The sequence of the structural genes in the lac operon is

The correct answer is lacZ-lacY-lacA.

Explanation:
The lac operon (lactose operon) is an operon required for the transport and metabolism of lactose in Escherichia coli and other bacteria. It consists of regulatory elements (promoter and operator) and three structural genes that are transcribed together as a single polycistronic mRNA.

The three structural genes are arranged in a specific linear sequence in the 5' to 3' direction:
1. lacZ: This is the first structural gene. It encodes the enzyme β-galactosidase, which cleaves the disaccharide lactose into the simple sugars glucose and galactose.
2. lacY: This is the second structural gene. It encodes lactose permease, a membrane-bound protein that facilitates the entry of lactose into the bacterial cell.
3. lacA: This is the third structural gene. It encodes thiogalactoside transacetylase, an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides.

Because of this linear arrangement, transcription by RNA polymerase proceeds sequentially through lacZ, then lacY, and finally lacA, making the correct sequence lacZ-lacY-lacA.