Chapter 18, 19, 20 Summaries

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

• RNA molecules play many roles in gene expression in eukaryotes

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

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

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

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

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)

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

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….

• 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

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

• 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

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

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

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

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

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

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)

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

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

• 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

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

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

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

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

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”