In an operon what is present in between the promoter and the structural genes.

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Hint: Operon is found in prokaryotic, eukaryotic cells. It is also found in viruses like bacteriophage. The operon is considered as the functional unit of DNA which consists of a cluster of genes which work under a single promoter. These genes are transcribed together as mRNA.

Complete answer:
To solve this question, we must know about the Operon.
Operon is a cluster of genes which work under a single promoter to transcribe into mRNA and gets translated in the cytoplasm. These mRNA can also undergo splicing to form monocistronic mRNAs. Thus in an operon, a group of genes are expressed together (Co-transcribed) or not expressed at all.

Structure of operon-
1.An operon is made up of three basic DNA components-
2.Promoter gene- It is a nucleotide sequence which helps in transcription of genes. The RNA polymerase recognises the promoter site and initiates transcription.
3.Operator- It is a segment of DNA at which the repressor binds. It is present in between the promoter site and the structural genes.
4.Structural gene- These are the genes that are co-regulated by the operon.

The operon in the prokaryotes leads to polycistronic mRNA and leads to monocistronic mRNAs in eukaryotes.

Note: Lac operon is the first discovered operon. The main operator in Lac operon is $O_1$ which is located downstream of the promoter. It has two additional operators $O_2$ and $O_3$. The repressor is a protein which binds to the operator gene and does not allow RNA polymerase to bind the promoter site.