a. Bacterial cells
Trp operon: repressible operon, normally on, build organic molecules, organic molecule produce acts as corepressor that binds to repressor to activate it. From on to off. No corepressor, inactive repressor. Corepressor, active repressor.
Lac operon: inducible operon, normally off, catabolic, break down food for energy. No inducer, active repressor, operon off. Inducer, inactive repressor, operon on.
b. Eukaryotic cells
RNA polymerase attaches to a promoter region （TATA boxes） on the DNA to begins to unzip the DNA into two strands. RNA polymerase assemble RNA nucleotides using one strand of the DNA. 5' to 3'. RNA polymerase reaches a special sequence of nucleotides that serve as a stop sign.