Bio102: Introduction to Cell Biology and Genetics

Regulation of Gene Expression

Key Terms:

Gene expression / lactose / trp repressor / peptide hormone
Activator / b-galactosidase / enhancer / phosphorylation
Inhibitor / CAP protein / steroid hormone / kinase
Operon / cAMP / receptor
Feedback inhibition / lac repressor / transcription factor

Key Questions:

·  All cells in the human body have the same DNA; how can they have such different functions?

·  How can a cell respond to changing environmental conditions?

·  What is an operon? Why don't we find operons in eukaryotic cells?

·  How is the trp operon shut down when tryptophan levels are high?

·  Why is the lac operon not transcribed very efficiently when lactose and glucose are both available?

·  How might a eukaryotic gene be turned "on" in one cell type but not in another?

·  What are two different ways that a eukaryotic gene could be regulated after transcription?

Lecture Outline:

Cells regulate which genes are used when to allow differentiation and environmental response

any step can be regulated, but transcription is the most common

Prokaryotes usually shut down metabolic genes when not needed.

genes of similar function are grouped together in an operon

one promoter and terminator for all the genes. one Shine-Dalgarno sequence for each gene

trp repressor protein binds to the DNA near the promoter for the trp operon

encodes five enzymes that synthesize the amino acid tryptophan (trp)

repressor binds only if the amino acid trp is present.

repressor binds the amino acid which slightly changes the protein’s shape

feedback inhibition. If the operon is turned off, trp levels fall until the operon is turned on

lac operon encodes b-galactosidase for the consumption of lactose (a sugar)

only want the operon ‘on’ if lactose is present and glucose is not

is b-galactoside being expressed?
glucose present / no glucose
no lactose / no / no
lactose present / no / yes

lac repressor binds near the promoter only if lactose is not present

if lactose is present, it binds the repressor which changes shape and releases DNA

only if no glucose is present, the cell makes cAMP which binds the CAP protein.

CAP-cAMP can bind to the DNA at a site near the lac promoter and turns transcription ‘on’

In eukaryotes, each gene has it’s own promoter with a unique combination of enhancers

only if all the correct transcription factors are present in the cell at the same time is the gene used

Steroid hormones can cross membranes and bind to their receptor proteins.

the receptor-hormone complex and bind to an enhancer and activate transcription

Peptide hormones bind a receptor on the cell membrane.

information is passed through the cytoplasm to activate a transcription factor