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Gene Expression

Gene Expression. AP Biology. Questions to Ponder…. How do your cells “know” what kind of cell they are? How do your cells “know” when to make a particular protein? When to stop making it? How does the environment affect your cells? ANSWER: Gene Expression. Gene Expression in Bacteria.

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Gene Expression

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  1. Gene Expression AP Biology

  2. Questions to Ponder….. • How do your cells “know” what kind of cell they are? • How do your cells “know” when to make a particular protein? When to stop making it? • How does the environment affect your cells? • ANSWER: Gene Expression

  3. Gene Expression in Bacteria • Bacteria are single-celled organisms who are surrounded on all sides by their environment. • They must be able to regulate expression of their genes in response to environmental changes.

  4. Bacteria Respond by Regulating Transcription • Bacteria cells that can conserve resources and energy have a selective advantage over cells that cannot do so. • Natural selection has favored bacteria that express only the genes they need.

  5. E. Coli Regulation of Tryptophan • An individual E. coli cell living in the erratic environment of the human colon, is dependent for its nutrients on the whimsical eating habits of its host—you! • If the environment is lacking in the amino acid tryptophan, which the bacterium needs to survive, it responds by activating a metabolic pathway that makes tryptophan from another compound. • If tryptophan becomes available, it shuts down this pathway.

  6. Regulation of a Metabolic Pathway • In the pathway for tryptophan synthesis, an abundance of tryptophan can both inhibit the activity of the first enzyme (a rapid response) OR repress expression of the genes encoding the enzymes in the pathway (a longer response). • This is an example of feedback inhibition. It allows for a cell to adapt to short-term fluctuation in the supply of a substance it needs.

  7. Fig. 18-2 Precursor Feedback inhibition trpE gene Enzyme 1 trpD gene Regulation of gene expression trpC gene Enzyme 2 trpB gene Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production

  8. Gene Expression Controls Which Enzymes are Made and When • In many cases, this occurs in the process of transcription. • Many genes may be switched on or off by changes in the metabolic status of the cell. • One example was discovered in 1961 by Francois Jacob and Jacques Monod at the Pasteur Institute in Paris. This method is called the Operon Model.

  9. Operons: The Basic Concept • A cluster of functionally related genes that can be under coordinated controlby a single on-off “switch”. • The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter. • An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control.

  10. The operon can be switched off by a protein repressor • The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase • The repressor is the product of a separate regulatory gene

  11. The repressor can be in an active or inactive form, depending on the presence of other molecules. • A corepressor is a molecule that cooperates with a repressor protein to switch an operon off.

  12. Bacteria can synthesize tryptophan by utilizing the trpoperon. • By default, the trpoperon is on and the genes for tryptophan synthesis are transcribed • When tryptophan is present, it binds to the trprepressor protein, which turns the operon off • The repressor is active only in the presence of its co-repressor tryptophan; thus the trpoperon is turned off (repressed) if tryptophan levels are high

  13. Fig. 18-3a trp operon Promoter Promoter Genes of operon DNA trpD trpR trpE trpC trpB trpA Operator Regulatory gene Stop codon Start codon 3 mRNA 5 RNA polymerase mRNA 5 B A D C E Protein Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on

  14. Fig. 18-3b-1 DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

  15. Fig. 18-3b-2 DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

  16. Video for Gene Expression • http://education-portal.com/academy/lesson/regulation-of-gene-expression-transcriptional-repression-and-induction.html

  17. Different Types of Operons • A repressible operon is one that is usually ON—binding a repressor to the operator turns off transcription. (The trp operon is like this) • An inducible operon is one that is usually OFF—a molecule called an inducer inactivates the repressor and starts transcription. (The lac operon is this type)

  18. The lac Operon • The lac operon is an inducible operon (usually off) and contains genes that code for enzymes that break down the sugar lactose (found in dairy products) • By itself, the lac repressor is active and therefore shuts the lac operon off most of the time. • A molecule called an inducer inactivates this repressor which turns the lac operon on.

  19. Fig. 18-4 Regulatory gene Promoter Operator lacZ lacI DNA No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon lacZ DNA lacI lacY lacA RNA polymerase 3 mRNA mRNA 5 5 Permease -Galactosidase Transacetylase Protein Inactive repressor Allolactose (inducer) (b) Lactose present, repressor inactive, operon on

  20. Fig. 18-4a Regulatory gene Promoter Operator lacI lacZ DNA No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off

  21. Fig. 18-4b lac operon lacY DNA lacI lacZ lacA RNA polymerase 3 mRNA mRNA 5 5 Permease Transacetylase -Galactosidase Protein Inactive repressor Allolactose (inducer) (b) Lactose present, repressor inactive, operon on

  22. Inducible Enzymes • Inducible enzymes (such as those found in the lac operon) are usually catabolic enzymes, which means they break things apart. • Their synthesis is usually induced by some kind of signal. • In the lac operon, the signal is the presence of the lactose sugar molecule.

  23. Repressible Enzymes • Repressible enzymes (such as those in the trp operon) usually function in anabolic pathways which build things or put things together. • Since these are almost always ON, they are repressed (shut down) when there are high levels of the end-product present.

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