Unit 2: #6 Genetics: Operons (copy)

Operon

• Regulatory sequence used to regulate genes and their products 

• Only prokaryotes use operons to regulate genes and their products

What does an Operon Include?

• A cluster of structural genes 

• Promoter: Binding site for RNA polymerase 

• Operator: Short sequence of bases controls access of RNA Polymerase to gene 

• Operator switched off by protein called repressor

Repressor Protein

• Binds to operator 

• Blocks attachment of RNA Polymerase to promoter, preventing transcription

Lac Operon

• Consists of 3 genes that code for proteins involved in metabolism of lactose (lacZ, lacY and lacA) 

• These genes are only required to be transcribed when lactose is present 

Lac Operons (How it works)

• Inducible operon (to start something) 

• Presence of lactose signals the cell when to synthesize the lactose-metabolizing enzymes 

• Inducer (lactose) inactivates repressor by changing its shape and allows genes to be transcribed

• Transcription proceeds and enzyme Ƃ-galactosidase (needed to break down lactose) is made 

trp Operons

• Tryptophan is an amino acid used by E.coli to produce proteins 

• May need to make their own 

• If enough tryptophan, it can stop producing it by inhibiting genes

trp Operons (How it works)

• Repressible operon 

• Presence of tryptophan causes transcription to stop 

• Tryptophan known as co-repressor

• When trp levels are high - Tryptophan binds to trp repressor protein, activating it, thus stopping transcription

• When trp levels low - Does not bind to operator