Molecular Biology

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Molecular Biology

• Part III
• Regulation of Gene Expression
• Operons

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Mueller-Hill, B. 1996. The lac Operon
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Mueller-Hill, B. 1996. The lac Operon
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Transcriptional Regulation

• Main mechanism is to control access by RNA
  polymerase to the structural genes
• Other mechanisms may be added

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General Rules for Gene Regulation (1)

• Synthesis (of gene product) occurs only when
  needed
• 2. Catabolic (breakdown) systems are inducible by
  their substrates
• Anabolic (synthetic) systems are repressible by
  their end products

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General Rules for Gene Regulation (2)

• 3. Most systems have more than 1 regulatory
  mechanism
• 4. If there is more than one catabolic pathway,
  the cell will use the pathway that produces ATP
  at the greatest rate

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General Rules for Gene Regulation (3)

• 5. Proteins bring information to the genes about
  the status of cellular metabolism and conditions
  outside the cell

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Positive Regulation No synthesis unless turned
ON Negative Regulation Constituitive synthesis
unless turned OFF
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BUT There can be more than one control
point, and a gene can be BOTH and
regulated! If so, the gene is ON when the
regulator is bound and the regulator is not
bound. sic!
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INDUCIBLE turned ON by the presence of
substrate REPRESSIBLE turned OFF by the
presence of (end) product
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The Central Dogma Legos

• http//gadgets.boingboing.net/2009/03/02/video-usi
  ng-lego-to.htmlcomments

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the lac operon Jacob Monod

• catabolic
• negatively controlled
• expression is inhibited by a regulatory protein
  (lac repressor)
• inducible

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Mapping the lac operon

• Operon mapping
• mapped by interrupted mating

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Mapping the lac operon

• constituitive synthesis
• occurs unless repressor is bound to operator

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Mapping the lac operon

• cis/trans test
• cis dominance means the gene is dominant only
  when cis, i.e. located on the same piece of DNA
• i.e. does not operate via a diffusible
  intermediate
• e.g. operator vs. repressor
• O- dominant only when cis
• i dominant cis or trans

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Mapping the lac operon

• Partial Diploids
• constructed with plasmids (naturally occurring,
  naturally transmitted)
• allow testing of interactions among regulatory
  elements, e.g. cis/trans test