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 AnimationTrpytophan 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.