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Questions
What is meant by an operon?
Explain the concept of operon.
Solution 1
A unit of genetic material that functions in a coordinated manner by means of a regulator, an operator, a promoter, and one or more structural genes that are transcribed together is called an operon.
Solution 2
Concept of operon:
- The concept of the operon was first proposed by Jacob and Monod. A unit of genetic material that functions in a coordinated manner by means of a regulator, an operator, a promoter, and one or more structural genes that are transcribed together is called an operon. The clusters of genes with related functions are called operons.
- Components of operon:
- Regulator gene:
i. This gene controls the operator gene in cooperation with an inducer present in the cytoplasm.
ii. Regulator gene precedes the promoter gene. It may not be present immediately adjacent to the operator gene.
iii. Regulator gene produces a protein called repressor protein.
iv. Repressor binds with operator gene and represses (stops) its action. Therefore, it is called regulator protein. - Promoter gene:
i. This gene precedes the operator gene. It is present adjacent to the operator gene.
ii. RNA polymerase enzyme binds to the promoter gene.
iii. Promoter gene base sequence determines which strand of DNA acts as a template.
iv. When the operator gene is turned on, the enzyme moves over the operator gene and transcription of structural genes starts. - Operator gene:
i. This gene lies adjacent to the structural genes and controls their functioning.
ii. When operator gene is turned on by an inducer, the structural genes produce mRNA.
iii. Operator gene is turned off by a product of repressor gene. - Structural gene:
i. When lactose is added to the E. coli culture, the structural genes produce mRNA which in turn produces polypeptides, on the ribosomes.
| Structural Gene | Enzyme produced | Function |
| lac z | β-galactosidase | \[\ce{Lactose →[β-galactosidase] Glucose + Galactose}\] |
| lac y | Permease | Entry of lactose in the cell |
| lac a | Transacetylase | Transfers acetyl group from Acetyl CoA to β-galactosidase |
RELATED QUESTIONS
Describe how the lac operon operates, both in the presence and absence of an inducer in E.coli.
Identify the event that would occur in 'lac operon' of E. coli when the growth medium has high concentration of lactose.
In which of the following conditions the lac operon in E. coli becomes "switched on"?
In lac operon concept of gene expression, allolactose acts as, ____________.
The wild type E. coli cells are growing in normal medium with glucose. They are transferred to a medium containing only lactose as sugar. Which of the following changes takes place?
Which of the following cannot act as inducer?
Select the two correct statements out of the four (A-D) given below about lac operon.
- Glucose or galactose may bind with the repressor and inactivate it.
- In the absence of lactose, the repressor binds with the operator region.
- The z-gene codes for permease.
- This was elucidated by Francois Jacob and Jacque Mono.
Which enzyme will produce in a cell in which there is a nonsense mutation in the lac Y-gene?
Select the correct pair.
In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
