Ch. 5 - Genetics

Norman-McKay- Microbiology: Basic and Clinical Principles 2nd Edition

Norman-McKay2nd EditionMicrobiology: Basic and Clinical PrinciplesISBN: 9780137661619Not the one you use?Change textbook

Norman-McKay 2nd Edition

Ch. 5 - Genetics

Problem 18

Chapter 5, Problem 18

Select all the true statements about repressible operons:
a. By default they are on until turned off.
b. An example is the lactose operon.
c. An example is the arginine operon.
d. A repressor must bind to the operator in order for the operon to be turned off.

Verified step by step guidance

Step 1: Understand what a repressible operon is. Repressible operons are typically 'on' by default, meaning the genes are being transcribed unless a specific molecule (corepressor) activates the repressor protein to bind the operator and turn the operon off.

Step 2: Evaluate statement (a): 'By default they are on until turned off.' This aligns with the definition of repressible operons, so this statement is true.

Step 3: Evaluate statement (b): 'An example is the lactose operon.' The lactose operon (lac operon) is actually an inducible operon, which is off by default and turned on in the presence of lactose, so this statement is false.

Step 4: Evaluate statement (c): 'An example is the arginine operon.' The arginine operon is a classic example of a repressible operon, where arginine acts as a corepressor to turn the operon off, so this statement is true.

Step 5: Evaluate statement (d): 'A repressor must bind to the operator in order for the operon to be turned off.' In repressible operons, the repressor protein binds to the operator only when activated by a corepressor, which stops transcription, so this statement is true.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Repressible Operons

Repressible operons are genetic regulatory systems that are usually active (on) but can be turned off (repressed) when a specific molecule, often the end product of a metabolic pathway, is abundant. This allows the cell to conserve resources by stopping the production of enzymes when they are not needed.

Examples of Operons: Lactose vs. Arginine

The lactose operon (lac operon) is an inducible operon, meaning it is usually off and turned on in the presence of lactose. In contrast, the arginine operon (arg operon) is a classic example of a repressible operon, which is normally on but can be turned off when arginine levels are high.

Role of the Repressor Protein and Operator

In repressible operons, a repressor protein binds to the operator region to block transcription and turn the operon off. This binding typically requires the presence of a corepressor molecule, which activates the repressor, enabling it to attach to the operator and inhibit gene expression.

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Related Practice

Textbook Question

In Griffith's classical experiments on transformation, which of the following scenarios led to a dead mouse? Select all that apply.

a. Infecting the mouse with a living strain of S. pneumoniae that makes a capsule

b. Infecting the mouse with a heat-killed strain of S. pneumoniae that makes a capsule

c. Infecting the mouse with a heat-killed strain of S. pneumoniae that makes a capsule and a living strain of S. pneumoniae that cannot make a capsule

d. Infecting the mouse with a living strain of S. pneumoniae that cannot make a capsule

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Textbook Question

Which of the following helps to prevent mutations? Select all that apply.

a. Conjugation

b. Transposons

c. Transduction

d. DNA proofreading

e. Specialized transduction

f. Excision repair

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Textbook Question