Biology A2 · Inheritance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A cell does not make all its proteins all the time. It switches genes on and off, so that energy and amino acids are not wasted.
A structural gene codes for a protein the cell uses, such as an enzyme. A regulatory gene codes for a protein that controls whether other genes are expressed. In bacteria, the lac operon is a group of structural genes with one promoter and one operator. A repressor protein sits on the operator and blocks transcription until lactose arrives.
In eukaryotes, transcription factors bind to DNA and raise or lower the rate of transcription. Gibberellin works through them: it removes the DELLA proteins that hold a transcription factor back.
Students say that lactose binds to the operator. Lactose binds to the repressor protein; it is the repressor that binds to the operator.
Answer: β-galactosidase is made all the time, whether or not lactose is present.
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