Question
Which of the following statements about lac-operon is correct ?
This question was previously asked in
NEET UG Re-Exam 2026 Question Paper (21-Jun-2026)
The correct answer is
Gene i is constitutively expressed
Lac-Operon Gene i Constitutive Expression Analysis
The lac-operon is a key example of gene regulation in prokaryotes. Understanding the function and expression patterns of its genes is crucial.
Analyzing Lac-Operon Statements
- Option 1: Lactose activates repressor to bind to the operator
This is incorrect. The repressor protein normally binds to the operator sequence, blocking transcription. Lactose (or its isomer, allolactose) binds to the repressor, causing a conformational change that makes the repressor *release* from the operator, thereby *allowing* transcription. - Option 2: Genes i, z, y and a share single common promoter
This is incorrect. The structural genes $z$, $y$, and $a$ are transcribed as a single unit (polycistronic mRNA) from one promoter. However, the regulatory gene $i$, which codes for the repressor protein, has its own distinct promoter and is transcribed separately. - Option 3: Galactose can act as an inducer of lac operon
This is incorrect. While galactose is a related sugar, it is not the natural inducer of the lac-operon. The inducer is typically allolactose, a structural isomer of lactose formed within the cell. - Option 4: Gene i is constitutively expressed
This is correct. The $i$ gene encodes the lac repressor protein. This repressor protein is required to regulate the operon, even when lactose is absent. Therefore, the $i$ gene is transcribed and translated continuously, meaning it is constitutively expressed, ensuring a baseline level of repressor is always present.
Conclusion
Based on the analysis, the only correct statement regarding the lac-operon is that the regulatory gene $i$ is constitutively expressed.
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Similar Questions
- Arrange the following steps of DNA fingerprinting in a correct sequence.
A. Isolation of DNA and its digestion by restriction endonucleases.
B. Hybridisation using a labelled VNTR probe.
C. Transferring of separated DNA fragments to synthetic membranes.
D. Detection of hybridised DNA fragments by autoradiography.
E. Separation of DNA fragments by electrophoresis.
Choose the correct answer from the options given below : - In the lac operon, the z gene codes for :
- Which of the following statements are correct with reference to packaging of DNA helix ?
A. Histones are organized to form a unit of eight molecules called histone octamer.
B. Histones are negatively charged basic proteins.
C. Histones are rich in the basic amino acid residues - lysine and arginine.
D. The positively charged DNA is wrapped around the histone octamer to form nucleosome.
E. The packaging of chromatin at higher levels requires an additional set of proteins called non-histone chromosomal proteins.
Choose the correct answer from the options given below : - The correct statement with regard to the secondary structure of DNA/RNA is :
- Which of the following statements are correct with reference to a transcription unit ?
A. A transcription unit in DNA is defined primarily by three regions : promoter, structural gene and terminator.
B. The promoter is said to be located towards the 5'-end of the structural gene.
C. The promoter is a DNA sequence that provides binding site for RNA polymerase.
D. The promoter defines the template and coding strands.
E. The terminator is located towards the 3'-end of the coding strand and it defines the end of the process of transcription.
Choose the correct answer from the options given below : - Which of the following enzymes synthesizes precursor mRNA ?
Important Questions from Molecular Biology
- Arrange the following steps of DNA fingerprinting in a correct sequence.
A. Isolation of DNA and its digestion by restriction endonucleases.
B. Hybridisation using a labelled VNTR probe.
C. Transferring of separated DNA fragments to synthetic membranes.
D. Detection of hybridised DNA fragments by autoradiography.
E. Separation of DNA fragments by electrophoresis.
Choose the correct answer from the options given below : - In the lac operon, the z gene codes for :
- Which of the following statements are correct with reference to packaging of DNA helix ?
A. Histones are organized to form a unit of eight molecules called histone octamer.
B. Histones are negatively charged basic proteins.
C. Histones are rich in the basic amino acid residues - lysine and arginine.
D. The positively charged DNA is wrapped around the histone octamer to form nucleosome.
E. The packaging of chromatin at higher levels requires an additional set of proteins called non-histone chromosomal proteins.
Choose the correct answer from the options given below : - The correct statement with regard to the secondary structure of DNA/RNA is :
- Which of the following statements are correct with reference to a transcription unit ?
A. A transcription unit in DNA is defined primarily by three regions : promoter, structural gene and terminator.
B. The promoter is said to be located towards the 5'-end of the structural gene.
C. The promoter is a DNA sequence that provides binding site for RNA polymerase.
D. The promoter defines the template and coding strands.
E. The terminator is located towards the 3'-end of the coding strand and it defines the end of the process of transcription.
Choose the correct answer from the options given below :
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