Gene Regulation

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Gene Regulation

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

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

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• 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
  lactose)
• 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

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• 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

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• 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

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• 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
  operon
• 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
  time)
• 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)

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• 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

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• 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)