Definitions
• Gene – sequence of DNA that is
expressed as a protein (exon)
• Genes are coded
– DNA →RNA→Protein→Trait
• Transcription – rewritting DNA into RNA
which is needed for a gene to be
expressed as a protein
• Operon – a sequence of DNA that will turn
transcription on and off
Parts of an Operon
• Regulatory gene
– Repressor
• Promoter
Operator
Structural Genes
Parts of an Operon
• Promoter – site where RNA polymerase
attaches to begin transcription
• Operator – on/off switch
– Repressor will attach turning off transcription
– If repressor doesn’t fit into operator, RNA
polymerase can transcribe structural genes
– Transcription occurs
Operator/Repressor
• Lock n’ Key
arrangement
Inducible Operon
• Transcription is normally off
• Repressor is active
– Fits into operator turning off transcription
• Inducer
– Molecule from the environment
– Attaches to the repressor
– Changes the shape of the repressor making it
inactive.
• Inducible Operon
– Repressor is active
• RNA polymerase can not attach to
promoter
• No proteins are translated (synthesized)
• Inducer needs to
be present to
start
transcription
• i.e. inducer is
lactose
Transcription is on
• The end proteins
produced are
enzymes
• i.e. lactase
• Operon is off
– Repressor is
active
– Transcription
does not occur
• Operon is on
– Inactive
repressor
– Proteins
(enzymes) are
made
Regulatory Gene makes an active repressor
Inducible Animation
Lac Operon
• Transcription is normally off
• Repressor is active (fits into operator)
• Need an outside molecule from
environment to inactivate repressor
• Inducer has the role of inactivating
repressor
• Proteins produced through transcription
are enzymes.
• Enzymes break down inducer.
• Inducer is absent which makes repressor
active again.
Repressible Operon
• Transcription is normally on
• Repressor is inactive
– Repressor does not fit into the operator
• CoRepressor is needed to stop
transcription
– COREPRESSOR is the end protein produced
by transcription
– Structural protein used by the cell
End product = corepressor
Tryptophan is the corepressor
Repressible Operon Animation
Trpytophan Operon
• Inactive repressor
• The build up of the end product turns off
transcription
• Usually makes structural proteins
– Proteins needed for cell function
• When structural proteins are used up by
the cell; the corepressor is absent
• Transcription will turn on again.