Lecture Outline for March 14, 2005


Topic: gene regulation

Chapter 13 sections 13.1

Figures used in lecture: 13.3, 13.4, 13.5

Additional resources:

            Prokaryotic Gene Regulation

            The Lac Operon

            The Operon


Key Terms

constituitive genes, inducible genes, operon, promoter, operator, structureal genes, inducer, repressor, beta-galactosidase, lactose permease, transacetylase


To express all of the genes of a cell all of the time would be a waste of valuable energy. Therefore, the expression of genes is regulated. Many genes are only transcribed and translated when their protein product is required in the cell.


You should be able to:

1.  Describe the general structure of an operon including the location of regulatory sequences and structural genes.


2.  Describe the state of the lac operon when the cell is using glucose as an energy source.  You can describe the state of the lac operon by answering the following questions:

Is the lac I gene being expressed to produce the repressor protein? 

Is the inducer present?  

Will the repressor bind to the operator?  Why or why not?

If the repressor binds to the operator, will the structural genes be transcribed?

If the structural genes are transcribed and translated, what enzymes are produced and what are their functions?


3.  Describe the state of the lac operon when the cell is using lactose as an energy source.

Ask yourself all of the same questions listed in #2 above.



Gene Regulation

genes are transcribed and translated to produce the protein product or they remain inactive.


Constituitive genes –


Inducible genes -


Operon -



lac Operon

- inducible gene system

- lac = lactose

- allows bacterial cells to use lactose as an energy source


Regulatory Sequences

Promoter -


Operator -



Structural genes produce 3 enzymes:

1. β-galactosidase –


2. permease –


3. transacetylase –



Repressor –



Fig.  13.3b

lac Operon  When glucose is present…

- bacterial cells prefer glucose

- lac operon is not expressed

- repressor protein -     binds to the operator to block transcription of the structural genes


Fig.  13.3c

lac Operon  When lactose is used…

- an inducer binds to the repressor

- shape of the repressor changes

- repressor can no longer bind to the operator

- transcription of structural genes

- inducer = lactose