Control of Transcription

DNA has “on” and “off” switches

Activator –protein that binds near gene’s promoter region

- allows RNA polymerase to transcribe (allows it to fit)

Repressor – protein that binds to DNA and prevents RNA polymerase from binding

-coded for by “regulator” gene

The Operon Model

Operon – region of DNA with group of genes for proteins with related functions

(see diagram)

-All of the genes in the operon are controlled by activity at the promoter & the operator

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Promoterfor I gene

Gene forrepressor protein

Regulatory region Coding region

CAP-bindingsite

Gene forpermease

Operator

Promoter forlac operon

Gene for-galactosidase

Gene fortransacetylase

PI CAP O

ZY A

PlacI

lac control system

The “Lac” Operon-in E. coli bacteria-genes for enzymes to break down lactose sugar

1. Regulator gene codes for Lac repressor protein2. Repressor binds with operator region

If no lactose present:3. RNA polymerase can not bind to promoter, no enzyme made

and operon is “off”If lactose is present:

3. Lactose binds with repressor, repressor no longer binds to operator

4. RNA polymerase transcribes the structural genes• Translation occurs, enzymes are made• Lactose is metabolized

*Conservation of resources – enzymes only made when needed*

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RepressorCAP

CAP

Promoter OperatorcAMP

RNApolymerase

OY

A

IZ

lac operon is "repressed"

Plac

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lac operon is "induced"

CAP

CAP

Promoter Operator

Allolactose(inducer)

cAMP

Plac

RNApolymerase

OY

A

IZ

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mRNA synthesis

CAP-bindingsite

RNA-polymrasebinding site(promoter) Operator

lacZ gene

Operon ? because CAP is not bound

Operon ? both because lac repressor is bound and CAP is not

Operon ? because lac repressor is bound

Operon ? because CAP is bound and lacrepressor is not

RNA polymerase

Repressor

RNA polymerase

CAP

CAP

+

+

+

+G

luco

se

Lac

tose

–

––

–

RNA in Gene regulation“Small RNA’s”

Process of RNA interference (RNAi) or “RNA silencing”-double stranded RNA is cut into small pieces by enzyme called dicer-pieces then unravel into single strands

1. miRNA – micro RNA-binds to a specific RNA thus blocking it from

being translated-reversible2. siRNA – small interfering RNA-binds to specific mRNA, destroys it

Purposes: (evolutionary origins)1. Inactivate viral DNA transcription2. Inactivate transposons – renegade “jumping genes”3. Regulation

Nucleosomes

X- Inactivation