Microbial Models: Viruses and Bacteria AP Biology 1/08/03

Viruses and Bacteria Discovery of viruses Science as a process Tobacco Mosaic Virus *Mayer 1883 Disease is contagious; small maybe bacteria *Ivanowsky 1893 Filtered and found pathogen to be very small bacteria or toxin *Beirjerinck 1897 Found it could reproduce only in the host; couldn’t be cultivated; not inactivated by alcohol *Stanley 1935 Crystalized the infectious agent Viruses and Bacteria Size comparison size demonstration

Basic structure of a virus Bacterio- phages Basic structure of a virus Some form of nucleic acid (DNA or RNA) Enclosed in a protein coat. (capsid) Viral envelopes -membranes that cloak their capsids. Often derived from host cell membrane.

Viral ‘life’ cycle Viruses are obligate intracellular parasites -they can reproduce only within a host cell Therefore, viruses are basically packages of genetic material that move from host to host. Something to ponder: should viruses be considered 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 and the receptors on the host cell

The basics of viral reproduction 1) Entry into the host cell -injection -membrane fusion 2) Replication and Translation of the genetic material -using the host cells genetic machinery 3) Assembly and release of the new viral particles -lysis of host cell -budding from the host cell Symptoms 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 growth control mechanisms (oncogenes)

A generalized viral reproduction cycle

Lytic and Lysogenic viral cycles: focusing on phages Lytic cycle: reproductive cycle that results in the death of the host cell as it breaks open (lyses), releasing the new viral 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 without destroying the host cell. Prophage: viral DNA that is incorporated into the genetic material of the host cell. Temperate viruses utilize both modes of reproduction

Lytic

The lytic cycle Protection for the bacterial cells: receptor variation restriction nucleases lysogenic evolution of viruses (?)

Lytic and Lysogenic Environmental trigger Protein represses most of the other phage genome

Lysogenic

Animal viruses Most vertebrate viruses have tissue specificity - cold virus : upper respiratory tract - HIV : lymphocytes -WNV : brain tissue Animal viruses Impact of the virus may depend on the type of tissue and the possibility of cell renewal - cold virus destroys epithelium that can be repaired -poliovirus attacks nerve cells, and therefore the damage is permanent. Defenses: Vaccines: Jenner and his faith in the milk maid -Stimulate the immune system to set up the defenses against the actual pathogen. Disruption of the genetic translation mechanisms - AZT for HIV

Reproductive cycle of animal viruses Viral envelopes Equipped with an outer membrane, outside of the capsid. - lipid bilayer and glycoproteins -often derived from the host cell Helps the viral particle to enter the host cell and also helps to disguise the viral particles to limit recognition by the host immune system. Genetic material: Animal viruses may contain DNA or RNA Provirus: DNA that is integrated into the host cells DNA -Herpes RNA – broad variety of RNA genomes in animal viruses -mRNA -retroviruses utilize reverse transcription (e.g. HIV) RNA -> DNA -> Provirus -> RNA

Enveloped virus reproductive cycle

HIV – enveloped retrovirus

Retrovirus

Outbreaks: Emerging viruses Three main components to ‘new’ viruses appearing 1) Mutation of existing diseases -RNA viruses have high mutation rates; lack proof-reading steps. -individuals may not have immunity to new strains -flu virus 2) Spread from a different host species - hantavirus outbreak in 1993 spread from rodents to humans -resulted from very high populations of rodents 3) Dissemination of a virus from an isolated population -international travel and tourism -AIDS

Bacteria Short generation time facilitates evolutionary adaptation. Binary fission -Asexual reproduction -under optimal conditions E. coli can reproduce every 20 minutes Spontaneous mutation rate: 1 * 107 per cell division, therefore: approx. 2000 mutations per gene per day

Genetic recombination in bacteria Combining the DNA from two individuals into the genome of a single individual. After 24 hrs, the # of cells that can synthesize both Arg and Trp excedes the rate of mutation…. Must be recombination.

Bacterial genome alteration Transformation -alteration of bacterial DNA by uptake of naked, foreign, DNA from the surrounding environment. 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

Plasmids Small self-replicating DNA molecule Can undergo reversible incorporation into the cell’s Chromosome…plays an important role in recombination Episome: genetic element that can replicate either as a plasmid or as part of the bacterial chromosome. (lambda phage also an episome…similarities and differences between plasmids and episomes?) F-plasmid R-plasmid ‘fertility’ ‘resistance’

Conjugation and plasmids Hfr can ‘mate’ with F- cell, sending DNA fragment. Crossing over can then occur between frament and the original DNA

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 cuts The DNA at that site, and at the target site.

Insertion sequence

Composite transposons Contain additional genes, such as those for antibiotic resistance May help bacteria adapt to new environments of harsh conditions packaging these genes on an R plasmid would be especially favorable. (also found in eukaryotic cells…chap 19)

Control of gene expression the lac operon model