What is the effect of a repressor binding to the operator in a gene regulatory region?

• A. It enhances RNA polymerase binding
• B. It recruits ribosomes to the mRNA
• C. It promotes mRNA splicing
• D. It binds to the operator to block RNA polymerase binding or progression, inhibiting transcription

Answer: (D)

Binding of a repressor to the operator blocks transcription by preventing RNA polymerase from binding to the promoter or from moving along the DNA to synthesize RNA. The operator is a DNA sequence near the promoter that serves as a docking site for the repressor. When the repressor is bound, RNA polymerase can’t properly engage with the promoter or cannot clear the promoter to begin elongation, so transcription doesn’t occur. This is a form of negative regulation in bacteria, where transcription is turned off until the repressor is released (often in response to an inducer). The other ideas describe steps that happen after transcription (like translating the mRNA) or processes not involved with the operator’s control (such as splicing, which is a post-transcriptional step in eukaryotes).