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Home »  Biology »  Genetics

Control of Gene Expression in Prokaryotes

Topic Review on "Title":

Introduction
Gene expressions are strictly controlled at many levels to ensure the organism having the appropriate response to its environment or internal changes.  This is important for prokaryotes because there are usually single-cell organisms, and they largely depend on their environment for all of their activities. 

Transcription in bacteria
In bacteria transcription often occur as polycistrons, i.e., many functional-related genes are clustered and transcribed under the same types of regulation.  These are called operons.  An operon usually contains regulatory genes and structure genes.  The gene expression can be induced under certain circumstances or be constitutive.  

Lac operon
Lac operon are activated by lactose, which binds to Lac I,  a Repressor, and removes it from the operator sequence, and therefore release the repression from Lac I, the consequence is that the structure genes lac Z. lac Y and Lac A are expressed, and the cells are able to use lactose as carbon source. 

Trp  operon
Similar to Lac operon, Trp operon has TrpR repressor for normal repression of the operon.  But in contrary to lac operon, TrpR is normally inactivate, only in the presence of large amount of Trp, TrpR is activated and the trp operon is suppressed.  Trp operon also has an attenuator sequence located within the operon to enable the operon sense the decrease or increase of the trp in the environment. 

Gene expression in Bacteriophage
When l phage switch between lysogeny and lytic cycles, two repressors play critical roles: cI and Cro.  cI encodes Lambda repressor. Cro encode a protein that controls the repressor (and other genes).  When cI proteins predominate, phage remains in the lysogenic state; When Cro proteins predominate, phage enters the lytic state. The regulation on cI and Cro is very artistic and delicate.


Rapid Study Kit for "Title":
Flash Movie Flash Game Flash Card
Core Concept Tutorial Problem Solving Drill Review Cheat Sheet

"Title" Tutorial Summary :

Gene expressions are strictly controlled at many levels to ensure the organism having the appropriate response to its environment or internal changes.  In bacteria many functional-related genes are clustered and transcribed under the same types of regulation (operons).  An operon usually contains regulatory genes and structure genes.  The most studied operons in bacteria are Lac operons and Trp operons, in both of which cases the activation or inactivation of repressors play key roles in gene regulation.  This is also true in l phage in terms of regulating its lytic and lysogenic cycles: When cI proteins predominate, phage remains in the lysogenic state; When Cro proteins predominate, phage enters the lytic state.


Tutorial Features:
  • Concept map to depict the gene regulation in prokaryotes.
  • Detailed step by step illustration of lac operons
  • Detailed illustration of Trp operon and its attenuator
  • Diagrammatic illustration of l phage gene regulation on cI and Cro
  • Concise key concept sheets

"Title" Topic List:

Gene regulation in prokaryotes

  • Introduction
  • Transcription in bacteria
  • Operons
  • Trans and cis-acting elements
  • Inducible and constitutive expression
  • Repressor vs activator

Lac operon

  • Structure
  • Repression
  • Removal of repression
  • Catabolic repression

Trp operon

  • Structure
  • TrpR and Trp: repressor and co-repressor
  • Attenuator

Gene expression in l phage

  • Life cycle of l phage
  • The phage switch: cI and Cro
  • Establish lysogenic cycle
  • Induction of lytic cycle

See all 24 lessons in Genetics, including concept tutorials, problem drills and cheat sheets:
Teach Yourself Genetics Visually in 24 Hours

Home »  Biology »  Genetics
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