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Biology · 2026 · 4 marks
CBSE 2026 · Region 4 · Set 1 · Q29
Read the following passage and answer the questions that follow: Gene expresses itself in a cell system as a protein/enzyme. How an expression of…
Read the following passage and answer the questions that follow : Gene expresses itself in a cell system as a protein/enzyme. How an expression of gene occurs in a cell system, when it needs to occur and how the gene expression is regulated in prokaryotic cell system was studied by the combined efforts of Francois Jacob, a geneticist and Jacques Monod, a biochemist. For their work on lactose metabolism in E. coli and introducing the concept of 'lac operon', they were awarded the Nobel Prize in 1965.
(a)
It is said that 'lac operon' has to be operational at a very low level in the bacterial cell all the time. Justify.
(b)(i)
How are the structural genes activated in the lac operon in E. coli ?
OR
(b)
(ii)
Why is the regulation of operon called negative regulation ?
(c)
Why is lac operon said to be a transcriptionally regulated system ?
Marking-scheme solution
(a)
It is necessary for the entry of lactose into the cell .
(b)(i)
In the presence of lactose the repressor is inactivated by the interaction with
the inducer that allow RNA polymerase access to promoter and transcription proceeds.
OR
(b)
(ii)
It is called negative regulation because the operon is switched off in the
absence of inducer and transcription is prevented.
(c)
In the absence of the inducer the repressor binds to the operator and prevents
transcription of the structural gene.
When inducer or lactose is present it prevents the repressor from binding to
the operator hence regulation is at the transcriptional level.
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CBSE Class 12 Biology past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.