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Chapter 18 Genetics of Viruses and Bacteria Virus Discovery • 1883: Adolf Mayer (German) was looking for the cause of the tobacco mosaic disease – Stunts growth of tobacco and gives leaves a mottled look – Disease was contagious and could be transmitted by spraying sap extracted from diseased leaves onto healthy ones – Thought it was caused by an unusually small bacteria that could not be seen by the microscope Magnification Demonstration • Dimitri Ivanowsky (Russia) passed sap from infected tobacco leaves through a filter designed to remove bacteria – Sap still caused disease – Thought that they were too small to be filtered, or made a toxin that caused the disease • 1897: Marntinus Beijerinck (Dutch) discovered the infectious agent could reproduce (therefore is not a toxin) – Pathogen could only reproduce within the host it infected • Sprayed and sprayed and sprayed – Could not be cultivated on petri dishes • 1935: Wendell Stanley (USA) crystallized the virus with the electron microscope Viral Anatomy • Infectious particles consisting of nucleic acid enclosed in a protein coat • Genome: may be double stranded DNA, single stranded DNA, double stranded RNA, or single stranded RNA – DNA virus or RNA virus – Usually organized in a single linear or circular molecule of nucleic acid • Capsid is the protein shell that encloses the genome – may be rod shaped, polyhedral, or more complex • Viral envelopes are membranes that cloak the capsid – Derived from the membrane of the host cell – Adds to offense Obligate Intracellular Parasites • Can reproduce only within a host cell • Viruses lack enzymes for metabolism and have no equipment for making their own proteins • Each virus can infect only a limited range of hosts, called host range – Can be wide (rabies) or narrow (only infect E. coli) • Viruses identify their host by the same way enzymes find their substrates- lock and key • Eukaryotic viruses are generally tissue specific – Cold virus attaches to respiratory tissue Reproduction • Lytic • Lysogenic Lytic Cycle • Phage reproduction that cumulates in death of the host cell • Virulent phage can only reproduce by the lytic cycle • Turns cell into a phage producing factory, and the cell lyses and releases the products Lysogenic Cycle • Replicates the phage genome without destroying the host • Could undergo both lytic and lysogenic = temperate phage • Phage DNA is incorporated into host DNA at the prophage • Every time the cell divides, it replicates the phage DNA also and passes it to daughter cells – Switch from lysogenic to lytic is environmental Clay Video Animal Viruses- Viral Envelope RNA as Genetic Material • Retroviruses undergo reverse direction of genetic information – Reverse transcriptase transcribes DNA from and RNA template (RNADNA) • New DNA integrates into a chromosome in the host cell – Called a provirus, and it never leaves the host cell – Examples: HIV and AIDS Cause of Viral Diseases • Some viruses cause the infected cells to produce toxins that lead to disease symptoms, and some have molecular components that are toxic • How much damage a virus causes depends on the ability of the infected tissue to regenerate – Cold virus repairs respiratory tissue relatively fast – Polio virus damage is permanent – Symptoms of viral infections actually result in the body’s efforts at defending against the infection Prevention • Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen – “Vacca” is Latin for cow • First vaccine, against smallpox, consisted of cowpox virus – Edward Jenner learned that milkmaids who had contracted cowpox were resistant to smallpox – Scratched a boy with a needle bearing fluid from an infected milkmaid’s sore and the boy resisted smallpox Treatment • Antibiotics don’t work • Most viruses need few nutrients to live • New medication is working on blocking the action of reverse transcriptase Emerging Viruses • AIDS – Sudden appearance in 1980’s • Nipah Virus – Malaysia; killed 105 in 1999 and destroyed pig markets How Does it Happen? • Mutation of existing diseases – Flu • Spread of existing viruses from one species to another – ¾ of new viruses come from animals – Hantavirus • Dissemination of virus from a small isolated population spreads to widespread epidemics – AIDS Viruses and Cancer • 1911: Peyton Rous discovered a virus that causes cancer in chickens • Virus that causes hepatitis B also causes liver cancer • Virus that causes mono has been linked to several types of cancer in parts of Africa • HPV cervical cancer • Viruses have oncogenes that mess with the cell cycle – Want more and more copies of its genome in the host Tiny Infectious Agents • Viroids are tiny molecules of naked circular RNA that infect plants – Disrupt the metabolism of a plant cell and stunt its growth – Caused 10 million coconut trees to die in Philippines – Cause errors in the regulatory systems that control growth • Prions are infections proteins – Cause mad cow disease and Creutzfeldt-Jakob disease – It is a misfolded form of a protein normally present in brain cells – When the prion gets into a cell containing the normal form, it converts it to the prion version Creutzfeldt-Jakob Disease • • • • • • • • • • • • • • Blurred vision (sometimes) Changes in gait (walking) Confusion, disorientation Dementia that occurs over a few weeks or months Hallucinations Lack of coordination (for example, stumbling and falling) Muscle stiffness Muscle twitching Myoclonic jerks or seizures Nervous, jumpy feelings Personality changes Sleepiness Speech impairment CJD is rarely confused with other types of dementia (such as Alzheimer's disease) because in CJD, the symptoms get worse much more quickly. Both forms of CJD are different than dementia because the symptoms progress quickly to disability and death Bacterial Genome • Major component of the bacterial genome is one double-stranded, circular DNA molecule • Consists of about 4.6 million base pairs coding for 4,300 genes – 100 times more than viruses, but one thousandth as much as eukaryotic cells • DNA is found in the nucleiod, a region not bound to the membrane like a nucleus • When DNA needs to be copied, it starts at a single origin of replication and proceeds both ways around the circular chromosome Proliferation • E. coli growing under optimal conditions can divide every 20 minutes • A single cell can produce a colony of 108 new cells in 12 hours • In the human colon, E. coli reproduces enough to replace the 2 x 1010 bacteria lost in feces daily Reproduction • Binary fission is a type of asexual reproduction • Therefore, they are essentially clones of the parent cell – Mutations occur every million divisions • 2,000 mutations in your colon replacements – New mutations can have a significant impact on the genetic diversity when reproductive rates are very high because of short generation times • Leads to the evolution of bacteria Genetic Recombination • Transformation • Transduction • Conjugation Transformation • Alteration of a bacterial cell’s genotype by the uptake of naked, foreign DNA from the surrounding environment – Oswald Avery Experiment • The foreign allele is incorporated into the bacterial chromosome, replacing the native allele • Many species of bacteria have surface proteins specialized for the uptake of naked DNA Transduction • Phages carry bacterial genes from one host cell to another – Generalized transduction: some of the host’s DNA gets packaged with the new phage during lytic cycle and transferred to a new host – Specialized transduction: requires infection from a temperate phage, and a piece of the prophage gets picked up and sent out when it turns to the lytic cycle Conjugation • Direct transfer of genetic material between two bacterial cells that are temporarily joined • “Bacterial Sex” • DNA transfer is one way • “Male” uses appendages called sex pilli to attach to the DNA recipient, the “female” – Pulls the two cells together, and a bridge is formed for DNA transfer Manly Bacteria • The “maleness” ability to form sex pilli and donate DNA during conjugation comes from a piece of DNA called the F factor – F stands for fertility – Could exist as a segment of DNA or as a plasmid Plasmids • Small, circular, self-replicating DNA molecule separate from the bacterial chromosome • Can undergo reversible incorporation into the cell’s chromosome • An episome is a genetic element that can exist wither as a plasmid or as a part of the chromosome • Generally beneficial to the cell – Contains F factor that may be advantageous in a changing environment that no longer favors existing strains in a bacterial population F Factor • “Males” are considered F+ • It is heritable and contagious – F- “females” that undergo conjugation become F+ R Plasmids • 1950- Japan; began to notice that patients suffering from bacterial dysentery did not respond to antibiotics that had once been effective • R plasmids transfer resistance to its cell – Some code for enzymes that attack antibiotics themselves • How do so many resistance genes become a part of one plasmid? – Transposon + Transposons • “Transposable genetic element” • Piece of DNA that can move from one location to another in a cell’s genome. • Never exist independently • Bring multiple genes for antibiotic resistance into a single R plasmid by moving the genes to that location from different plasmids • Do a copy and paste or cut and paste of the genes to put them in new locations Insertion Sequences • Simplest bacterial transposons • Consist only of the DNA necessary for the act of transposition • Uses inverted repeats to know where the Transposase gene is located Composite Transposons • Include extra genes other than ones only needed for transposition – Resistance genes • May help bacteria adapt to new environments • Transposons are found in other organisms as well – Barbara McClintock and maize Gene Expression Control • Bacteria live in crazy and unpredictable environments • The E. coli in our colons are dependent on the food we eat for their nutrients • However, if they do not get enough of a certain amino acids they need, they will make their own by changing more readably available amino acids into the one they are lacking Operons • When the bacteria must make their own amino acids, all the enzymes for that particular metabolic pathway are synthesized at one time – The switch to turn it on/off is called an operator • Can allow or block RNA polymerase to attach and synthesize the amino acids – The operator, the promoter, and the genes they control are operons Trp Operon • Pathway to make Tryptophan • Repressible Operon • Pathway is generally on – RNA polymerase can bind to the promoter and transcribe the genes of the operon • Can be switched off by a protein called the repressor – Binds to the operator and blocks RNA polymerase • Made by a regulatory gene • Tryptophan itself acts as a corepressor because when present, it will sit on the operator and turn the switch off Inducible Operons • lac operon is usually OFF, but in the presence of lactose, lactose will bind to the inducer to inactivate the repressor • Lac operon codes for genes that allow bacteria to take in lactose, which is not normally in the nutrient medium