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 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

3. 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.

4. 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

5. 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)

6. A generalized viral reproduction cycle

7. 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

8. Lytic

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

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

11. Lysogenic

12. 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

13. 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

14. Enveloped virus reproductive cycle

15. HIV – enveloped retrovirus

16. Retrovirus

17. 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

18. 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 mutations per gene per day

19. 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.

20. 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

21. 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’

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

24. Insertion sequence

25. 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)

26. Control of gene expression the lac operon model