chapter 18, 19, 20 summaries
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Chapter 18, 19, 20 Summaries. Gene Expression Viruses Biotechnology. Gene Expression (Chapter 18). Regulated by altering gene expression in response to a changing environment Regulates both development and differentiation of cells - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 18, 19, 20 Summaries
Gene Expression
Viruses
Biotechnology
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Gene Expression (Chapter 18)
• Regulated by altering gene expression in response to a changing environment
• Regulates both development and differentiation of cells
• RNA molecules play many roles in gene expression in eukaryotes
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Prokaryotes (Bacteria)
• Often respond to their environment by regulating transcription
• This regulation can be done by feedback inhibition or gene regulation
• Gene expression in bacteria is controlled by the operon model
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Operons
• A cluster of functionally related genes can be under the control of a single on-off switch
• This switch is a segment of DNA called an operator (usually positioned within the promoter sequence)
• So an operon is the entire stretch of DNA including the promoter the operator and the genes they control
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How it works
• The operon can be switched off by a repressor (protein)
• The repressor prevents gene transcription by binding to the operator and blocking the action of RNA polymerase
• The repressor is the product of a separate regulatory gene
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How it Works 2
• The repressor may have an active or inactive form, depending on the presence of a particular molecule
• A corepressor is a molecule that cooperates with a repressor protein to switch an operon off
• Example: E. coli can make the amino acid tryptophan
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How it works (3)
• The “default “ setting allows the genes for tryptophan synthesis to be tanscribed
• If tryptophan is present, especially in large amounts, it binds to a tryptophan repressor protein and turns the operon off, no longer transcribing genes that make tryptophan
• This repressor is only active when the corepressor tryptophan is present
• This prevents the bacteria from making too much tryptophan (form of feedback inhibition)
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Repressible and Inducible Operons
• The tryptophan operon is a repressible operon and is repressed in the presence of tryptophan
• Inducible operons are ones that are usually off
• A molecule called an inducer inactivates the repressor and turns on transcription
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The lac Operon
• An inducible operon; contains genes that code for enzymes used in the hydrolysis and metabolism of lactose
• By itself, the lac repressor is active and switches the lac operon off
• A second molecule called an inducer then inactivates the repressor to turn the lac operon on….
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• Inducible operons usually function in catabolic pathways and their synthesis is induced by a chemical signal
• Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product
• This type of regulation is referred to as negative gene regulation because operons are turned off by the active form of the repressor
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Positive Gene Regulation
• Some operons can be stimulated by a protein (ex:CAP or catabolite activator protein) to activate transcription
• When glucose is scarce, the CAP binds with Cyclic AMP
• Activated CAP attaches to the promoter of the lac operon and increases the chemical affinity of RNA polymerase, thus accelerating transcription
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• When glucose levels increase, CAP detaches from the lac operon and transcription continues at its normal rate
• CAP helps regulate other operons that encode enzymes used in catabolic pathways
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Differential Gene Expression
• All multicellular organisms gene regulation is essential for cell specialization which makes it important in development of embryos
• Almost all the genes in a cell are genetically identical, so how do we get our many types of cells?
• Differential gene expression is the expression of different genes by cells with the same genome
• Errors in gene expression can lead to cancer and other diseases
• Gene expression is regulated at many stages
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Control Elements & Transcription Factors
• Control elements are segments of noncoding DNA that help regulate transcription by binding certain proteins
• Transcription factors are proteins that act along with RNA polymerase to start transcription
• There are both general and specific transcription factors
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Post-Transcriptional Regulation
• Transcription alone cannot account for gene expression
• Regulatory mechanisms can operate after transcription
• These allow the cell to fine-tune its response to changes in the environment
• There are several things that can be involved in this type of regulation
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Alternative RNA Splicing
• Different mRNA molecules are produced from the same primary mRNA transcript, depending on which RNA segments are treated as introns and which are treated as exons
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Differential Gene Expression and Embryonic Development
• Development of multicellular organisms is controlled by gene expression
• Materials in the egg can set up gene regulation that is carried out as cells divide
• Cytoplasmic determinants are maternal substances in the egg that influence early development
• Early mitotic divisions still contain these and lead to different gene expression
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Induction
• Process by which signal molecules from cells in the environment cause transcriptional changes in nearby target cells
• So interactions between cells cause differentiation into particular cell types
• Cell differentiation is marked by production of tissue specific proteins (ex. Muscle-specific proteins for muscle cells and tissue)
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Setting up the Body Plan
• Pattern formation is the development of a spatial (3D) formation of tissues and organs
• It begins with the formation of axes and body areas such as ventral and dorsal
• Positional Information-molecular clues that control pattern formation by telling a cell where it is in relation to other cells or tissues or axes
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Viruses (Chapter 19)
• Viruses consist of either DNA or RNA surrounded by a protein coat
• They were detected before they were able to be seen
• They are not cells• 1935 Wendell Stanley discovered the
Tobacco Mosaic Virus while researching the disease that stunted tobacco plants
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• Viruses are sometimes referred to as obligate intracellular parasites because they can reproduce only within a host cell
• Each virus has a Host Range or limited number of host cells that it can infect
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Viral Envelopes
• Some viruses have membranous envelopes that help them infect host cells
• These surround the capsids of influenza
viruses
• These can be derived from the host’s cell membrane and contain a combination of host cell and viral molecules
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Reproductive Cycles
• Once the viral genome has entered the cell, it begins to manufacture viral proteins
• It makes use of host cell enzymes, ribosomes, tRNA’s, amino acids, ATP and other molecules
• Viral parts spontaneously self-assemble into new viruses
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Bacteriophages
• Viruses that infect bacteria are called bacteriophages or phages
• They are the most studied of all viruses and have directly and indirectly provided us with many tools we now use in biotechnology
• They have two reproductive cycles: Lytic and Lysogenic
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The Lytic Cycle of A Bacteriophage
• The lytic cycle of a virus destroys the bacterial cell
• The viral DNA or RNA enters the bacterium and takes over the host cell DNA
• It begins producing viral parts• The viral parts self-assemble and can cause
the cell membrane to rupture in several places
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Lysogenic Cycle
• This is a reproductive cycle where the virus enters the cell, becomes a part of the host cell genome and remains dormant for a time
• If something stimulates it to become virulent it will enter the lytic cycle and destroy the cell
• If not, its genome is reproduced along with the host cell and it “hitches a ride”