Gene regulation controls gene expression and involves turning genes on and off in response to environmental changes in bacteria and bringing about cellular differentiation in multicellular eukaryotes. An operon is a single transcriptional unit that includes a series of structural genes, a promoter for the structural genes, and an operator site where a regulator gene product binds. There are two types of transcriptional control - negative control where a repressor protein binds DNA and inhibits transcription, and positive control where an activator protein binds DNA and stimulates transcription. Negative control can be inducible, where transcription is normally off and an inducer renders the repressor inactive, or repressible, where transcription is normally on and a co-repressor activates the repressor.

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Gene regulation: The mechanisms and systems that
control the expression of genes.
- In bacteria, gene regulation maintains internal
flexibility, turning genes on and off in response to
environmental changes.
- In multicellular eukaryotic organisms, gene
regulation brings about cellular differentiation.
Genes and Regulatory Elements:
Genes include DNA sequences that encode
proteins (Structural genes), as well as sequences
(Regulatory genes) that encode rRNA, tRNA,
snRNA, and other types of RNA.

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- An operon: is a single transcriptional unit that
includes:
1- A series of structural genes.
2- Promoter for the structural genes.
3- An operator site where the product of a regulator
gene binds.

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- A regulator gene: helps to regulate transcription of
the structural genes of the operon.
1- Not considered as a part of the operon, although it
affects operon function.
2- Has its own promoter and transcribed into mRNA,
which translated to a small protein. This regulator
protein may bind to a region of DNA called the
operator and affect whether transcription can take
place or not.
-The operator: usually overlaps the 3 end of the
promoter and sometimes the 5 end of the first
structural gene.

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B- Positive control:
In which a regulatory
protein acts as an
activator, a regulatory
protein binds to DNA and
stimulates transcription.
A- Inducible operons:
Transcription is normally off
and must be turned on.
B- Repressible operons:
Transcription is normally on
and must be turned off.
There are two types of transcriptional control:
A- Negative control:
In which a regulatory
protein acts as a repressor,
binding to DNA and inhibiting
transcription.

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(A) Negative inducible operons:
- In a negative inducible operon, transcription and translation
of the regulator gene produce an active repressor that readily
binds to the operator so physically blocks the binding of RNA
polymerase to the promoter and prevents transcription.
- Transcription is turned on when a small molecule (inducer)
binds to the repressor rendering the repressor inactive and
unable to bind to the operator site. Now RNA polymerase can
bind to the promoter and transcribe the structural genes.

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(B) Negative repressible operons:
- In a negative repressible operon, transcription and
translation of the regulator gene produce an inactive repressor
that cannot by itself bind to the operator so RNA polymerase
binds to the promoter and transcription of the structural genes
takes place
- To turn transcription off, when a small molecule (co-
repressor) binds to the repressor and activate the repressor,
preventing transcription.

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- Transcription would normally off because the regulator
protein would be produced in an inactive form.
- Transcription would be on when an inducer became
attached to the regulatory protein, rendering the regulator
protein active.
(C) Positive inducible operon:

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- Transcription would normally on. because the regulator
protein would be produced in an active form.
- Transcription would be off when a substance became
attached to the regulatory protein, rendering the regulator
protein inactive.
(D) Positive repressible operon :

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Type of
Control
Transcripti
on
Normally
Regulat
or
Protein
Effect of
Regulatory
Protein
Action of
Modulator
Negative
inducible
Off
Active
repressor
Inhibits
Transcription
Substrate makes
repressor
inactive
Negative
repressible
On
Inactive
repressor
Inhibits
Transcription
Product makes
repressor active
Positive
inducible
Off
Inactive
activator
Stimulates
Transcription
Substrate makes
activator active
Positive
repressible
On
Active
activator
Stimulates
transcription
Product makes
activator
inactive

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