BISC 130 - Chapter 16: Regulating Gene Expression

• Gene --> mRNA --> Protein --> Cellular activities

• Expressed  = "on"

• Not expressed = "off"

LECTURE STOPPED - 42:27 (5/6)

• Not all genes need to be expressed at all times

  • Different cell types express different genes in order to fulfill different functions

    • Brain cells or neurons contain instructions to build muscle proteins, but they don't use it because they only build brain matter

    • Your muscle has instructions to build liver proteins, but it only expresses the genes necessary to build muscle.

• To respond to the environment.

• In Prokaryotes, most control is at the transcriptional stage.

  • Transcription factors used

    • A protein that binds to DNA and influences transcription of a gene or genes.

    • Two Types:

      • Repressors

        • Type of transcription factor that shut down transcription levels

        • Prevents or reduces levels of transcription

      • Activators

        • Type of transcription factor that increases transcription levels.

• In Eukaryotes, control is at ALL stages - including the transcriptional stage.

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• If lactose is absent:

  • Lac repressor protein binds to operator

    • Sequence specific

    • Physically blocks RNA Pol. From transcribing.

      • Expression of lac operon is repressed, "off"

• What if lactose is present:

  1. Lactose binds to the lac repressor protein and causes a shape change

     • Lac repressor + lactose cannot bind DNA

• RNA Pol. can transcribe Lac operon.

  • Expression of lac operon is "on", but low.

    • Low expression levels because promoter is "weak" - Not easily bound by RNA Pol.

1. Only want to express lac operon if a better sugar source is not available.

   1. Takes some energy to break down lactose (pineapple analogy)

   2. More efficient to use glucose, if present (blueberry analogy)

   3. Uses a transcription factor called "Catabolite Activator Protein" (CAP)

• High glucose --> low cAMP

  • cAMP is a small signaling molecule

  • Low glucose --> high camp

• CAP (big protein) needs cAMP (small signaling molecule) to activate

  • In other words, it needs low glucose to activate.

• If glucose is low/absent.

  • CAP binds cAMP, then binds close to promoter.

    • Sequence specific!

• CAMP + cAMP complex stabilizes RNA Pol. and helps start transcription

  • Expression is "on" and strong.

• If glucose is high/present:

  • CAP has no cAMP

  • CAP cannot bind DNA without cAMP

  • No stabilization of RNA Pol.

    • Expression "on" but low

• The two transcription factor mechanisms work together

  • *Lac operon is only expressed strongly if lactose is present and there is low glucose.

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