If a nonsense mutation is present in the Lacz gene of the lac operon, the mRNA of βgalactosidase would
1. be expressed in response to binding of the lac repressor to the CAP protein
2. be expressed in response to the presence of lactose
3. not be expressed at all
4. always be expressed

 Understanding the Lac Operon and LacZ Gene

The lac operon is a classic model in molecular biology that describes how E. coli bacteria regulate genes responsible for the digestion of lactose. The operon includes:

• LacZ (codes for β-galactosidase)

• LacY (codes for lactose permease)

• LacA (codes for transacetylase)

Role of LacZ:

The LacZ gene produces β-galactosidase, an enzyme that breaks down lactose into glucose and galactose.

 What Is a Nonsense Mutation?

A nonsense mutation is a point mutation in a DNA sequence that introduces a premature stop codon, leading to the production of a truncated (incomplete) protein.

In the case of LacZ:

• The mRNA will still be transcribed in response to lactose presence

• But the protein (β-galactosidase) produced will be non-functional or incomplete

 Why Option 2 Is Correct

Despite the nonsense mutation:

• The lac operon remains inducible by lactose

• The regulatory mechanisms (involving the lac repressor and CAP protein) still function

• Therefore, mRNA for LacZ will still be expressed when lactose is present

 However, due to the mutation, the resulting β-galactosidase enzyme will not function properly.

 Clarifying the Other Options

1. Incorrect: Lac repressor does not bind to the CAP protein. CAP (Catabolite Activator Protein) binds to the promoter in the presence of cAMP to enhance transcription.

2. Incorrect: The mRNA is still made; it’s the protein that’s faulty.

3. Incorrect: Expression is inducible, not constant.

 Conclusion

A nonsense mutation in the LacZ gene doesn’t stop mRNA expression, but it prevents proper enzyme formation. This highlights how gene regulation and protein synthesis are distinct processes, and mutations can impact them in different ways.