundamental Processes

Allolactose binds to the Lac repressor and triggers a change in the shape (conformation) of that protein.