Polycistronic message: Into how many functional coding segments are the structural genes of the lac operon transcribed on a single mRNA?

Introduction / Context:
The lac operon in Escherichia coli is a classic example of a polycistronic transcript, meaning a single mRNA carries multiple open reading frames that are translated separately.

Given Data / Assumptions:

• Structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase).
• They are under control of one promoter and operator.
• Regulatory gene lacI is separate and not part of the transcript.

Concept / Approach:
One promoter produces an mRNA with three coding regions. Each cistron has its own ribosome-binding site, enabling independent translation of Z, Y, and A from the same mRNA.

Step-by-Step Solution:
List structural genes → Z, Y, A. Confirm they are co-transcribed but separately translated. Conclude the transcript contains 3 coding segments.

Verification / Alternative check:
Genetic mapping and mRNA analyses confirm three distinct coding regions; mutations in each gene yield distinct phenotypes.

Why Other Options Are Wrong:
Two-segment and one-segment options contradict established operon structure; operator/regulator are DNA/protein control elements, not translated cistrons in the mRNA.

Common Pitfalls:
Confusing lacI with structural genes; thinking operators are transcribed as protein-coding segments.

Final Answer:
3 segments (lacZ, lacY, and lacA).