The lac operon is a system that allows some bacteria to digest lactose, or milk sugar, under certain conditions. The operon ensures that lactase, the enzyme that digests lactose, is produced only in the presence of the lactose sugar.
Lactose
Lactose is the chemical name for milk sugar, a disaccharide molecule -- meaning it is made of two smaller sugar units -- composed of glucose and galactose, explain Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry." The enzyme lactase breaks down lactose into its constituent units to provide cells with energy. Most humans make lactase all the time. Many bacteria, such as Escherichia coli, make lactase only when it is needed, which helps to conserve resources.
Lac Operon
The system in E. coli and some other bacterial species that helps them to produce lactase when necessary is called the lac operon. This is a sequence of DNA that contains the genetic information for making lactase. Lactase is a protein; to make it, bacteria need to produce a working copy of the information contained in the DNA using RNA, which is similar to DNA. The cell's "machinery" then reads the information contained in the RNA and uses it to make the functional lactase protein.
Lac Repressor
Under normal circumstances, a large protein called a repressor prevents a cell from making RNA using the information contained in the lac operon region of DNA. The repressor does not prevent the cell from making proteins using other regions of DNA; it affects only the lactase-encoding region. When E. coli encounters lactose -- and thus needs to produce lactase -- the repressor moves off the DNA and allows the cell to make RNA, and eventually functional lactase, using the information on the DNA.
Other Considerations
For E. coli to make lactase, two conditions must be met: the bacteria must have access to lactose, and they must be low on glucose. Glucose, like lactose, can provide E. coli with energy; it is not worthwhile for bacteria to produce lactase, which uses up resources, if glucose is available for use. Activating the lac operon requires both removing the repressor and activating a promoter site, which happens in the absence of glucose, explain Drs. Thomas Pollard and William Earnshaw in their book, "Cell Biology."
References
- "Biochemistry"; Reginald Garrett, Ph.D., and Charles Grisham, Ph.D.; 2007
- "Cell Biology"; Thomas Pollard, M.D., and William Earnshaw, Ph.D.; 2007
Member Comments