Q.16 Which of the following is a cis-acting element?
1. cAMP response binding protein
2. Operator
3. Repressor
4. Terminator
What Are Cis-Acting Elements in Gene Regulation?
Cis-acting elements are specific DNA sequences that regulate gene expression on the same DNA molecule, acting as binding sites for regulatory proteins. They include promoters, operators, enhancers, and terminators, influencing transcription without diffusing away.
The correct answer to the question “Which of the following is a cis-acting element?” is 2. Operator.
Correct Answer Breakdown
The operator is a classic cis-acting element, a short DNA sequence near the promoter where repressor proteins bind to block transcription initiation in operons like the lac operon.
It functions on the same DNA molecule (“cis” means “same side”), controlling nearby genes without encoding proteins itself.
Option Explanations
| Option | Description | Cis-Acting? | Reason |
|---|---|---|---|
| 1. cAMP response binding protein | A trans-acting protein (transcription factor) that binds cAMP and activates genes like those in catabolite repression. | No | Proteins like CRP diffuse and act from elsewhere; they bind cis elements. |
| 2. Operator | DNA sequence where repressors bind to prevent RNA polymerase access, as in bacterial operons. | Yes | Stays on the same DNA molecule, regulating adjacent genes. |
| 3. Repressor | A trans-acting protein that binds operators to inhibit transcription. | No | Encoded by separate genes, it acts across DNA molecules. |
| 4. Terminator | DNA or RNA sequence signaling transcription end by releasing RNA polymerase. | Yes (sometimes debated) | Functions locally on the same transcript, but primarily classified as cis in prokaryotes. |
Why Operator Fits Best
In standard molecular biology MCQs, operator exemplifies cis-acting elements due to its role in prokaryotic regulation. Terminators are cis but less emphasized as “regulatory elements” compared to operators/promoters.
This distinction aids exam prep in genetics and microbiology, aligning with topics like operon models.
