Chapter 1. Chapter 16 Online Worked Problem: Gene Expression in Bacteria

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Think about the steps listed and how they logically flow from one to another, from very general, information-gathering steps to more specific steps related directly to the question.

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There are generally two types of regulated operons. Review your text, page 448 and Figure 16.4, to recall the difference between these two types.

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Review the terms regulated and constitutive to compare and contrast their meanings.

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Review your notes and Chapter 18 of your text.

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Review the terms regulated and constitutive to compare and contrast their meanings.

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The lac operon is a regulated, inducible operon where Lac I is the repressor. How does LacI block transcription?

Look at figure below to help understand the structural layout of the operon and the roles of the various components.

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The lac operon is a regulated, inducible operon where Lac I is the repressor. How does LacI block transcription?

Look at figure below to help understand the structural layout of the operon and the roles of the various components.

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Review Figure 16.6 regarding what the repressor binds and when, as well as how the binding of the repressor influences transcription.

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Review Figure 16.6 regarding what the repressor binds and when, as well as how the binding of the repressor influences transcription.

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Incorrect. Review your notes paying particular attention to the activity/location of LacI when an inducer is absent and whether transcription will occur as a result of that activity.

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Review your notes on the meaning of constitutive.

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Review your notes, text, or drawings of an inducible operon and under what circumstances transcription normally occurs.

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Review your notes, text, or drawings of an inducible operon and what DNAs/proteins, chemicals are necessary for blocking transcription.

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Review your notes, text, or drawings of an inducible operon and what DNAs/proteins, chemicals are necessary for blocking transcription. Please see Figure 16.12 for review.

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Review your notes, text, or drawings of an inducible operon and what DNAs/proteins, chemicals are necessary for blocking transcription. Please see Figure 16.10 for review.

For the promoter to always be available, the repressor can never be bound to the operator. So a mutant repressor that can’t even bind DNA would be one option. With nothing to hide the promoter, RNA polymerase can initiate transcription over and over.
Another option is a mutation in the operator DNA, as this is where the repressor normally binds to hide the promoter from RNA polymerase. With nothing to hide the promoter, RNA polymerase can initiate transcription over and over.

Final feedback: Some common errors and tips…
There are many components to keep track of in this type of complex question. You must understand the operon control system before you can even begin to think about the mutants. Making a diagram can really help you keep track of what is going on—where the repressor, inducer, and DNA components are, whether they are present or absent. Then once you are clear on how things work normally, look at what the mutants can no longer do correctly. The proteins/DNAs in that part of the process no longer function correctly, i.e., they are the ones that have been mutated.

Once you understand the system of control, you can also think of it in terms of whether the repressor is bound to the operator (no transcription) or whether the repressor is bound to inducer (transcription).

Then you can think of what mutations might cause transcription to occur non-stop. Thinking backwards along the path of “no transcription” on the right side, you can see that both the repressor and operator are involved. So making a mutant in either of those will affect the outcome (transcription).