Class 12th Biology 2017 Set1 Outside Delhi Board Paper Solution
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)
• Gene z codes for β-galactosidase
• Gene y codes for permease.
• Gene a codes for enzymes transacetylase.
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.
Popular Questions of Class 12th biology
- Q:- What is spermatogenesis? Briefly describe the process of spermatogenesis.
- Q:- Why is reproduction essential for organisms? Answer
- Q:- Write a short note on
(a) Adaptations of desert plants and animals
(b) Adaptations of plants to water scarcity
(c) Behavioural adaptations in animals
(d) Importance of light to plants
(e) Effect of temperature or water scarcity and the adaptations of animals. - Q:- With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.
- Q:- What is DNA fingerprinting? Mention its application.
- Q:- Describe the structure of a seminiferous tubule.
- Q:- What is oogenesis? Give a brief account of oogenesis.
- Q:- What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion.
- Q:- With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.
- Q:- Explain the Law of Dominance using a monohybrid cross.
Recently Viewed Questions of Class 12th biology
- Q:- An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?
- Q:- Identify True/False statements. Correct each false statement to make it true.
(a) Androgens are produced by Sertoli cells. (True/False)
(b) Spermatozoa get nutrition from Sertoli cells. (True/False)
(c) Leydig cells are found in ovary. (True/False)
(d) Leydig cells synthesise androgens. (True/False)
(e) Oogenesis takes place in corpus luteum. (True/False)
(f) Menstrual cycle ceases during pregnancy. (True/False)
(g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience. (True/False) - Q:- Why is reproduction essential for organisms? Answer
- Q:- What is DNA fingerprinting? Mention its application.
- Q:- Differentiate between gametogenesis from embryogenesis.
- Q:- Suggest the aspects of reproductive health which need to be given special attention in the present scenario.
- Q:- What do you think is the significance of reproductive health in a society?
- Q:- Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q:- What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?
- Q:- Discuss with your teacher and find out how to distinguish between
(a) Plasmid DNA and chromosomal DNA
(b) RNA and DNA
(c) Exonuclease and Endonuclease