Daily Newsletter April 4, 2012
Today's Topic: Lac Operon
One of the most well studied gene regulation systems is the Lac Operon found in Escherichia coli. To understand this system, it is important to first understand that bacteria do not experience Transcription and Translation in exactly the same way as eukarotes (the mechanics are very similar, but there are some distinct differences).
Bacteria lack a nucleus, thus transcription takes place in the cytosol. There is no need for a tag to get out of the nucleus, so there is no need for capping. Also, there are no introns, so no need for splicing. Basically, there is no processing of RNA to make mRNA. What is transcribed is mRNA.
Since there is no membrane separating transcription from translation, you can couple these to processes. As mRNA is made, it can be translated.
Bacteria also have a single circular molecule of DNA (a genophore, not a chromosome). They have to conserve their genetic space, so bacteria combine genes in a single mRNA. IMPORTANT: bacteria can combine genes for a metabolic pathway into a sequential sequence with a single promoter. Thus, when you transcribe, you get all the genes for a given metabolic pathway.
The word OPERON describes this unique arrangement of prokaryotic genes: One promoter and one operator for a given series of metabolically linked genes.
The Lac Operon holds three genes that give the cell the ability to take in and use the sugar LACTOSE. For E. coli, glucose is the preferred sugar. When glucose is present, there is no need to use lactose: these genes are not transcribed. When there is no glucose, E. coli has to use other sugars. IF lactose is present, the genes for lactose utilization will be made. Conversely, if there is no lactose, they genes remain locked down.
Here are three articles that go over different aspects of prokaryotic regulation and the Lac Operon:
- Operons and Prokaryotic Gene Regulation
- Negative Transcription Regulation in Prokaryotes
- Positive Transcription Control: The Glucose Effect
1) You can block the operator of the gene. This prevents RNA polymerase from making RNA.
2) You can alter the promoter (or the interaction between transcription factors and DNA) to prevent binding of the Transcription Complex (RNA polymerase).
Daily Challenge:
In your own words, describe the regulation of the Lac Operon. Start with a brief discussion of the nature of an operon using the Lac Operon as your model. Then discuss the control of the Lac Operon expression.
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