Question

Promotor and operator are cis sites on DNA for binding of

• A. RNA polymerase binds to promotor and repressor binds to operator
• B. Repressor binds to promotor and RNA polymerase binds to operator
• C. DNA polymerase binds to promotor and repressor binds to operator
• D. DNA polymerase binds to promotor and RNA polymerase binds to operator

Hint:

• Cis-acting elements DNA sequences (like promoter, operator, enhancers) that regulate genes on the same DNA molecule.
• Trans-acting factors Proteins (like RNA polymerase, repressors, activators) that bind to cis-acting elements to regulate gene expression. They are encoded by genes that can be located elsewhere.
• Promoter: Binding site for RNA polymerase (initiates transcription).
• Operator: Binding site for repressor protein (blocks transcription).

Answer 1

The Correct Option is A

In gene regulation, particularly in prokaryotic operons (like the lac operon or trp operon):

• Promoter: A specific DNA sequence located upstream of a gene or operon where RNA polymerase binds to initiate transcription. It is a cis-acting regulatory site.
• Operator: A specific DNA sequence, usually located near or overlapping the promoter, where a repressor protein can bind. Binding of the repressor to the operator typically blocks RNA polymerase from transcribing the downstream genes, thus inhibiting gene expression. It is also a cis-acting regulatory site.

Let's analyze the options: (a) "RNA polymerase binds to promotor and repressor binds to operator": Correct. This describes the standard roles. (b) "Repressor binds to promotor and RNA polymerase binds to operator": Incorrect. Roles are swapped. (c) "DNA polymerase binds to promotor...": Incorrect. DNA polymerase is involved in DNA replication, not transcription initiation at promoters. (d) "DNA polymerase binds to promotor...": Incorrect, as in (c). Therefore, RNA polymerase binds to the promoter, and a repressor protein (if present and active) binds to the operator. \[ \boxed{\text{RNA polymerase binds to promotor and repressor binds to operator}} \]