Regulation of Gene Expression and Cell Specialization
Transcription
Translation
DNA
mRNA
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Bacterial Gene Expression in Transcription
Gene Expression Prokaryotes and eukaryotes must be able to regulate which genes are expressed at any given time. Genes can be turned "on" or "off" based on environmental and internal cues. This on/off regulation refers to whether or not transcription will take place, allowing for cell specialization.
Bacterial Gene Expression Example
Monocistronic mRNA
5'
AUG
Polycistronic mRNA
AUG
Peptide
STOP
Peptide A
AUG
STOP
Peptide B
Gelling Down Wit Science
STOP
3'
Bacterial Gene Expression Steps
Operons: a group of genes that can be turned on or off. Operons have three parts:
- Promoter: where RNA polymerase can attach
- Operator: the on/off switch
- Genes: code for related enzymes in the pathway
Inducible and Repressible Operons
Operons can be repressible or inducible. Repressible operons have transcription usually on, but can be repressed (stopped). Inducible operons have transcription usually off, but can be induced (started).
Regulatory Gene
The regulatory gene produces a repressor protein that binds to the operator to block RNA polymerase from transcribing the gene. It is always expressed but at low levels, and the binding of a repressor to an operator is reversible.
Allosteric Regulation in Gene Expression
Before going through examples, let's review allosteric enzymes. Allosteric activators stabilize the shape of the enzyme so that the active sites remain open, while allosteric inhibitors stabilize the enzyme shape so that the active sites are closed.
Allosteric Regulation Example
Repressible Operons
An example is the trp operon in bacteria, which controls the synthesis of tryptophan. It is repressible, and transcription is active unless repressed by a trp repressor. When too much tryptophan builds up in bacteria, tryptophan is more likely to bind to the repressor, turning it active (off) and temporarily shutting off transcription for tryptophan.
Inducible Operon Example
The lac operon allows for the effective digestion of lactose when glucose is not available. While it is inducible, transcription is usually off, with a lac repressor bound to the operator. When lactose is present, allolactose binds to the lac repressor and makes it let go of the operator, allowing RNA polymerase to transcribe the operon.
Overview: trp Operon
When tryptophan is present, the trp repressor binds to the operator, blocking RNA synthesis. In the absence of tryptophan, the repressor dissociates from the operator, and RNA synthesis proceeds.
Overview: lac Operon
The lac repressor is active and bound to the operator. When allolactose is present, it binds to the lac repressor and makes it inactive, allowing the digestive genes to be transcribed.
Both regulate transcription of genes. The trp operon is repressible, while the lac operon is inducible.
Understanding the steps involved in bacterial gene expression is essential for comprehending the regulation of gene expression in bacteria. Through the examples of the trp and lac operons, we can see how gene expression can be repressed or induced based on the environmental and internal cues the bacteria encounter. This knowledge is crucial for understanding how bacteria adapt to their surroundings.
