Viruses
Ebola
Influenza
West Nile Virus
Rabies
HIV
General Characteristics
1.Very small
- 10 -1000 nanometers
- 10 – 1000 x 10 -9 m
- Cannot be seen without an
electron microscope
General Characteristics
2. Various
Shapes
General Characteristics
Protein
Coat
3. Structure =
Two Parts:
Nucleic acid
+
Protein Coat
(Capsid)
Nucleic
Acid
General Characteristics
4. Acellular
- no cell
- no organelles
- no metabolism
- cannot be killed with antibiotics
General Characteristics
5. Nucleic acid
- Has DNA
- or RNA (retro viruses)
DNA
- (never both)
?s 1-6
RNA
General Characteristics
 Nucleic Acids
 1. DNA = deoxyribonucleic acid
- Double strand
- Contains genes for making viral
proteins (requires 2 steps)
General Characteristics
 Nucleic Acids
2. RNA – ribonucleic acid
- Single strand
- Retro virus
- contains code for directly
producing viral proteins
General Characteristics
6. Mutates very easily
- no cell
- no protection from mutagens
E.g. chemicals, radiation,
uv light exposure
General Characteristics
7. Virus = Latin for poison
General Characteristics
8. Obligate intercellular Parasite
- Cannot reproduce EXCEPT inside
a host cell
- No metabolism
outside host cell
- Inside host,
uses cell’s reproduction
& metabolic mechanisms
General Characteristics
9. Some have an envelope
Membrane surrounding protein
coat
Made of glycoproteins
Examples of Viruses






Common Cold
Polio
Rabies
HIV
Hepatitis A,B*,& C
Herpes ( E.g.
Chicken
pox)
 Measles
 Smallpox
?s 8-13
 Ebola
 Hanta
 HPV (Human
papilloma virus)*
 Mumps
 Rubella
 Epstein-Barr*
 SARS
* can cause cancer
Viral Cycles
Two Types of Viral Cycles
1. Lytic
2. Lysogenic
Lytic Cycle

1.
2.
3.
4.
5.
Immediate viral replication
Viral DNA injected into host cell
Viral DNA put into host genome
Synthesis & Assembly of virus
Cell lysis (rupture) = tissue damage
New viruses emerge to infect new
cells
Lysogenic Cycle
 Viral replication is delayed
1. Viral DNA put into host genome
2. Host cells divide, including viral genes
3. When organism is stressed →
→ lytic cycle begins
→ viral replication (cell lysis) etc.
?s 7-12
Lytic vs Lysogenic Cycle
New viruses
emerge
Injection
Viral
DNA
Bacterial
genome
Lysis
Assembly
Many divisions
Synthesis
How do Viruses do Harm?
Possible effects:
1. Lyse cells (Tissue damage)
2. Release viral toxins
3. Prevent host’s cell division
4. Disrupt formation of host
chromosomes
How do Viruses do Harm?
5. Disrupt lysosomal function
6. Can cause uncontrolled cell division
(cancer)
7. Can cause changes in cell
membrane
(immune system destroys host
cells)
Duration of Viral Infections
 Acute – rapid onset, death or recovery
(most viruses)
 Chronic – recurs again & again
- latent period = virus dormant
between flare-ups
E.g. Herpes simplex
(cold sores)
Prevention
Vaccines
 Live or killed virus injected
 Host immune system makes
antibodies
 When infected, host immune
system ready to kill virus
Treatment
1. Antiviral drugs
 Interfere with viral replication
 E.g. AZT ( for AIDS)
 E.g. Interferon (for Hepatitis C)
2. Supportive Therapy
 Reduce fever (E.g. aspirin, Tamiflu)
 Make host comfortable
 Wait until virus runs its course ?s 14-19
Viroids
1. Nucleic acid (RNA) only
2. No protein coat
3. Common in plants
Prions
1. Protein only
2. No nucleic acid
3. Replicate by bumping proteins,
causing refolding in shape of
prion
4. Cause diseases of nervous
system
Prions
E.g.
 Kreutzfeld-Jakob’s disease
Bovine Spongiform Encephalopathy
(BSE) aka Mad cow disease)
Prions
2. Chronic wasting disease
Deer and elk
Prions
3. Scrapie – usually fatal sheep disease
Prions
4. Kuru
Papua New Guinea
Headhunters (cannibalistic tribes)
Word to your
mama!
Viruses
 Are they living or nonliving ?
 Are they smart nucleic acids?
 Why can’t we kill them?
?s 20-25