Regulation of Gene Expression

Page
1

Slide 1

Conducting the Genetic Orchestra

Prokaryotes and eukaryotes alter gene expression in response to their changing environment

In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types

RNA molecules play many roles in regulating gene expression in eukaryotes

Slide 2

Fig. 18-1

Slide 3

Concept 18.1: Bacteria often respond to environmental change by regulating transcription

Natural selection has favored bacteria that produce only the products needed by that cell

A cell can regulate the production of enzymes by feedback inhibition or by gene regulation

Gene expression in bacteria is controlled by the operon model

Slide 4

Fig. 18-2

Regulation

of gene

expression

trpE gene

trpD gene

trpC gene

trpB gene

trpA gene

(b) Regulation of enzyme

production

(a) Regulation of enzyme

activity

Enzyme 1

Enzyme 2

Enzyme 3

Tryptophan

Precursor

Feedback

inhibition

Slide 5

Operons: The Basic Concept

A cluster of functionally related genes can be under coordinated control by a single on-off “switch”

The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter

An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control

Slide 6

The operon can be switched off by a protein repressor

The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase

The repressor is the product of a separate regulatory gene

Slide 7

The repressor can be in an active or inactive form, depending on the presence of other molecules

A corepressor is a molecule that cooperates with a repressor protein to switch an operon off

For example, E. coli can synthesize the amino acid tryptophan

Slide 8

By default the trp operon is on and the genes for tryptophan synthesis are transcribed

When tryptophan is present, it binds to the trp repressor protein, which turns the operon off

The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high