Presentation on theme: "Microbial Models: Viruses and Bacteria AP Biology 1/08/03"— Presentation transcript:
1 Microbial Models: Viruses and Bacteria AP Biology 1/08/03
2 Viruses and Bacteria Discovery of viruses Science as a process Tobacco Mosaic Virus*Mayer 1883Disease is contagious; smallmaybe bacteria*Ivanowsky 1893Filtered and found pathogen tobe very small bacteria or toxin*Beirjerinck 1897Found it could reproduce only inthe host; couldn’t be cultivated;not inactivated by alcohol*Stanley 1935Crystalized the infectious agentViruses and BacteriaSize comparisonsize demonstration
3 Basic structure of a virus Bacterio-phagesBasic structure of a virusSome form ofnucleic acid(DNA or RNA)Enclosed in a protein coat. (capsid)Viral envelopes-membranes that cloak their capsids. Often derived from hostcell membrane.
4 Viral ‘life’ cycle Viruses are obligate intracellular parasites -they can reproduce only within a host cellTherefore, viruses are basically packages of genetic materialthat move from host to host.Something to ponder: should viruses beconsidered to be alive?(…are computer viruses or chain letters alive?)Host range: the potential hosts that a given virus can infect.-Host specificity like a ‘lock-and-key’ system-depend on proteins on the outside of the virus andthe receptors on the host cell
5 The basics of viral reproduction 1) Entry into the host cell-injection-membrane fusion2) Replication and Translation of the genetic material-using the host cells genetic machinery3) Assembly and release of the new viral particles-lysis of host cell-budding from the host cellSymptoms from a viral infection:-Host response to the viral infection (immune response)-Prolific cell death-Proteins produced by viral genetic material (e.g. diptheria)-Cancer resulting from disruption of cell growthcontrol mechanisms (oncogenes)
7 Lytic and Lysogenic viral cycles: focusing on phages Lytic cycle: reproductive cycle that results in the death of thehost cell as it breaks open (lyses), releasing the newviral particles.-lysis may be brought on by the release of lysozyme,from the newly assemble viral particles,that weakens the bacterial cell wall.‘Virulent’ viruses utilize this reproductive cycle.Lysogenic cycle: replicates the viral genome withoutdestroying the host cell.Prophage: viral DNA that is incorporated into thegenetic material of the host cell.Temperate viruses utilize both modes of reproduction
12 Animal viruses Most vertebrate viruses have tissue specificity - cold virus : upper respiratory tract- HIV : lymphocytes-WNV : brain tissueAnimal virusesImpact of the virus may depend on the type of tissue andthe possibility of cell renewal- cold virus destroys epithelium that can be repaired-poliovirus attacks nerve cells, and therefore thedamage is permanent.Defenses:Vaccines: Jenner and his faith in the milk maid-Stimulate the immune system to set up thedefenses against the actual pathogen.Disruption of the genetic translation mechanisms- AZT for HIV
13 Reproductive cycle of animal viruses Viral envelopesEquipped with an outer membrane, outside of the capsid.- lipid bilayer and glycoproteins-often derived from the host cellHelps the viral particle to enter the host cell and also helpsto disguise the viral particles to limit recognition bythe host immune system.Genetic material: Animal viruses may contain DNA or RNAProvirus: DNA that is integrated into the host cells DNA-HerpesRNA – broad variety of RNA genomes in animal viruses-mRNA-retroviruses utilize reverse transcription (e.g. HIV)RNA -> DNA -> Provirus -> RNA
17 Outbreaks: Emerging viruses Three main components to ‘new’ viruses appearing1) Mutation of existing diseases-RNA viruses have high mutation rates; lackproof-reading steps.-individuals may not have immunity to new strains-flu virus2) Spread from a different host species- hantavirus outbreak in 1993 spread from rodentsto humans-resulted from very high populations of rodents3) Dissemination of a virus from an isolated population-international travel and tourism-AIDS
18 Bacteria Short generation time facilitates evolutionary adaptation. Binary fission-Asexual reproduction-under optimal conditions E. colican reproduce every 20 minutesSpontaneous mutation rate: 1 * 107 per cell division,therefore: approx mutations per gene per day
19 Genetic recombination in bacteria Combining theDNA from twoindividuals intothe genome ofa single individual.After 24 hrs, the #of cells that cansynthesize bothArg and Trpexcedes the rateof mutation….Must be recombination.
20 Bacterial genome alteration Transformation-alteration of bacterialDNA by uptake of naked, foreign,DNA from the surroundingenvironment.Transduction-DNA transfer via phages*Generalized-random pieces of host DNA gets transfered*Specialized-prophage exits chromosome and carried pieces of host DNA with it
21 Plasmids Small self-replicating DNA molecule Can undergo reversible incorporation into the cell’sChromosome…plays an important role in recombinationEpisome: genetic element that can replicate either as a plasmidor as part of the bacterial chromosome.(lambda phage also an episome…similarities anddifferences between plasmids and episomes?)F-plasmid R-plasmid‘fertility’ ‘resistance’
22 Conjugation and plasmids Hfr can ‘mate’ with F- cell, sending DNA fragment.Crossing over can then occur between frament and the original DNA
23 Transposons -a piece of DNA that can move from one location to another; moves genes into new areas (target sites)Insertion sequence: simplest transposon-one gene that codes for transposase-transposase recognizes the inverted repeates and cutsThe DNA at that site, and at the target site.
25 Composite transposons Contain additional genes, such as those for antibiotic resistanceMay help bacteria adapt to new environments of harsh conditionspackaging these genes on an R plasmid would be especiallyfavorable.(also found in eukaryotic cells…chap 19)
26 Control of gene expression the lac operon model