Microbial Models: Viruses and Bacteria AP Biology1/08/03. 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
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Discovery of viruses
Science as a process
Tobacco Mosaic Virus
Disease is contagious; small
Filtered and found pathogen to
be very small bacteria or toxin
Found it could reproduce only in
the host; couldn’t be cultivated;
not inactivated by alcohol
Crystalized the infectious agent
Some form of
(DNA or RNA)
Enclosed in a protein coat. (capsid)
-membranes that cloak their capsids. Often derived from host
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
1) Entry into the host cell
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)
Lytic cycle: reproductive cycle that results in the death of the
host cell as it breaks open (lyses), releasing the new
-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
most of the other
Most vertebrate viruses have tissue specificity
- cold virus : upper respiratory tract
- HIV : lymphocytes
-WNV : brain tissue
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.
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
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
RNA – broad variety of RNA genomes in animal viruses
-retroviruses utilize reverse transcription (e.g. HIV)
RNA -> DNA -> Provirus -> RNA
Three main components to ‘new’ viruses appearing
1) Mutation of existing diseases
-RNA viruses have high mutation rates; lack
-individuals may not have immunity to new strains
2) Spread from a different host species
- hantavirus outbreak in 1993 spread from rodents
-resulted from very high populations of rodents
3) Dissemination of a virus from an isolated population
-international travel and tourism
Short generation time facilitates evolutionary
-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
DNA from two
the genome of
a single individual.
After 24 hrs, the #
of cells that can
Arg and Trp
excedes the rate
Must be recombination.
-alteration of bacterial
DNA by uptake of naked, foreign,
DNA from the surrounding
-DNA transfer via phages
-random pieces of host DNA gets transfered
-prophage exits chromosome and carried pieces of host DNA with it
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?)
Hfr can ‘mate’ with F- cell, sending DNA fragment.
Crossing over can then occur between frament and the original DNA
-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.
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
(also found in eukaryotic cells…chap 19)