More than 350 different type II endonucleases with over 100 different recognition sequences are known.

TLS Online TPP Program

#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. 

TLS Online TPP Program

#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.

TLS Online TPP Program

#Id: 2451

#Unit 3. Fundamental Processes

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

TLS Online TPP Program

#Id: 2452

#Unit 3. Fundamental Processes

In the absence of allolactose, the repressor is present in a form that binds its site on DNA (and so keeps the lac genes switched off ). 

TLS Online TPP Program

#Id: 2453

#Unit 3. Fundamental Processes

Allolactose binds to a part of the Lac repressor distinct from its DNA-binding domain.

TLS Online TPP Program

#Id: 2454

#Unit 3. Fundamental Processes

Negative control of the lac operon. 

No lactose; repression. The lacI gene produces repressor, which binds to the operator and blocks RNA polymerase from transcribing the lac genes.