ch 18.1

precursor

substance, cell, or molecule that is like a building block for the formation of a more complex substance, cell, or molecule

tryptophan

amino acid required to make protein

feedback inhibition

cellular control mechanism where the activity of an enzyme is restrained by the end product of a pathway

operator

segment of DNA usually positioned within the promoter

promoter

region of DNA where transcription of a gene starts (binding site for RNA polymerase)

operon

stretch of DNA that includes the operator, promoter, and genes they control

repressor

protein that binds to the operator and stops transcription by blocking RNA polymerase, can be in active or inactive form, (product of a separate regulatory gene)

corepressor

molecule that cooperates with a repressor protein to switch an operon off

trp operon

usually on, genes for tryptophan synthesis are transcribed, when there is enough, it binds to the trp repressor protein which turns the operon off