Which of following statement is true for lac operon in E-coli
(a) lac operon consists of one regulatory gene and five structural gene.
(b) The i gene codes for inducer
(c) The z-gene codes for `beta` -galacosidase
(d) A very low level of expression of lac operon has to be present in cell all the time

Options

A. a, b, c, d
B. a, c, d
C. c, d
D. b, d

Answer

To solve the question regarding the lac operon in E. coli, we will analyze each statement provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding the Components of the Lac Operon**:
- The lac operon consists of several genes involved in the metabolism of lactose in E. coli. It includes one regulatory gene (i gene) and three structural genes (z, y, and a).

2. **Analyzing Statement (a)**:
- Statement: "lac operon consists of one regulatory gene and five structural genes."
- **Evaluation**: This statement is incorrect. The lac operon has one regulatory gene (i) and three structural genes (z, y, and a). Therefore, this statement is false.

3. **Analyzing Statement (b)**:
- Statement: "The i gene codes for inducer."
- **Evaluation**: This statement is incorrect. The i gene codes for the repressor (inhibitor) protein, not the inducer. The inducer for the lac operon is allolactose, which binds to the repressor. Therefore, this statement is false.

4. **Analyzing Statement (c)**:
- Statement: "The z-gene codes for beta-galactosidase."
- **Evaluation**: This statement is correct. The z gene indeed codes for the enzyme beta-galactosidase, which is responsible for breaking down lactose into glucose and galactose. Therefore, this statement is true.

5. **Analyzing Statement (d)**:
- Statement: "A very low level of expression of lac operon has to be present in cell all the time."
- **Evaluation**: This statement is also correct. A low level of expression of the lac operon is necessary for maintaining the permeability of the cell membrane to lactose through the permease (coded by the y gene). Therefore, this statement is true.

6. **Conclusion**:
- From the analysis, we find that statements (c) and (d) are true. However, since the question asks for the true statement among the options given, the correct answer is:
- **(c) The z-gene codes for beta-galactosidase.**