12th Class Biology Molecular basis of inheritance

  • question_answer 81)     Define an operon, giving an example, explain an inducible operon.


                      The concept of operon was first proposed in 1961, by Jacob and Monad. An operon is a unit of prokaryotic gene expression which includes coordinately regulated (structural)genes and control elements which are recognized by regulatory gene product. Components of art Operon (i) Structural gene The fragment of DNA which transcribe mRNA for polypeptide synthesis. (ii) Promoter The sequence of DNA where RNA polymerase binds and initiates transcription of structural genes is called promoter. (iii) Operator The sequence of DNA adjacentto promoter where specific repressor protein binds is called operator. (iv)RegulatorgeneThe gene that codes for the repressor protein that bind to the operator and suppresses its activity as a result of which transcription will be switched off. (v)InducerThe substrate that prevents the repressor from binding to the operator, is called an inducer. As a result transcription is switched on. It is a chemical of diverse nature like metabolite, hormone substrate, etc. Inducible Operon System An inducible operon system is a regulated unit of genetic material which is switched on in response to the presence of a chemical, e.g., the lactose or lac-operon of E. coli. The lactose operon Thelac z, y, a genes aretranscribed from a lac transcription unit under the control of a single promoter. They encodeenzyme required for the use of lactose as a carbon source. The laci gene product, the lacrepressor, is expressed from a separate transcription unit upstream from the operator.lac operon consists of three structural genes(z, y and a), operator, promoter and aseparate regulatory gene. The three structural genes (a, y and a) transcribe a polycistronic mRNA.     Gene z codes for (3-galactosidase (-gal) enzyme which breaks lactose into galactose and glucose. Gene y codes for permease, which increases the permeability of the cell to lactose. Gene a codes for enzyme transacetylase, which catalyses the transacetylation of lactose in its active form. When Lactose is Absent (i) When lactose is absent, i gene regulates and produces repressor mRNA which translate repression. (ii) The repressor protein binds to the operator region of the operon and as a result prevents RNA polymerase to bind to the operon. (iii) The operon is switched off. When Lactose is Present (i) Lactose acts as an inducer which binds to the repressor and forms an inactiverepressor. (ii) The repressor fails to bind to the operator region. (iii) The RNA polymerase binds to the operator and transcript lac mRNA. (iv) lac mRNA is polycistronic. i.e., produces all three enzymes, b-galactosidase, permeaseandtransacetylase. (v) The lac operon is switched on.

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