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Gene expression in Bacteria

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Gene Expression in Prokaryotes


Gene expression is taking the instructions from DNA and turning them into the proteins to do or be something within a cell.


The gene expression in bacteria is controlled by an operon. 


Remember that DNA is translated into RNA and then the RNA is translated into a protein.  The protein may be used as an enzyme.  The purpose of an enzyme is to lower the activation energy and encourage a reaction to take place.



An operon is a segment of DNA that contains structural genes.  Structural genes can be transcribed in to RNA and the translated into a protein or polypeptide.  This takes place using the enzyme RNA polymerase.  The polypeptide or protein made can then become enzyme, be integrated into a cell membrane, used for transport, release energy, or another function in the cell.



The promoter is an area on the DNA before the structural genes.  The RNA polymerase must bind to the promoter to begin transcribing the structural genes.



Between the promoter and the structural genes is a segment of DNA called the operator.  The operator controls whether or not transcription will take place.  The operator is on when nothing is bound to it but it can be shut off if a repressor binds.



The repressor is coded for by a regulatory gene.  The regulatory gene is located some distance in front of the promoter.  Repressors are always being made but in constant numbers.  Once the repressor binds to the operator, the RNA polymerase no long “fits” on the DNA strand and therefore, transcription cannot take place.  This process is reversible.  Also, the repressor is synthesized in an inactive form that will not allow it to attach to the operator. 



The corepressor is a molecule that binds to the repressor, changes its conformation (shape), and allows it to fit into the operator.  Once this takes place, the RNA polymerase cannot pass down the DNA.  The corepressor is often the end product of the enzyme chain.  For instance in the tryptophan chain, tryptophan is the corepressor that binds to the repressor to tell the operator to stop which inhibits the RNA polymerase from transcribing the DNA strand.  This usually happens in response to a high concentration of tryptophan in the cell.  Once the level of tryptophan is decrease, the repressor is removed and transcription begins again.