Question

All of the following are part of an operon except
A. An enhancer
B. An operator
C. A promoter
D. Structure genes

Answer 1

The operon is the DNA segment. It helps in the transcription of structural genes. The initiation of the transcription starts at the promoter sites. The structural genes present in the operon helps to transcribe the enzymes for the metabolism of the certain compounds.

Complete answer:
The functional unit of the DNA, which contains several structural genes under the control of a single promoter, is known as operon. Earlier, operon was thought to exist solely in prokaryotes but in the 1990s the first operon in eukaryotes were observed. The generation of polycistronic mRNAs occurs in prokaryotes while in eukaryotes, operons lead to monocistronic mRNAs.
The operon is mainly made up of three basic DNA components: The promoter, operator and the structural genes. Promoter region is a nucleotide sequence that enables a gene to be transcribed. The RNA polymerase recognized by the RNA polymerase. It initiates the transcription.
Operator is the DNA segment which binds to the repressor. The lac operon is the generally known operon.
The structural genes are co-regulated by the operon. The structural genes in the lac operon activate specific enzymes for the metabolism of lactose.
From the above information we have found that operon does not contain an enhancer in their structure. An enhancer has no specific role in the functioning of the operon.

So, the correct answer is option A.

Note:
Bacteriophages also have an operon segment in the DNA. For ex – two operons are found in the T7 phages. The regulatory gene is also present in operon which is not important in its function. An inducer binds to the repressor to inhibit its binding to the operator so that the transcription of structural genes does not stop. The two other types of operons are His operon and Trp operon.