1. The Genetics of Viruses
Background
Structure
Life Cycles

2. Viral infections, past & present
Viral infection have been one of the major infectious challenges of the human species, for as far back as we can tell.

3. Polio
Viral infection have been one of the major infectious challenges of the human species, for as far back as we can tell. If one looks as far back as the Egyptian hieroglyphics, one can find evidence there of polio infection.

4. Ebola Virus

5. Herpes Virus

6. Severe acute respiratory syndrome (SARS)
Figure A, B
(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.
(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.

7. Let’s size them up…

8. Compare the size of: eukaryotic cell, bacterial cell and a virus

9. I. Background Everyone is at risk for infection! Size 20nm-250nm
Bacteriophages infect bacteria only
All eukaryotes (animals, plants, fungi, protist) all vulnerable
Size 20nm-250nm
0.5 m
Ghost phage
100 to 1000 times smaller than the cells they infect
Can pass through filters
To put viral size into perspective, a medium sized virion next to a flea is roughly equivalent to a human next to a mountain twice the size of Mount Everest.

10. Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known.
Spots on leaves and fruits, stunted growth, and damaged flowers or roots

11. Tobacco Mosaic Virus

12. I. Background Virus Discovery: Tobacco mosaic disease (1930’s)
Stunts growth produces the speckled coloration

13. I. Background
Viral Evolution proposal: Fragments of cellular nucleic acid
Reproduce within host cells only (non-living)
Obligate intracellular parasites
Host range

14. II. Structure of viruses

15. II. Structure of Viruses
1. Nucleic acid
Enclosed in a protein coat
Genomes may be
ds/ss DNA
ds/ss RNA

16. II. Structure of Viruses
2. Capsids
Protein
Various shapes & structures
Capsids are produced by host
18  250 mm
70–90 nm (diameter)
20 nm
50 nm
(a) Tobacco mosaic virus
(b) Adenoviruses
RNA
DNA
Capsomere
Glycoprotein
Capsomere of capsid

17. Viral structure

18. II. Structure of Viruses
3. Envelopes
Found in many animal viruses
Glycoprotein and lipids
Derived by host
“Spikes” fuse with membrane or receptor mediated entry
80–200 nm (diameter)
50 nm
(c) Influenza viruses
RNA
Glycoprotein
Membranous envelope
Capsid

19. Receptor mediated model

20. Fusion Model

21. Basic Infection: Just Genome and Capsid

22. Bacteria and Eukaryotic Models
III. Life Cycles
Bacteria and Eukaryotic Models

23. III. Life Cycles:
1. Bacteriophages complete two reproductive mechanisms:
lytic cycle
lysogenic cycle
Figure 18.4d
80  225 nm
50 nm
(d) Bacteriophage T4
DNA
Head
Tail fiber
Tail sheath

24. III. Life Cycles A. Lytic Cycle:
Digests the host’s cell wall, releasing the progeny viruses
Kills host
Virulent phage

25. Phage assembly 1 2 5 4 3 Head Tails Tail fibers
Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 1
Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. 2
Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 5
Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. 4
Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. 3

27. Life Cycles B. Lysogenic cycle
Incorporate viral DNA into bacteria genome (propahge)
Phage genome is replicated (for free!) w/o destroying the host
Temperate phages capable of using both cycles

28. The lytic and lysogenic cycles of phage , a temperate phage
Many cell divisions produce a large population of bacteria infected with the prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into the bacterial chromosome, becoming a prophage.
New phage DNA and proteins are synthesized and assembled into phages.
Occasionally, a prophage exits the bacterial chromosome,
initiating a lytic cycle.
Certain factors
determine whether
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
The cell lyses, releasing phages.
Lytic cycle
is induced
Lysogenic cycle
is entered or
Prophage
Bacterial
chromosome
Phage
DNA
Figure 18.7

30. III: Life Cycles 2. RNA viruses typically infect animals
Retroviruses (HIV), use reverse transcriptase to make cDNA
Integrated into genome, provirus

32. The reproductive cycle of HIV, a retrovirus
Figure 18.10
mRNA
RNA genome for the next viral generation
Viral RNA
RNA-DNA hybrid
DNA
Chromosomal DNA
NUCLEUS
Provirus
HOST CELL
Reverse transcriptase
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIV
Membrane of white blood cell
The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the viral proteins and RNA. 1
Reverse transcriptase
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA. 2
catalyzes the synthesis of a second DNA strand
complementary to the first. 3
The double-stranded DNA is incorporated
as a provirus into the cell’s DNA. 4
Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. 5
The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 6
Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane. 7
Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules. 8
New viruses bud
off from the host cell. 9

33. III: Life Cycles Life after infection Viruses may damage or kill cells
Tissue damage
Toxins that lead to disease symptoms may be produced
Asymptomatic

34. Sources http://www.aapsj.org/
Campbell Reece Mitchell. Biology, Prentice Hall 1999, 2001
Sherris. Medical Microbiology: An introduction to infectious disease, Appleton and Lange,1990

35.
Good links