Gene ControlProkaryotes vs. Eukaryotes

Gene regulation Two types of genes:

1)Structural genes – code specific proteins

2)Regulatory genes – control activity of structural genes (gene expression)

Gene regulation in prokaryotes

Operons control rate of transcription. group of genes working together

code for enzymes regulating specific metabolic pathway.

Regulator gene Promoter Operat

orStructural gene A

Produces the repressor

RNA polymerase binding site

Repressor binding site

OPERON

Structural gene B

The Operon Model Operon – group of genes with

related functions

Genes – code for specific proteins Promoter – RNA pol binding site

– controls transcription of ALL genes in operon

– Single mRNA produced

Operator – binding site of repressor protein (turns off gene)

These 3 make up an operon!

Quick Review in Metabolic regulation

Allosteric inhibitors – provide feedback inhibition (enzyme regulators)

Product of pathwaysignals continuation ofpath to STOP

Making all these enzymes is wasteful

Induction: Ex-The Lac Operon

Regulator gene

Codes for

Repressor protein

Promoter

Operator

RNA Polymerase

1 2 3

*Presence or absence of lactose regulates production of enzymes from

structural genes of the lac operon*

**NO lactose present**

Regulator gene

Codes for

Repressor protein

Promoter

Operator 1 2 3

**Lactose present** Lactose

molecules

Lactose is an inducer

*Why do cells have these “on/off” switches?*

Animation

mRNA

enzyme1 enzyme2 enzyme3 enzyme4operator

promoter

Repression: Ex – Tryp operon

DNATATA

RNApolymerase

tryptophan

repressorrepressor protein

repressortryptophan – repressor proteincomplex

Excess tryptophan present, binds to tryp repressor protein triggering repressor to bind to DNA– blocks (represses) transcription– tend to be anabolic pathways

gene1 gene2 gene3 gene4

Tryp is an effector - activates repressor

1 2 3 4

repressortrpRNApolymerase

trp

trp

trp trp

trp trp

trptrp

trptrp

trp

Gene regulation in prokaryotes -

summary Genes for metabolic pathways linked

together in operons with a common switch mechanism (operator).

No introns – no RNA processing

Structural genes undergo transcription & translation simultaneously.

Regulation occurs by switching all genes of a pathway on or off.

What about Eukaryotes? How are they different? What might the

process have to accommodate for? How might it do this? Eukaryotes often multicellular

Must maintain homeostasis

Coordinate body as a whole Differentiated & specialized cells

Battle changing environment

When does gene control occur???

1. Packing/unpacking DNA

2. Transcription3. mRNA processing4. Translation5. Protein processing6. Protein degradation

1. DNA PackingIf all 46 of your chromosomes were lined up

in a row, your DNA would be over 3 feet long. How can your cells contain this large amount of material when cells are microscopic?

Coils & Folds– Double helix– Nucleosomes– Chromatin fiber– Looped domains– Chromosomes

from DNA double helix to condensed chromosome

Degree of DNA packing regulates transcription– Tightly packed = no transcription =

genes OFF “Dark” DNA = tight “Light” DNA = loose

Repressors - (ex: adding -CH3’s) block transcription factors no transcription genes OFF!

Activators -(ex: adding –COCH3’s) unwind DNA coils loosen transcription genes ON!

2. Transcription Initiation Control regions on DNA

– Promoter nearby control

sequence – “standard” rate

bind RNA pol bind transcription factors

– Enhancer distant control

sequence – “enhanced” rate

bind activator proteins

Transcription complex…

ActivatorActivator

Activator

CoactivatorRNA polymerase II

AB F E

HTFIID

Activator Proteins• regulatory proteins bind to DNA at distant enhancer sites

• increase the rate of transcription

Coding regionT A T A

Enhancer Sitesregulatory sites on DNA distant from gene

Initiation Complex - transcrip activated when “hairpin loop” brings TF’s on enhancer sequence (activators) to TF’s bound to RNA pol on promoter protein-protein interactions KEY!

Core promoter

and initiation complex

3. Post-transcriptional control Alternate splicing pattern

– ↑ variation in protein family

4. Regulation of mRNA degradation

Lifespan of mRNA controls amt of protein synthesized– mRNA can last from hrs to weeks!

What if degradation is interfered with???

RNAiSmall interfering RNAs

(siRNA)short RNA (21-28 bases)

bind to mRNAcreate sections of double-stranded mRNA

“death” tag for mRNAtriggers degradation

gene “silencing”post-transcriptional controlturns off gene = no protein

5. Control of Translation Block initiation of translation

– Regulatory proteins attach to 5’ end Prevent attachment of ribosome &

initiator tRNA Synthesis turned OFF

6/7. Protein Processing & Degradation

Protein processing– Folding, cleaving, adding sugar

groups, targeting for transport Protein degradation

– “death tags” (ubiquitin -76 aa’s)– Proteasomes – degradation

machinery

Regulation Prokaryotes Eukaryotes

1. Genes for metabolic pathways linked together in operons w/common switch mechanism (operator).

2. No introns - no RNA processing

3. Structural genes undergo transcription & translation simultaneously.

4. Regulation occurs by switching all genes in pathway on or off.

1. Pathways separated, no operons.

2. Genes switched on separately.

3. Introns on genes removed in RNA processing.

4. Transcription & translation do not occur simultaneously.

5. Large number of control elements.