Question

Contrast repressors and activators. How do they contribute to positive or negative gene regulation?

Answer 1

repressors and activators are DNA-interacting proteins that bind to a given operon or regulating DNA sequence. there are many ways they can influence gene expression rate, I will illustrate the most commone one.

so usually, the repressor is bound to the operon, hindering the ribosomal complex to bind. now if you have a stimulus (for the lac-operon this would be lactose), this activator will bind to the repressor, inducing a conformation change and thus causing it to disengage from the DNA. now, ribosomal complex can bind and you got your gene expression.

now, most of the time, the protein made out of your gene will act as a feedback repressor on its own gene. now this is kinda obvious. you build a protein that digests lactose -> activator gone -> gene repressed again.