Which enzyme will be produced in a cell in which there is a non-sense mutation in the lac Y gene ?
- Transacetylase
- Lactose permease and transacetylase
- Lactose permease
- -galactosidase
Answer
(4) -galactosidase
Solution
The lac operon has three structural genes in the order z y a, transcribed as one polycistronic mRNA:
z codes for -galactosidase, y codes for permease and a codes for transacetylase.
⇒ A non-sense mutation in the y gene creates a premature stop codon, so a complete, functional permease is not formed.
∵ Translation of the downstream a gene is also stopped (polar effect), transacetylase is not formed either.
∴ Only -galactosidase, coded by the upstream z gene, is produced.
Answer: (4)