The question asks to identify the correct statements about the $lac$ operon in $E. coli$ when both glucose and lactose are present.
Understanding the $lac$ Operon Regulation
The $lac$ operon controls the breakdown of lactose. Its expression is regulated by two main factors:
- Glucose: High glucose levels lead to low cyclic AMP (cAMP) levels, which reduces the binding of the CAP protein complex to the promoter, thus repressing the operon (catabolite repression). Low glucose levels lead to high cAMP levels, favoring CAP binding and promoting transcription.
- Lactose: Lactose acts as an inducer. In the absence of lactose, the $lac$ repressor protein binds to the operator site, blocking transcription. When lactose is present, it binds to the repressor, causing the repressor to detach from the operator, allowing transcription.
Analysis of Conditions and Options
The specific condition is the presence of both glucose and lactose.
- Glucose effect: The level of glucose is critical.
- Lactose effect: Lactose is present, meaning the repressor should be inactivated (detached from the operator).
Let's analyze the provided correct options:
Option A Analysis: "At low glucose level, the operon is activated"
This statement is correct. When glucose is low, cAMP levels rise. The CAP-protein binds cAMP, forming a complex that binds to the promoter region near the $lac$ operon's start site. This binding enhances RNA polymerase activity. Since lactose is also present, it ensures the repressor is not blocking the operator. Therefore, low glucose conditions combined with the presence of lactose lead to the activation of the $lac$ operon.
Option C Analysis: "The $lac$ repressor binds to operator region inactivating the operon"
This statement is considered correct in the context of understanding the repressor's fundamental role. The $lac$ repressor protein's primary function is to bind to the operator sequence. This binding event physically obstructs RNA polymerase, thereby preventing transcription and inactivating the operon. Although the presence of lactose (as stated in the question's scenario) causes the repressor to detach from the operator, this statement accurately describes the *mechanism* by which the repressor *can* inactivate the operon under different conditions (i.e., when lactose is absent). The statement describes the repressor's inherent capability and purpose in regulation.
Other Options Analysis (Briefly)
Other options are not selected as correct based on the provided answer key. For instance, Option B is incorrect because high glucose levels lead to repression, not activation. Option 4 describes the induction mechanism but might be considered incomplete or less fitting compared to the selected options in this specific context.
Therefore, statements A and C are identified as correct.
Important Questions from Regulation of Gene Expression
- Which of the following conditions induce(s) the expression of $ \beta$-galactosidase gene in the lac operon?
Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: In multicellular organisms, cells of different lineages have different gene expression profiles.
Reason [r]: Alternative splicing is the only mechanism to generate protein diversity.- The event(s) that lead(s) to inactivation of tumor suppressor genes in cancer cells is(are)
Determine the correctness or otherwise of the following Assertion [a] and the Reason [r]
Assertion: Ab initio gene finding algorithms that predict protein coding genes in eukaryotic genomes are not completely accurate.
Reason: Eukaryotic splice sites are difficult to predict.
A recombinant protein is to be expressed under the control of the lac promoter and operator in a strain of E. coli having the genotype $lacI^+ \ crp^+$. Even in the absence of inducer IPTG, low levels of expression of the recombinant protein are seen (leaky expression). Which one of the following should be done to minimize such leaky expression?
