The Blueprint of Life DNA to Protein

1
The Blueprint of Life - DNA to Protein

• Chapter 7 (Part II)

2
Regulation of Gene Expression

• Microorganisms possess mechanisms to
• synthesize a maximum amount of cell material
• from a limited amount of energy source
• Two general mechanisms
• Allosteric inhibition of enzymes
• Controlling the synthesis of enzymes

3

• Principles of regulation
• Not all genes are subjected to regulation
• Enzymes can be classified as follows
• Constitutive enzymes
• Enzyme is constantly produced regardless of
  conditions
• Inducible enzymes
• Not regularly produced
• Synthesis is activated by certain conditions
• Repressible enzymes
• Routinely synthesized but can be turned off by
  certain conditions
• Generally involved in biosynthesis

4

• Transcription is frequently controlled by a
  regulatory
• region near the genes promoter
• A specific protein binds to region and acts as
  on/off switch
• The protein can act as repressor or activator
• Repression blocks transcription
• Activation facilitates transcription
• Set of genes controlled by protein is called an
  operon

5
Operon structure
Operator
Gene 1
Gene 3
Gene 2
Promoter
Operator binding site for the repressor protein
for the regulation of gene expression
Promoter binding site for RNA polymerase
Structural Genes DNA sequence for specific
proteins
6

• Repressors
• Control mechanisms that inhibit gene expression
  and
• decrease the synthesis of enzymes
• Repression is usually in response to the
  overabundance
• of an end product
• Repression decreases the rate synthesis of
  enzymes, leading to
• the formation of the particular end product
• Repressors may work alone or require a
  corepressor
• Regulatory proteins called repressors mediate
  repression
• Repressors block the ability of RNA polymerase to
  bind and initiate
• protein synthesis

7
(No Transcript)
8

• Activators
• Control mechanisms that induce the transcription
  of a gene or set
• of genes
• Enzymes synthesized in the presence of inducers
  are called inducible
• enzymes

9

• Lac Operon
• The Lac Operon is a well studied example of
  induction of
• gene expression
• Near the operon on the DNA is a regulatory gene
  called
• the I gene
• The I gene encodes the repressor protein
• When lactose is absent, the repressor protein
  binds to the operator gene
• When lactose is present, the repressor binds to
  lactose instead of the operator and RNA
  polymerase is able to bind to the promoter and
  transcribes the structural genes
• Lactose acts as an inducer by keeping the
  repressor from binding to the operator

10
Lac Operon
Operator
Gene 1
Gene 3
Gene 2
1.
Promoter
2.
Repressor
3.
Lactose
11
Affect of Glucose on the Lac Operon
12
Diauxic Growth Curve