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Genetic Regulatory Mechanisms. Control of Gene Expression. Transcriptional control Clustering of genes with related function Coordinate control of genes with related function Polycistronic mRNA. Inducible Genes - Operon Model.

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Presentation Transcript
control of gene expression
Control of Gene Expression
  • Transcriptional control
  • Clustering of genes with related function
  • Coordinate control of genes with related function
  • Polycistronic mRNA
inducible genes operon model
Inducible Genes - Operon Model
  • Definition: Genes whose expression is turned on by the presence of some substance
    • Lactose induces expression of the lac genes
    • An antibiotic induces the expression of a resistance gene
  • Catabolic pathways
lactose operon

RegulatoryGene

Operon

DNA

i

p

o

z

y

a

m-RNA

Protein

-Galactosidase

Transacetylase

Permease

Lactose Operon
  • Structural genes
    • lac z, lac y, & lac a
    • Promoter
    • Polycistronic mRNA
  • Regulatory gene
    • Repressor
  • Operator
  • Operon
  • Inducer - lactose
lactose operon1

Absence of lactose

z

y

a

i

p

o

Active

No lac mRNA

Presence of lactose

z

y

a

i

p

o

Inactive

-Galactosidase

Permease

Transacetylase

Lactose Operon
  • Inducer -- lactose
    • Absence
      • Active repressor
      • No expression
  • Presence
    • Inactivation of repressor
    • Expression
  • Negative control
catabolite repression glucose effect

- glucose

Glucose added

Units of -galactosidase

+ glucose

Time (hr)

+ lactose

Catabolite Repression (Glucose Effect)
  • Definition: Control of an operon by glucose
  • Catabolic operons
mechanism of catabolite repression

Adenyl cyclase

Absence of glucose

c-AMP

ATP

CAP

i

z

y

a

p

o

Active

Inactive

-Galactosidase

Permease

Transacetylase

Maximum expression

Mechanism of Catabolite Repression
  • c-AMP
  • CAP (CRP) protein
  • CAP-cAMP complex
    • Promoter activation
  • Positive control
mechanism of catabolite repression1

Presence of glucose

CAP

Adenyl cyclase

X

ATP

Low level expression

z

y

a

i

p

o

Inactive

-Galactosidase

Permease

Transacetylase

Mechanism of Catabolite Repression
  • Glucose:cAMP
  • CAP (CRP) protein
  • No CAP-cAMP complex
    • No Promoter activation
repressible genes operon model
Repressible Genes - Operon Model
  • Definition: Genes whose expression is turned off by the presence of some substance (co-repressor)
    • Tryptophan represses the trp genes
  • Biosynthetic pathways
    • Co-repressor is typically the end product of the pathway
tryptophan operon

RegulatoryGene

Operon

A

B

L

R

P

O

E

D

C

Inactive repressor (apo-repressor)

5 Proteins

Tryptophan Operon
  • Structural genes
    • trp E, trpD, trpC trpB & trpA
    • Common promoter
  • Regulatory Gene
    • Apo-Repressor
      • Inactive
  • Operator
  • Leader
  • Operon
  • Co-repressor
    • Tryptophan
tryptophan operon1

Absence of Tryptophan

A

B

L

R

P

O

E

D

C

Inactive repressor (apo-repressor)

5 Proteins

Presence of Tryptophan

A

B

L

R

P

O

E

D

C

No trp mRNA

Inactive repressor (apo-repressor)

Trp

(co-repressor)

Tryptophan Operon
  • Co-repressor -- tryptophan
    • Absence of tryptophan
      • Gene expression
  • Presence of tryptophan
    • Activates repressor
    • No gene expression
  • Negative control
attenuation

L

DNA

3

4

P

O

1

2

E

RNA

ATG

TGA

3

4

1

2

2 trp codons

Attenuation
  • Definition: Premature termination of transcription
  • Leader region
    • Leader transcript
    • Translation start
    • Translation stop
    • Tryptophan codons
attenuation1

4

1

2

3

UUUUUUU

3

2

UUUUUUU

4

1

Attenuation
  • Mutually exclusive mRNA secondary structure
    • region 1 : region 2
    • region 2 : region 3
    • region 3 : region 4
  • Coupled transcription and translation
attenuation2

High tryptophanyl-t-RNA

Low tryptophanyl-t-RNA

3

3

4

4

1

1

2

2

ATG

TGA

ATG

TGA

UUUUUUU

2

1

4

2 trp codons

3

2 trp codons

3

2

UUUUUUU

4

1

No Attenuation

Attenuation

Attenuation
regulation of enzyme activity
Regulation of Enzyme Activity
  • Feed back inhibition
  • Epigenetic modification
    • Post translational modifications
      • Phosphorylation/dephosphorylation
      • Adenylation/deadenylation
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