CBSE 2026 · Region 1 · Set 3 · Q32 · 5 marks
Describe how the lac operon operates both in the presence and absence of an inducer in E. coli.(b)Differentiate between Mendelian and Chromosomal disorders.(ii)Which disorder is known as"Inborn Error of Metabolism"? Mention its cause and any one symptom. Name the harmful compounds formed.
Describe how the lac operon operates both in the presence and absence of an inducer in E. coli.
(b)
Differentiate between Mendelian and Chromosomal disorders.
(ii)
Which disorder is known as"Inborn Error of Metabolism"? Mention its cause and any one symptom. Name the harmful compounds formed.
Marking-scheme solution
Presence of Inducer `
-Repressor protein is synthesised from the i gene
Lactose act as inducer
-Repressor protein is inactivated by interaction with the inducer molecule ie lactose molecule and thus it does not bind to operator.
-Operon consist of three structural genes z,y and a
-RNA polymerase binds to promoter and initiates transcription ,
Producing enzymes β – galactosidase, permease and transacetylase from
the genes (z, y, a) thus helping in metabolism of lactose.
Absence of Inducer
The regulatory gene i of lac operon produces a repressor protein and this protein binds to operator region of operon, and prevents RNA polymerase from transcribing the structural genes (z, y, a) not transcribed and thus enzymes are not produced.
(b)
| Mendelian Disorders | Chromosomal Disorders |
| Due to alteration or mutation in a single gene. | Absence or excess or abnormal arrangement of chromosomes. |
| Inheritance pattern can be studied by pedigree analysis | Cannot be studied by pedigree analysis |
(ii)
• Phenylketonuria
• Cause –Mutation in the gene that codes for the enzymes phenyl alanine hydroxylase (single gene mutations )
• One symptoms -Mental retardation, Hair loss, Skin pigmentation
• Harmful Compounds Formed
Phenylpyruvic acid and other derivatives.
Molecular Basis of InheritanceRegulation of Gene ExpressionUnderstandlong_answermedium
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CBSE Class 12 Biology past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.