Bio2051ch10_12 - Chapter 10 cont The amount of an enzyme in...

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Chapter 10 cont. The amount of an enzyme in the cell can be controlled by decreasing transcription ( repression ) or increasing transcription ( induction ) and thus the amount of mRNA that encodes the enzyme For negative control of transcription, the regulatory molecule is called a repressor protein and it functions by inhibiting mRNA synthesis The E. coli lac Operon - Lactose (milk sugar) is used as nutrient source Cannot pass through cell membrane Lactose permease allows entry Disaccharide lactose must be cleaved into monosaccharides (glu and gal) to be digested b-galactosidase cleaves lactose People also make b-galactosidase If not, person is lactose-intolerant The lacZ gene encodes b-galactosidase The lacY gene encodes lactose permease Need both proteins to digest lactose Operon Multiple genes transcribed from one promoter Both genes are transcribed together Repressor protein LacI blocks transcription Repressor binds to operator Blocks sigma factor from binding promoter Repressor responds to presence of lactose Binds inducer (allolactose) or DNA, not both Add lactose repressor falls off operator
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lac operon is an example of negative control of transcription. (allolactose) For positive control of transcription an activator protein binds to activator-binding sites on the DNA to stimulate transcription. For transcription of the maltose operon: 1. Maltose (inducer) binds to the maltose activator protein 2. maltose activator protein binds to the binding site on the DNA RNA polymerase can then proceed with transcription. The
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Bio2051ch10_12 - Chapter 10 cont The amount of an enzyme in...

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