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Gene Regulation


Gene Regulation Gene Regulation in Prokaryotes the Jacob-Monad Model certain genes are transcribed all the time constitutive genes synthesis of some proteins ... – PowerPoint PPT presentation

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Title: Gene Regulation

Gene Regulation
  • Gene Regulation in Prokaryotes the Jacob-Monad
  • certain genes are transcribed all the time
    constitutive genes
  • synthesis of some proteins is regulated and are
    produced only when needed under special
  • The Lac Operon
  • Jacob and Monad demonstrated how genes that code
    for enzymes that metabolize lactose are regulated

  • An operon consists of three elements
  • the genes that it controls
  • a promotor region where RNA polymerase first
  • an operator region between the promotor and the
    first gene which acts as an on-off switch.

  • Intestinal bacteria (E. coli) are able to absorb
    the disaccharide, lactose, and break and break it
    down to glucose and galactose (E. coli will only
    make these enzymes when grown in the presence of
  • Requires the production of 3 enzymes
  • ? - galactosidase breaks down the lactose to
    glucose and galactose
  • galactose permease needed to transport lactose
    efficiently across bacterial cell membrane
  • galactoside transacetylase function is not clear

  • Production of these enzymes is controlled by
    three structural genes and some closely linked
    DNA sequences responsible for controlling the
    structural genes entire gene complex is called
    the operon
  • Structural genes of the lactose operon
  • lacZ codes for ? - galactosidase
  • lacY codes for galactose permease
  • lacA codes for galactoside transacetylase

  • Next to the structural genes are 2 overlapping
    regulatory regions
  • promotor region to which RNA polymerase binds
    to initiate transcription
  • operator region of DNA that acts as the switch
    that controls mRNA synthesis sequence of bases
    that overlaps part of the promotor region

  • when lactose is absent, a repressor protein (in
    this case the lactose repressor) binds to the
    operator region repressor protein is large
    enough to cover part of the promotor sequence,
    too, and blocks RNA polymerase from attaching to
    promotor transcription is blocked
  • when lactose is present, it acts as an inducer
    and turns on the transcription of the lactose
  • lactose binds to repressor protein, inactivates
    it, and unblocks the promotor region allowing RNA
    polymerase to attach and begin transcription

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Gene Regulation in Eukaryotes
  • most cells in a multicellular organism contain
    the same DNA but they dont all use the DNA all
    the time
  • individual cells express only a small fraction of
    their genes those genes that are appropriate to
    the function of that particular cell type
  • transcription of a cells DNA must be regulated
  • factors such pregnancy may affect gene expression
    (genes for milk production are not used all the
  • the environment may affect which genes are
    transcribed (length of day may increase a change
    in size of sex organs affecting the production of
    sex hormones in birds)

  • Gene expression may be regulated by
  • 1. the rate of transcription of genes
  • the cell may regulate the transcription of
    individual genes through regulatory molecules
    (ex. steroids may stimulate the production of
    certain proteins)
  • certain parts of eukaryotic chromosomes are in a
    highly condensed, compact state making it
    inaccessible to RNA polymerase
  • some of these areas are structural and dont
    contain genes
  • other of these regions are functional genes that
    are not currently being transcribed
  • entire chromosomes may be inactivated
  • ex Female mammals have two X chromosomes in
    each cell but only one is available for
    transcription the other chromosome is condensed
    into a tight mass called a Barr body

  • 2. mRNAs may be translated at different rates
  • mRNAs vary in stability (how long they last
    before they are degraded) and in the rate at
    which they are translated into protein
  • 3. Proteins may require modification before they
    can carry out their functions in a cell
  • 4. The rate of enzyme activity may be regulated
    (previously discussed in organic chemistry)
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