Chapter 15 - Regulation

Genetics

Gene regulation/ genetic control

determines when, where, and how much a gene is expressed

Regulation of transcription

• Every gene = RNA coding region

• Regulatory regions are not transcribed 

• transcription factors are DNA-binding proteins that recognize specific sequences within the regulatory region(s) near a gene and either activate or repress it 

In bacteria the sigma factor helps core RNA polymerase bind to the promoter and start transcription at +1 forming the holoenzyme 

Operon

Common in prokaryotes, rare in eukaryotes

• A cluster of structural genes with similar functions under the control of a regulatory system that responds based on changes in environmental conditions

Transcription of the polycistronic mRNA (a single mRNA that contains genetic information from multiple genes) from the lac operon

1. The lac operon has three structural genes: lacZ, lacY, lacA.

2. Transcription produces a single polycistronic mRNA.

3. Ribosomes move along this mRNA to code for three lac operon proteins: β-galactosidase, Permease, and Transacetylase.

The lac operon (specific operon)

The genes of the lac operon are only expressed when glucose is absent and lactose is present 

• The gene will only be expressed if the cell needs to use lactose for energy BUT glucose is the cells main source of energy 

• B- galactosidase is made when glucose is absent but lactose is present 

Products of the structural genes of the lac operon (products made when the lac operon is on)

lacZ gene product: B- galactosidase

• an enzyme that breaks down lactose (a disaccharide) into galactose and glucose

• can isomerize lactose into allolactose 

lacY gene product: permease

• membrane transporter for lactose 

• facilitates lactose entering the bacterial cell 

lacA gene product: transacetylase 

• NOT involved in lactose metabolism 

• involved in the removal of by- products of lactose digestion form the cell