I.    An Operon:

   A.    A group of genes that are transcribed as a single unit; they are transcribed into the same mRNA. The genes are located next to each other.

   B.    Is composed of the following: a promoter, operator, transcription regulators (a repressor and an activator), and a set of genes. 

   C.    An operon’s set of genes is transcribed into the same mRNA using the operon’s promoter: a DNA sequence which RNA polymerase binds to in order to                    synthesize mRNA.

   D.    Is generally found in prokaryotes.

   E.    Allows prokaryotes to control gene expression because it permits a set of genes to be under the influence of 1 promoter. Eukaryotes, on the other hand,                    generally regulate each gene individually because each gene has its own promoter.

   F.    The set of genes is controlled by the operon’s operator: a sequence of DNA that lies within an operon that turns on and off, depending on the transcription               regulator(s) bound to it. When it’s turned on, it increases the gene expression of the genes it’s associated with. When it’s turned off, it leads to the decrease. 

   G.    Different types of operons exist. For instance, the Lac operon, the Tryptophan operon.

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II.    A Promoter: a DNA sequence which RNA polymerase binds to in order to synthesize mRNA.

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III.    An Operator:

   A.    A sequence of DNA, lying within an operon, that turns on and off, depending on the transcription regulator(s) it’s bound to.

   B.    Because transcription regulators bind to operators, operators are operons’ regulatory sequences.

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IV.    A Repressor: a protein that decreases the rate of a gene’s transcription.

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V.    An Activator: a protein that increases the rate of a gene’s transcription.

  

VI.    The Lac Operon:

   A.    A type of operon found in bacteria (e.g. E.coli) that allows bacteria to break down/digest the sugar lactose, which is a disaccharide composed of galactose               and glucose.

   B.    Generally, bacteria prefer glucose as a sugar source. When glucose is not available, bacteria turn to lactose.

   C.    Its repressor is called the Lac repressor, and its activator is called the CAP activator.

   D.    Is only expressed when glucose is absent in a bacterial cell and lactose is present. Memorize the following rules below (they are also written in Table 1):

      1.    When both glucose and lactose are present in a bacterial cell, the Lac operon is turned off, so it is not expressed. Also, no regulators are bound. 

      2.    When both glucose and lactose are absent in a bacterial cell, the Lac operon is turned off, so it is not expressed. Also, both the Lac activator and the Lac                   repressor are bound.

      3.    When glucose is present in a bacterial cell but lactose is not, the operon is turned off. Also, the Lac repressor is bound.

      4.    When glucose is absent in a bacterial cell but lactose is not, the Lac operon is on. Also, the CAP activator is bound to it.

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VII.    The Tryptophan/Trp Operon:

   A.    A type of operon found in bacteria (e.g. E.coli). 

   B.    It consists of 5 genes that each encode a different enzyme. The enzymes that each gene encodes are used to make the amino acid tryptophan.

   C.    This operon is transcribed when the tryptophan amino acid is low. When the amino acid is abundant, the operon is not transcribed.

   D.    Because operators are also regulatory sequences, the Tryptophan repressor binds to the Tryptophan operator when the amino acid is abundant to prevent the            operon from promoting its production. It’s important to note, however, that in order for the Tryptophan repressor to bind to the Tryptophan operator, the                  repressor has to be bound to Tryptophan first. 

   E.    The gene for the tryptophan repressor is expressed constitutively. Therefore, the Tryptophan repressor is always present in a bacterial cell. When the                        repressor isn’t needed, it’s just produced at a lower level.