AP Biology: Control of Prokaryotic and Eukaryotic Genes, Lecture notes of Biology

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AP Biology 2007-

Control of

Prokaryotic (Bacterial) Genes

What you must know….

 That genes can be activated by inducer molecules or that they

can be inhibited by the presence of a repressor as they interact

with regulatory proteins or sequences

 That a regulatory gene is a sequence of DNA that codes for a

regulatory protein such as a repressor protein

 How the components of an operon function to regulate gene

expression in both repressible and inducible operons

 How positive and negative control function in gene expression

 The impact of DNA methylation and histone acetylation on gene

expression

 How timing and coordination of specific events are regulated in

normal development, including pattern formation and induction

 The role of microRNA in control of cellular functions

 The role of gene regulation in embryonic development and

cancer

Bacterial metabolism

 Bacteria need to respond quickly to

changes in their environment

 if they have enough of a product,

need to stop production

 why? waste of energy to produce more

 how? stop production of enzymes for synthesis

 if they find new food/energy source,

need to utilize it quickly

 why? metabolism, growth, reproduction

 how? start production of enzymes for digestion

STOP

GO

AP Biology

Prokaryotic Control of Metabolism

 Natural selection has favored

bacteria that express only the

genes whose products are

needed by the cell (not

wasting resources).

 Metabolic Controls

 Cells can adjust the activity of

enzymes already present.

 Quick response.

 Feedback inhibition.

 Cells can adjust the

production level of enzymes.

 Regulating expression of genes.

 Occurs at transcription.

 Operon model.

= inhibition

Now, that’s a

good idea from a

lowly bacterium!

Bacteria Group Genes Together

 Operon

 Genes grouped together with related functions (such as all

enzymes in a metabolic pathway).

 Two types: INDUCIBLE and REPRESSIBLE.

 Structures of an Operon

 Promoter = RNA polymerase binding site

 Single promoter controls transcription of all genes in operon.

 Transcribed as one unit and a single mRNA is made.

 Operator = DNA binding site of repressor protein.

So how can these genes be turned off?

 Repressor protein

 binds to DNA at operator site

 blocking RNA polymerase

 blocks transcription

Trp Operon

E. coli makes tryptophan from a

precursor in a 3-step pathway.

 Three enzymes encoded by five genes.

 Single promoter for all genes.

 Creates one long mRNA.

promoter operator

DNA

trpE

trpD trpC trpB trpA

Trp Operon is Always Turned On

 Trp repressor turns operon off.

 Repressor is product of regulatory gene (trpR).

 Regulatory gene is some distance from gene and has its

own promoter.

 Regulatory gene expressed continuously.

 To bind to operator, repressor needs a

corepressor (tryptophan).

 Binds to an allosteric site.

promoter operator

DNA

trpE

trpD trpC trpB trpA

Lac Operon

 Inducible operon usually off but can be induces

when a small molecule binds to a regulatory

protein (function in catabolic pathways).

 Lactose metabolism begins with hydrolysis of a

disaccharide by β-galactosidase.

 If lactose in environment increases, β-galactosidase

increases.

 β-galactosidase encoded by lacZ gene.

promoter operator

DNA

lacZ lacY lacA

AP Biology

mRNA

enzyme 1 enzyme 2 enzyme 3 enzyme 4

promoter operator

Inducible operon: lactose

TATA DNA

RNA

polymerase

repressor repressor protein

repressor

lactose – repressor protein

complex

lactose

lac repressor gene 1 gene 2 gene 3 gene 4

Digestive pathway model

When lactose is present, binds to

lac repressor protein & triggers

repressor to release DNA

 induces transcription

RNA

polymerase

lac lac

lac

lac

lac

lac

lac

conformational change in

repressor protein!

lac

lac

Operon summary

 Repressible Operon

 Usually functions in anabolic pathways.

 Synthesizing end products.

 When end product is present in excess,

cell allocates resources to other uses.

 Inducible Operon

 Usually functions in catabolic pathways.

 Digesting nutrients to simpler molecules.

 Produce enzymes only when nutrient is

available.

 Cell avoids making proteins that have nothing to do,

cell allocates resources to other uses.

AP Biology 2007-

Control of

Eukaryotic Genes

Evolution of gene regulation

 Prokaryotes

 single-celled

 evolved to grow & divide rapidly

 must respond quickly to changes in

external environment

 exploit transient resources

 Gene regulation

 turn genes on & off rapidly

 flexibility & reversibility

 adjust levels of enzymes

for synthesis & digestion

Evolution of gene regulation

 Eukaryotes

 multicellular

 evolved to maintain constant internal

conditions while facing changing

external conditions

 homeostasis

 regulate body as a whole

 growth & development

 long term processes

 specialization

 turn on & off large number of genes

 must coordinate the body as a whole rather

than serve the needs of individual cells