Understanding Gene Regulation and Control of Gene Expression

 
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Controlling gene expression is often accomplished by controlling transcription
initiation.
Regulatory proteins 
bind to DNA to either block or stimulate transcription,
depending on how they interact with RNA polymerase.
Prokaryotic organisms regulate gene expression in response to their
environment.
Eukaryotic cells regulate gene expression to maintain 
homeostasis
 in the
organism.
Gene expression is often controlled by regulatory proteins binding to specific
DNA sequences.
regulatory proteins gain access to the bases of DNA at the 
major groove
regulatory proteins possess 
DNA-binding motifs 
(
regions of regulatory proteins
which bind to DNA)
 
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Bacterial genes can be classified according to their expression  into:
constitutive gene
 
is a gene that is transcribed continually as opposed to an
inducible gene
, which is only transcribed when needed.
housekeeping gene
 is typically a constitutive gene that is transcribed at a
relatively constant level. The housekeeping gene's products are typically
needed for the maintenance of the cell. It is generally assumed that their
expression is unaffected by experimental conditions.
An 
inducible gene 
is a gene only transcribed when needed as opposed to a
constitutive gene. An inducible gene is a gene whose expression is either
responsive to environmental change or dependent on the position in the
cell cycle.
 
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Constitutive Genes 
= unregulated
essentially constant levels of expression (often required in the cell all
the time )
•  Regulation can occur at:
Transcription
 (regulatory proteins; attenuation)
Translation
 (repressors; antisense RNA)
Post translational 
(feedback inhibition)
 
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Control of transcription initiation can be:
positive control
 – increases transcription when 
activators
 bind DNA
negative control
 – reduces transcription when 
repressors
 bind to
DNA regulatory regions called 
operators
Prokaryotic cells often respond to their environment by changes in
gene expression.
Genes involved in the same metabolic pathway are organized in
operons
.
Some operons are 
induced 
when the metabolic pathway is needed.
Some operons are 
repressed
 when the metabolic pathway is no longer
needed
 
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is a functioning unit of DNA containing a cluster of genes under the
control of a 
single promoter
. The genes are transcribed together into an
mRNA strand and either 
translated
 together in the cytoplasm called
polycistronic mRNA
, or undergo splicing to create 
monocistronic
 mRNAs
that are translated separately, i.e. several strands of mRNA that each
encode a single gene product. The result of this is that the genes
contained in the operon are either expressed together or not at all.
 
operons were thought to exist solely
in 
prokaryotes
, but operons exist also
in 
eukaryotes
 ( 
nematodes
 such
as  
Caenorhabditis
 elegans
 and the fruit
fly, 
Drosophila melanogaster
). In general, the
expression of prokaryotic operons leads to
the generation of polycistronic mRNAs(
a
single mRNA molecule that codes for
more than one 
protein
), while eukaryotic
operons lead to monocistronic mRNA (
a
single mRNA molecule that codes for
one 
protein
). Operons are also found in
viruses such as bacteriophages
General structure of operon:
Promoter
 
– a 
nucleotide
 sequence recognized by 
RNA polymerase
, which then
initiates transcription. In RNA synthesis, promoters indicate which genes
should be used for messenger RNA creation – and, by extension, control
which proteins the cell produces.
Operator
 – a segment of 
DNA
 that a repressor binds to. It is classically defined
in the 
lac operon
 as a segment between the promoter and the genes of the
operon.
 
 In the case of a repressor, the repressor protein physically obstructs
the RNA polymerase from transcribing the genes.
Structural genes
 – the genes that are co-regulated by the operon.
 
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The 
lac
 operon
 (lactose operon) is an 
operon
 required for the transport
and 
metabolism
 of 
lactose
 in 
Escherichia coli
 and many other 
enteric
bacteria
. Although glucose is the preferred carbon source for most bacteria,
the 
lac
 operon allows for the effective digestion of lactose when glucose is
not available. Bacterial operons are polycistronic transcripts that are able to
produce multiple proteins from one mRNA transcript. In this case, when
lactose is required as a sugar source for the bacterium, the three genes of
the lac operon can be expressed and their subsequent proteins
translated: 
lacZ
lacY
, and 
lacA
. The gene product of 
lacZ
 is 
β-
galactosidase
which cleaves lactose, a disaccharide,
into 
glucose
 and 
galactose
lacY
 encodes 
lactose permease
, a protein that
becomes embedded in the cytoplasmic membrane to enable the transport
of lactose into the cell. Finally, 
lacA
 encodes 
galactoside O-acetyltransferase
.
 
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•   The lac operon can be transcriptionally regulated
– 1.  By a repressor protein
– 2.  By an activator protein
 
•   The first method is an inducible, negative control mechanism
– It involves the lac repressor protein
– The inducer is allolactose
  It binds to the lac repressor and inactivates it
 
Constitutive
expression
 
The lac operon now is
repressed
 
Therefore no allolactose
The lac
operon now is
induced
 
The conformation of the repressor is now
altered Repressor can no longer
bind to operator
 
Some gets converted to allolactose
 
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catabolite repression
•   When exposed to both lactose and
glucose
– E. coli uses glucose first, and
catabolite repression prevents the use
of lactose
– When glucose is depleted, catabolite
repression is alleviated, and the lac
operon is expressed
 
•   The sequential use of two sugars by
a bacterium is termed 
diauxic 
growth
 
Regulation involves a small molecule, cyclic AMP (
cAMP
)
–   produced from ATP via the enzyme adenylyl cyclase
–   cAMP binds an activator protein known as the Catabolite Activator
Protein (CAP)
cAMP-CAP complex 
is an example of genetic regulation that is inducible
and under positive control
–   The cAMP-CAP complex binds to the CAP site near the   lac promoter and
increases transcription
•In the presence of glucose, the enzyme adenylyl cyclase is inhibited
–   This decreases the levels of cAMP in the cell
–   Therefore, cAMP is no longer available to bind CAP
–   And Transcription rate decreases
 
b) Lactose but no cAMP
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This comprehensive content delves into the intricate mechanisms of gene regulation and control of gene expression. It covers topics such as transcriptional regulation, bacterial genes classification, and the role of regulatory proteins. Explore how genes are regulated at transcription, translation, and post-translational levels. Understand the significance of constitutive genes and operons in maintaining cellular homeostasis.


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  1. Gene regulation Gene regulation

  2. Control of Gene Expression Control of Gene Expression Controlling gene expression is often accomplished by controlling transcription initiation. Regulatory proteins bind to DNA to either block or stimulate transcription, depending on how they interact with RNA polymerase. Prokaryotic organisms regulate gene expression in response to their environment. Eukaryotic cells regulate gene expression to maintain homeostasis in the organism. Gene expression is often controlled by regulatory proteins binding to specific DNA sequences. regulatory proteins gain access to the bases of DNA at the major groove regulatory proteins possess DNA-binding motifs (regions of regulatory proteins which bind to DNA)

  3. Bacterial genes Bacterial genes can be classified according to their expression into: A constitutive gene is a gene that is transcribed continually as opposed to an inducible gene, which is only transcribed when needed. A housekeeping gene is typically a constitutive gene that is transcribed at a relatively constant level. The housekeeping gene's products are typically needed for the maintenance of the cell. It is generally assumed that their expression is unaffected by experimental conditions. An inducible gene is a gene only transcribed when needed as opposed to a constitutive gene. An inducible gene is a gene whose expression is either responsive to environmental change or dependent on the position in the cell cycle.

  4. Gene regulation Gene regulation Constitutive Genes = unregulated essentially constant levels of expression (often required in the cell all the time ) Regulation can occur at: Transcription (regulatory proteins; attenuation) Translation (repressors; antisense RNA) Post translational (feedback inhibition)

  5. Transcriptional Regulation Transcriptional Regulation Control of transcription initiation can be: positive control increases transcription when activators bind DNA negative control reduces transcription when repressors bind to DNA regulatory regions called operators Prokaryotic cells often respond to their environment by changes in gene expression. Genes involved in the same metabolic pathway are organized in operons. Some operons are induced when the metabolic pathway is needed. Some operons are repressed when the metabolic pathway is no longer needed

  6. Operon Operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm called polycistronic mRNA, or undergo splicing to create monocistronic mRNAs that are translated separately, i.e. several strands of mRNA that each encode a single gene product. The result of this is that the genes contained in the operon are either expressed together or not at all.

  7. operons in in as fly, Drosophila melanogaster). In general, the expression of prokaryotic operons leads to the generation of polycistronic mRNAs(a single mRNA molecule that codes for more than one protein), while eukaryotic operons lead to monocistronic mRNA (a single mRNA molecule that codes for one protein). Operons are also found in viruses such as bacteriophages were thought but ( to exist exist solely also such prokaryotes, eukaryotes Caenorhabditis elegans and the fruit operons nematodes

  8. General structure of operon: Promoter a nucleotide sequence recognized by RNA polymerase, which then initiates transcription. In RNA synthesis, promoters indicate which genes should be used for messenger RNA creation and, by extension, control which proteins the cell produces. Operator a segment of DNA that a repressor binds to. It is classically defined in the lac operon as a segment between the promoter and the genes of the operon. In the case of a repressor, the repressor protein physically obstructs the RNA polymerase from transcribing the genes. Structural genes the genes that are co-regulated by the operon.

  9. Lac operon Lac operon The lac operon (lactose operon) is an operon required for the transport and metabolism of lactose in Escherichia coli and many other enteric bacteria. Although glucose is the preferred carbon source for most bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available. Bacterial operons are polycistronic transcripts that are able to produce multiple proteins from one mRNA transcript. In this case, when lactose is required as a sugar source for the bacterium, the three genes of the lac operon can be expressed and their subsequent proteins translated: lacZ, lacY, and lacA. The gene product of lacZ is - galactosidasewhich cleaves into glucose and galactose. lacY encodes lactose permease, a protein that becomes embedded in the cytoplasmic membrane to enable the transport of lactose into the cell. Finally, lacA encodes galactoside O-acetyltransferase. lactose, a disaccharide,

  10. Enzymes of Lac operon

  11. The lac Operon Is Regulated By a Repressor Protein The lac Operon Is Regulated By a Repressor Protein The lac operon can be transcriptionally regulated 1. By a repressor protein 2. By an activator protein The first method is an inducible, negative control mechanism It involves the lac repressor protein The inducer is allolactose It binds to the lac repressor and inactivates it

  12. Constitutive expression The lac operon now is repressed Therefore no allolactose

  13. The lac operon now is induced The conformation of the repressor is now altered Repressor can no longer bind to operator Some gets converted to allolactose

  14. The lac Operon Is Also Regulated By an Activator The lac Operon Is Also Regulated By an Activator Protein Protein catabolite repression When exposed to both lactose and glucose E. coli uses glucose first, and catabolite repression prevents the use of lactose When glucose is depleted, catabolite repression is alleviated, and the lac operon is expressed The sequential use of two sugars by a bacterium is termed diauxic growth

  15. Regulation involves a small molecule, cyclic AMP (cAMP) produced from ATP via the enzyme adenylyl cyclase cAMP binds an activator protein known as the Catabolite Activator Protein (CAP) cAMP-CAP complex is an example of genetic regulation that is inducible and under positive control The cAMP-CAP complex binds to the CAP site near the lac promoter and increases transcription In the presence of glucose, the enzyme adenylyl cyclase is inhibited This decreases the levels of cAMP in the cell Therefore, cAMP is no longer available to bind CAP And Transcription rate decreases

  16. b) Lactose but no cAMP

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