Which enzyme will be produced in a cell in which there is a non-sense mutation in the lac Y gene ?

Question

Accepted Answer

D. 1. **Recall the gene order in the lac operon.**

Given that the structural genes of the lac operon are arranged in the order $z$, $y$, $a$, coding respectively for beta-galactosidase, permease, and transacetylase, transcription and translation proceed sequentially along this order.

2. **Understand the effect of a nonsense mutation.**

Given that a nonsense mutation introduces a premature stop codon, translation halts at that point, meaning any genes coming **after** the mutated gene in the sequence also fail to be expressed properly.

3. **Apply this to a mutation in the $y$ gene.**

Since the mutation occurs in the **$y$ gene**, translation stops there, preventing the production of **permease** and the downstream **transacetylase ($a$ gene)**.

4. **Determine what is still produced.**

Given that the **$z$ gene** comes **before** the $y$ gene in the sequence, its translation is unaffected, so **beta-galactosidase** is still produced normally.

**Conclusion:** Hence, the answer is **D, $\beta$-galactosidase**.