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The Lac operon

THE lac OPERON
Rudrakshi B.Raut
The Institute Of Science,Mumbai
M.sc-2 (sem:3)
Paper-2
Roll no.17

CONTENT
Introduction
Concept of lac operon
Operon model
Functioning of lac operon
Different Scenarios
Lac mutations ...

INTRODUCTION
• Operon is operating units which can be
defined as the cluster of genes located
together on the chromosomes ...

The lac operon
• The lactose operon designated as lac operon.
• The lac operon codes for enzymes involved in
the catabolis...

Operon model

Designation
of gene
Codes for
enzyme
Function of the enzyme
lac Z β-galactosidase Breaks down lactose into
glucose & galac...

Element Purpose
Operator (lacO) Binding site for repressor
Promoter (lacP) Binding site for RNA Polymerase
Repressor Gene ...

FUNCTIONING OF LAC OPERON
• In the absence of lactose(inducer), the
regulator gene produce a repressor protein
which bind ...

• When lactose(inducer), introduce in the
medium, binds to the repressor the repressor
now fails to binds to the operator....

A simplified E. coli
bacterial cell.
The lac operon gene
sequence.

The repressor
molecule, bound to
the controlling
region.
Lactose molecules added to
the environment outside of
the cell.

Lactose molecules bound
to the repressor.
This releases the
repressor from the DNA.
RNA polymerase
transcribing the genes ...

Ribosomes translating
the mRNA into proteins.
One of the proteins
(yellow) encoded by the
lac operon allows
lactose to ent...

A second protein
(orange) digests the
lactose as it enters the
cell.
The lactose molecules
bound to the repressor
are rele...

s
Repressor again binds to the controlling
region of the DNA.

Different Scenarios
1. Lactose (-)
2. Lactose (+)
3. Lactose (+) and glucose (+)
4. Lactose (+) and glucose (-)

1. When lactose is absent
• A repressor protein is continuously synthesised. It
sits on a sequence of DNA just in front of...

2. When lactose is present
• A small amount of a sugar allolactose is
formed within the bacterial cell. This fits onto
the...

2. When lactose is present
• A small amount of a sugar allolactose is
formed within the bacterial cell. This fits onto
the...

3. When both glucose and lactose are present
• When glucose and lactose are present RNA
polymerase can sit on the promoter...

4. When glucose is absent and lactose is present
• Another protein is needed, an activator protein.
This stabilises RNA po...

Summary
Carbohydrates Activator
protein
Repressor
protein
RNA
polymerase
lac Operon
+ GLUCOSE
+ LACTOSE
Not bound
to DNA
L...

LAC MUTATIONS
• Jacob & Monod workout the structure &
function of lac operon by analyzing mutations
that affects lactose m...

STRUCTURAL-GENE MUTATION
• Jacob and Monod first discovered some
mutant strains that had lost the ability to
synthesize ei...

a) In the absence of inducer, the lacO+ operon is
turned off, whereas the lacOc operon produces
functional β-galactosidase...

b) In the presence of inducer the functional β-
galactosidase and defective permease are produce
from the lacOc operon, wh...

OPERATOR MUTATIONS
• Jacob & Monod find another constitutive mutants to a
site adjacent to lacZ.
• This mutations occurred...

PROMOTER MUTATION:
• Mutations affecting lactose metabolism have also been
isolated at the promoter site; these mutations ...

• The lac operon is under two forms of
control; positive and negative control.
• Negative control occurs when the binding
...

POSITIVE CONTROL
• When glucose is available, gene that participate in the
metabolism other sugars are repressed, in a
phe...

NEGATIVE CONTROL
• The lac repressor bind to the operator.
• The DNA sequence cover by the repressor
overlaps the DNA sequ...

POSITIVE VS NEGATIVE CONTROL
Regulatory
protein is
present
Mutate
regulatory
gene to lose
function
Positive control
Negati...

REFERENCE
Books :
• Genetics by Benjamin Pierce
• iGenetics by Peter J.Russell
Internet :
• Www.google.com
• https://www.g...

The Lac operon

The Lac operon

The Lac operon

The Lac operon

The Lac operon

The Lac operon
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    The Lac operon

    1. 1.
      THE lac OPERON
      Rudrakshi B.Raut
      The Institute Of Science,Mumbai
      M.sc-2 (sem:3)
      Paper-2
      Roll no.17

    2. 2.

      CONTENT
      Introduction
      Concept of lac operon
      Operon model
      Functioning of lac operon
      Different Scenarios
      Lac mutations :
       Structural Mutation
       Operator Mutation
       Promoter Mutation
      Positive and Negative control
      References

    3. 3.

      INTRODUCTION
      • Operon is operating units which can be
      defined as the cluster of genes located
      together on the chromosomes &
      transcribed together.
      • It is group of closely linked structure
      genes & associated control gene which
      regulate the metabolic activity.
      • All the genes of an operon are
      coordinately controlled by a mechanism
      1st described in 1961 by Francois Jacob
      & Jaques Monod of the Pasture institute
      of Paris.
      Jacob, Monod & Lwoff

    4. 4.

      The lac operon
      • The lactose operon designated as lac operon.
      • The lac operon codes for enzymes involved in
      the catabolism (degradation) of lactose.
      • lactose is the disaccharide which is made up
      of glucose & galactose.
      • It is the inducible operon since the presence
      of lactose induce the operon to switched on.

    5. 5.

      Operon model

    6. 6.

      Designation
      of gene
      Codes for
      enzyme
      Function of the enzyme
      lac Z β-galactosidase Breaks down lactose into
      glucose & galactose.
      lac y galactose
      permease
      This protein, found in the
      E.coli cytoplasmic membrane,
      actively transports lactose
      into the cells
      lac a Thio-galactoside
      trans acetylase
      The function of this
      enzyme is not known. It is
      coded for by the gene
      lacA.

    7. 7.

      Element Purpose
      Operator (lacO) Binding site for repressor
      Promoter (lacP) Binding site for RNA Polymerase
      Repressor Gene encoding the lac repressor
      protein. Binds to DNA at the
      operator & blocks binding of RNA
      Polymerase at the promoter.
      lacI Controls production of the
      repressor protein

    8. 8.

      FUNCTIONING OF LAC OPERON
      • In the absence of lactose(inducer), the
      regulator gene produce a repressor protein
      which bind to the operator site & prevent the
      transcription as a result, the structural gene
      do not produce mRNA & the proteins are not
      formed.

    9. 9.

      • When lactose(inducer), introduce in the
      medium, binds to the repressor the repressor
      now fails to binds to the operator.
      • Therefore the operoter is made free &
      induces the RNA polymerase to bind to the
      initiation site on promoter which results in
      the synthesis of lac mRNA.
      • This mRNA codes for three enzyme necessary
      for lactose catabolism.

    10. 10.

      A simplified E. coli
      bacterial cell.
      The lac operon gene
      sequence.

    11. 11.

      The repressor
      molecule, bound to
      the controlling
      region.
      Lactose molecules added to
      the environment outside of
      the cell.

    12. 12.

      Lactose molecules bound
      to the repressor.
      This releases the
      repressor from the DNA.
      RNA polymerase
      transcribing the genes in
      the lac operon into mRNA.

    13. 13.

      Ribosomes translating
      the mRNA into proteins.
      One of the proteins
      (yellow) encoded by the
      lac operon allows
      lactose to enter the cell
      at a high rate.

    14. 14.

      A second protein
      (orange) digests the
      lactose as it enters the
      cell.
      The lactose molecules
      bound to the repressor
      are released.

    15. 15.

      s
      Repressor again binds to the controlling
      region of the DNA.

    16. 16.

      Different Scenarios
      1. Lactose (-)
      2. Lactose (+)
      3. Lactose (+) and glucose (+)
      4. Lactose (+) and glucose (-)

    17. 17.

      1. When lactose is absent
      • A repressor protein is continuously synthesised. It
      sits on a sequence of DNA just in front of the lac
      operon, the Operator site
      • The repressor protein blocks the Promoter site
      where the RNA polymerase settles before it starts
      transcribing
      Regulator
      gene
      lac operonOperator
      site
      z y a
      DNA
      I
      O
      Repressor
      protein
      RNA
      polymeraseBlocked

    18. 18.

      2. When lactose is present
      • A small amount of a sugar allolactose is
      formed within the bacterial cell. This fits onto
      the repressor protein at another active site
      (allosteric site)
      • This causes the repressor protein to change its
      shape (a conformational change). It can no
      longer sit on the operator site. RNA
      polymerase can now reach its promoter site
      z y a
      DNA
      I O

    19. 19.

      2. When lactose is present
      • A small amount of a sugar allolactose is
      formed within the bacterial cell. This fits onto
      the repressor protein at another active site
      (allosteric site)
      • This causes the repressor protein to change its
      shape (a conformational change). It can no
      longer sit on the operator site. RNA
      polymerase can now reach its
      promoter site
      Promotor site
      z y a
      DNA
      I O

    20. 20.

      3. When both glucose and lactose are present
      • When glucose and lactose are present RNA
      polymerase can sit on the promoter site but it
      is unstable and it keeps falling off.
      Promotor site
      z y a
      DNA
      I O
      Repressor
      protein removed
      RNA polymerase

    21. 21.

      4. When glucose is absent and lactose is present
      • Another protein is needed, an activator protein.
      This stabilises RNA polymerase.
      • The activator protein only works when glucose is
      absent
      • In this way E. coli only makes enzymes to
      metabolise other sugars in the absence of glucose
      Promotor site
      z y a
      DNA
      I O
      Transcription
      Activator
      protein steadies
      the RNA
      polymerase

    22. 22.

      Summary
      Carbohydrates Activator
      protein
      Repressor
      protein
      RNA
      polymerase
      lac Operon
      + GLUCOSE
      + LACTOSE
      Not bound
      to DNA
      Lifted off
      operator site
      Keeps falling
      off promoter
      site
      No transcription
      + GLUCOSE
      – LACTOSE
      Not bound
      to DNA
      Bound to
      operator site
      Blocked by the
      repressor
      No transcription
      – GLUCOSE
      – LACTOSE
      Bound to
      DNA
      Bound to
      operator site
      Blocked by the
      repressor
      No transcription
      – GLUCOSE
      + LACTOSE
      Bound to
      DNA
      Lifted off
      operator site
      Sits on the
      promoter site
      Transcription

    23. 23.

      LAC MUTATIONS
      • Jacob & Monod workout the structure &
      function of lac operon by analyzing mutations
      that affects lactose metabolism.
      • To help define the role of the different
      components of the operon, they use partial
      diploid stain of E.coli.
      • They determine that some part of the lac
      operon are cis acting where other are trans
      acting.

    24. 24.

      STRUCTURAL-GENE MUTATION
      • Jacob and Monod first discovered some
      mutant strains that had lost the ability to
      synthesize either β-galactosidase or
      permease.
      • The mutation which occurred on lacZ and LacY
      structural genes altered the amino acid
      sequences of the proteins encoded by the
      genes.

    25. 25.

      a) In the absence of inducer, the lacO+ operon is
      turned off, whereas the lacOc operon produces
      functional β-galactosidase from the lacZ+ gene and
      nonfunctional permease molecules from the lacY-
      gene with missense mutation.

    26. 26.

      b) In the presence of inducer the functional β-
      galactosidase and defective permease are produce
      from the lacOc operon, whereas the lacO+ operon
      produces nonfunctional β-galactosidase from the
      lacZ- gene & functional permease from lacY+ gene.

    27. 27.

      OPERATOR MUTATIONS
      • Jacob & Monod find another constitutive mutants to a
      site adjacent to lacZ.
      • This mutations occurred at the operator site & were
      referred to as lacOc.
      • The lacOc mutations altered the sequence of DNA at
      the operator so that the repressor protein was no
      longer able to bind.
      • A partial diploid with genotype lacI+ lacOc lacz+ /lacI+
      lacO + lacz+ exhibited constitutive synthesis of β-
      galactosidase, indicating that lacOc is dominant over
      lacO +.

    28. 28.

      PROMOTER MUTATION:
      • Mutations affecting lactose metabolism have also been
      isolated at the promoter site; these mutations are
      designated lacP- ,and they interfere with the binding of
      RNA polymerase to the promoter.
      • This binding is essential for the transcription of the
      structural gene.
      • E.coli strain with lacP- mutation does not produce lac
      proteins either in a presence or absence of lactose.
      • lacP- mutations are cis acting.

    29. 29.

      • The lac operon is under two forms of
      control; positive and negative control.
      • Negative control occurs when the binding
      of a protein prevents an event.
      • Positive control is when the binding
      causes the event.

    30. 30.

      POSITIVE CONTROL
      • When glucose is available, gene that participate in the
      metabolism other sugars are repressed, in a
      phenomenon known as catabolite repression.
      • Catabolite repression Is a type of +ve control in the lac
      operon.
      • The catabolite activator protein(CAP), complex cAMP,
      binds to a site near the promoter & stimulates the
      binding of RNA polymerase.
      • A cellular level of cAMP are controlled by glucose;
      allolactose level increases the abundance of cAMP &
      enhance the transcription of the lac structural genes.

    31. 31.

      NEGATIVE CONTROL
      • The lac repressor bind to the operator.
      • The DNA sequence cover by the repressor
      overlaps the DNA sequence recognized by the
      RNA polymerase.
      • Therefore, when the repressor is bound to the
      operator, RNA polymerase cannot bind to the
      promoter & transcription can not occur, the lac
      operon is said to be under –ve control.

    32. 32.

      POSITIVE VS NEGATIVE CONTROL
      Regulatory
      protein is
      present
      Mutate
      regulatory
      gene to lose
      function
      Positive control
      Negative control
      Example of
      regulatory
      protein
      Operon ON
      Operon OFF
      Operon OFF
      Operon ON
      Activator
      Repressor

    33. 33.

      REFERENCE
      Books :
      • Genetics by Benjamin Pierce
      • iGenetics by Peter J.Russell
      Internet :
      • Www.google.com
      • https://www.google.co.in/search?q=The+lac+operon
      +in+e.coli.ppt&client=opera&hs=OtG&biw=1366&bih
      =586&source=lnms&tbm=isch&sa=X&ei=OzQ0VJu1N4
      2xuATqjIH4BQ&ved=0CAYQ_AUoAQ

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