Write the different components of a lac-operon in E. coli. Explain its expression while in an ’open’ state.
The lac operon (Inducible operon) Operon : - The concept of operon was first proposed in 1961, by Jacob and Monod Components of an operon : - (i) Structural genes : - The fragment of DNA which transcribe mRNA for polypeptide synthesis.
(ii) Promoter : - The sequence of DNA where RNA polymerase binds and initiates transcription.
(iii) Operator : - The sequence of DNA adjacent to promoter is called OPERATER.
(iv) Regulator gene : - It is the gene that codes for repressor protein which binds to operator due to which operon is switched ‘‘off’’
(v) Inducer : - Lactose is inducer which helps in switching ‘‘on’’ of operon. Lac operon consists of there structural genes (z, y, a), operator (o), promoter (p), regulatory gene (i)
When lactose is absent:- When lactose is absent, i.e. gene produces repressor protein. This repressor protein binds to operator and as a result prevents RNA polymerase to bind to operon. The operon is switched off. When lactose is present : -
• Lactose act as inducer which binds to the repressor and forms inactive repressor.
• The repressor cannot bind to operator.
• Now the RNA polymerase binds to operator and transcribes lac mRNA.
• Lac mRNA is polycistronic i.e. produces all three enzymes β−galactosidase, permease and trans-acetylase.
• The lac operon is switched on.
(a) Organisms may be conformers or regulators. Explain this statement and give one example of each.
(b) Why are there more conformers than regulators in the animal world ?
(a) Compare, giving reasons, the J-shaped and S-shaped models of population growth of a species.
(b) Explain ‘fitness of a species’’ as mentioned by Darwin.
Describe the inter-relationship between productivity, gross primary productivity and net productivity.
It is commonly observed that parents feel embarrassed to discuss freely with their adolescent children about sexuality and reproduction. The result of this parental inhibition is that the children go astray sometimes.
(a) Explain the reasons that you feel are behind such embarrassment amongst some parents to freely discuss such issues with their growing children.
(b) By taking one example of a local plant and animal, how would you help these parents to overcome such inhibitions about reproduction and sexuality ?
(a) When a seed of an orange is squeezed, many embryos, instead of one are observed. Explain how it is possible.
(b) Are these embryos genetically similar or different ? Comment.
