Studyledge

Inheritance: Question 1

Syllabus 16.3

Multiple choice A2 1 mark

The lac operon in the bacterium Escherichia coli controls the production of enzymes that allow the bacterium to digest the sugar lactose. The operon includes a regulatory gene that produces a repressor protein, and an operator sequence that this repressor protein can bind to. When no lactose is present, the repressor protein binds tightly to the operator, physically blocking RNA polymerase from transcribing the structural genes lacZ, lacY and lacA.

Which statement correctly describes what changes when lactose becomes available in the bacterium's environment?

Choose an answer to check it, then compare with the worked solution below.
Show worked solution Hide worked solution

Worked solution

Step 1: How the lac operon is normally kept switched off

In the absence of lactose, the regulatory gene of the lac operon is expressed and produces a repressor protein. This repressor protein binds to the operator, a short DNA sequence next to the promoter, and by doing so it physically blocks RNA polymerase from binding to the promoter and transcribing the structural genes (lacZ, lacY and lacA). Because the enzymes needed to digest lactose are not made unless lactose is present, they are described as inducible enzymes.

Step 2: What lactose changes

When lactose becomes available, it acts as an inducer: it binds to the repressor protein at a site away from the part that binds DNA. This binding changes the three-dimensional shape of the repressor protein, so it can no longer fit onto the operator and detaches from the DNA. With the operator free, RNA polymerase can now bind to the promoter and transcribe the structural genes, producing the enzymes needed to metabolise lactose. The regulatory gene itself continues to be transcribed throughout this process. It is the repressor protein’s ability to bind DNA, not the regulatory gene’s activity, that is switched off.

Step 3: Evaluate each option

  • A: incorrect. The regulatory gene is not disabled; it keeps producing repressor protein, which simply cannot bind the operator while lactose (the inducer) is present.
  • B: correct, lactose binds to the repressor protein, changing its shape so it detaches from the operator, allowing RNA polymerase to transcribe the structural genes.
  • C: incorrect. Lactose does not interact with RNA polymerase, and it does not increase transcription of the regulatory gene.
  • D: incorrect, lactose binds to the repressor protein, not to the operator DNA sequence itself.

Final answer

  • Lactose binds to and changes the shape of the repressor protein, releasing it from the operator so that RNA polymerase can transcribe the structural genes, option B.