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The Genetics of Viruses and Bacteria Microbial Models Viruses come in many shapes and sizes Compare the size of a Eukaoryotic cell, Bacterial Cell and a Virus Herpes Virus Measles Polio Ebola Virus Discovery of the Virus Adolph Meyer a German Scientist studied the Tobacco Mosaic Virus. Thought it was caused by a very small bacteria because it could not be viewed through the microscope. Tobacco Mosaic Virus Tobacco Mosaic Virus Dimitri Ivanosky a Russian Scientist Filtered the sap to get rid of the bacteria. The plants still received the infection when sprayed with the filtered sap. Still thought the pathogen were very small bacteria. Martinus Beijerink a Dutch Botanist Discovered that this infectious particle could reproduce. Sprayed plants with filtered sap and their sap infected other plants. Infection was not diluted on subsequent infections. Could not grow outside the host in culture medium. Could not be inactivated with alcohol like bacteria Wendell Stanley an American Scientist Finally crystallized this infectious particle and viewed it under the electron microscope. Viral Composition Capsid – protein coat Sometimes an envelope – glycoproteins acid – DNA or RNA. Never both. Can be single or double stranded. Nucleic Some have tail fibers – Bacteriophage T4 Viruses Are Obligate Intracellular Parasites They lack their own enzymes to perform metabolism and reproduction. They utilize the host’s enzymatic machinery to accomplish these tasks. Viruses have a host range or are host specific. Rabies infects more than one host Eukaryotic viruses are usually tissue specific. • Rhinoviruses, Adenoviruses, Herpes, HIV Reproductive Cycles of Virus Lytic Cycle – destroys the host cell Viral proteins are translated by host enzymes and new viral particles are produced. Viral particles are assembled and the host cell is lysed. Host cell death occurs. Bacterial cells possess restriction endonucleases that destroy foreign DNA. The bacterial DNA is methylated to protect from destruction. Lytic Cycle Lysogenic Cycle Can Be Used For Cloning Viruses can infect without destroying the host cell. They integrate their DNA into the host cell and turn off their own genes. These types of viruses are called temperate viruses. Bacterial cells that possess these viral genes are celled prophages. Viral DNA can be replicated along with the host cell’s DNA. Lysogenic Cycle 18-05-PhageLambdaReproduct.mov Lysogenic Viruses can be Triggered to Become Lytic Viruses Radiation, chemicals or any host cell stress can cause the virus to enter the lytic cycle and destroy the host cell. Some prophages express prophage genes that alter the phenotype of the host cell. Bacteria produce endotoxins that originate from viral genes.(Diptheria, Scarlet Fever and Botulism) Genes can be inserted into bacterial cells using viruses in a process called Transduction. HIV Infection HIV Life Cycle 18-07-HIVreproduction.mov HIV Infection RNA as Viral Genetic Material Messenger RNA serves as the template for new genetic material. Reverse transcriptase produces DNA from mRNA. Newly made DNA integrates into the host chromosome. Unlike prophages, proviruses never leave. The virus now is referred to as a provirus. Viruses that do this are called retroviruses. The host’s RNA polymerase transcribes viral RNA from the DNA. RNA serves as both a template and mRNA. RNA viruses mutate more rapidly because replication of RNA does not have to undergo the same proofreading steps as replicating DNA. Transduction Viroids Viroids are tiny molecules of naked circular RNA that infect plants. - only several hundred nucleotides long. - a molecule can be an infectious agent. - disrupt metabolism by interferring with the host genome. Prions Prions are infectious proteins. - cause degenerative brain diseases like scrapes and Creutzfeldt-Jakob disease. - abnormal shaped brain proteins induce normal proteins to assume an abnormal shape propagating itself. Prions The Genetics of Bacteria Bacterial genome Circular double stranded DNA in the nucleoid region. (remember prokaryotes do not have a nucleus) Plasmids are in addition to the genome • Circular double stranded DNA • May carry extra chromosomal DNA called plasmids. Bacteria have about 4,300 genes, 500 x more than a virus but about 1/10th of that of a typical eukaryotic cell. Divide by binary fission DNA synthesis is bidirectional and can occur very quickly. • E.coli can divide in 20 minutes Genetic recombination of Bacteria Transformation Bacteria take up naked DNA from the environment. • If expressed this can change the phenotype of the bacterial cell. • Remember how the smooth strain of streptococcus pneumonia was transformed into the rough strain of streptococcus pneumonia. • Biotechnologist stimulate bacteria to take up foreign DNA with CaCl2. • Human genes like insulin can be transferred into bacteria and large quantities of insulin can be produced. Genetic Recombination of Bacteria Transduction Viruses (phages) are used to carry bacterial genes from one host cell to another. Two kinds of transduction • Generalized Transduction During the lytic cycle of the virus some of the host DNA can be accidentally packaged in the viral particle. When the virus infects a new host cell it injects the DNA from the previous host. It is called generalized transduction because the bacterial DNA is taken up randomly by the virus. Transduction Genetic Recombination in Bacteria Specialized Transduction • Involved the lysogenic cycle • The viral DNA integrates into the host bacterial cell and is called a prophage. • The virus then enters the lytic cycle and accidentally takes DNA that is flanking the insertion site of the viral DNA. • When the virus infects another host cell only certain genes are transferred to the new host cell. Conjugation Genetic Recombination in Bacteria Conjugation Direct transfer of genetic material between bacteria. Involves a sex pilus • A bridge that forms between two cells. Involves the transfer of a small segment of DNA or a small circular segment of DNA called a plasmid containing genes that code for the formation for the sex pili. • The plasmid is called the F (fertility) factor • Bacteria that have the F factor are ( F + ) and are considered male. The f factor replicates and is donated to the recipient cell. • The recipient cells are ( F - ) and considered to be female. • The recipient cell becomes ( F + ) when it receives the F factor. When the F factor integrates into the main chromosome then the cell is called an Hfr cell. Hfr cells have the tendency to recombine with the host chromosome of another ( F - ) cell. Conjugation Other types of Plasmids Episomes A genetic element that can either exist as a plasmid or part of a bacterial chromosome. Usually beneficial to bacteria because: • it allows then to survive adverse conditions • F factors promote recombination which introduces new genes that can perhaps facilitate survival. R Plasmids These plasmids carry genes for antibiotic resistance. Bacteria can transfer antibiotic resistance from one cell to other by transferring the R plasmid. Very dangerous antibiotic resistant bacteria have been created in hospitals Transposons Transposons Often called “jumping genes” Code for its own enzymes to excise itself from the genome and insert itself somewhere else. Transposons also copy themselves and insert the copy somewhere else in the genome. Transposons can jump from a plasmid to the main chromosome and visa versa. One main difference between genetic recombination through transformation, transduction, conjugation and transposons are: • In transformation, transduction, conjugation depends on base pairing between homologous regions of DNA. Transposons do not require similar or identical sequences of DNA to pair with. Operons A mechanism that bacteria use to control their metabolic needs. A group of genes are either turned on or off depending the metabolic needs of the organism. Involve catabolic and anabolic pathways. • Catabolic pathways break down substances and typically are turned off until the substance to be metabolized is present. These are known as inducible operons • Anabolic pathways are usually turned on until there is enough product is made. These are known as repressible operons. Operons Anatomy of a Repressible Operon These operons are normally turned on until the repressor is activated and binds to the operator to block RNA polymerase from binding to the promoter. Operator • regulatory switch • If a molecule is bound to the operator, then RNA polymerase cannot bind to the promoter and mRNA cannot be transcribe and thus the protein product cannot be made. Repressor • The molecule that binds to the operator is the repressor. Corepressor • Binds to the repressor and activates it so that it can bind to the operator. Regulatory Gene • Synthesizes the repressor molecule Trp operon Repessible operon present in E.coli that synthesizes tryptophan. When enough tryptophan is made it acts as a corepressor and binds to the repressor to and converts the repressor to its active form. The activated repressor binds to the operator and blocks RNA polymerase from binding to the promoter thus preventing transcription. Trp operon Operons Anatomy of a Inducible Operon These operons are normally turned off until the repressor is inactivated. At this point the repressor falls off the operator and RNA polymerase and binds to the promoter. Operator • regulatory switch • If a molecule is bound to the operator, then RNA polymerase cannot bind to the promoter and mRNA cannot be transcribed and the protein product cannot be synthesized. Repressor • The molecule that binds to the operator is the repressor. Inducer • Binds to the repressor and converts it to the inactive from and it falls off the operator. Regulatory Gene • Synthesizes the repressor molecule lac operon The lac operon is only turned on when lactose is present because it produces enzymes necessary to break down lactose. It is an inducible operon The inducer in allolactose which binds to the repressor which is already bound to the operator and inactivates it. The inactivated repressor falls off the operator and the genes are transcribed by RNA polymerase. lac operon An Example of Positive Gene Regulation E.coli would prefer to use glucose as an energy source. It will produce the enzymes to break down lactose into its components galactose and glucose if glucose is not available. How does E.coli sense the glucose concentration and send a message to the operon to produce or not produce the enzyme necessary to break down lactose (Beta galactosidase)? A molecule called cyclic AMP(cAMP) builds up when glucose is in low concentrations. cAMP binds to cAMP receptor protein (CRP) and the cAMP/CRP complex binds to a site upstream from the promoter and bends the DNA in such a way that it is easier for RNA polymerase to bind to the operon. This stimulates transcription and is positive regulation. When there is an abundance of glucose the cAMP levels drop and the cAMP/CRP complex cannot form. Up regulation of the lac operon