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Viruses
Chapter 19
Bacteria vs. Viruses
Bacteria
Prokaryotic cell
 Most are free-living
(some parasitic)
 Relatively large size
 Antibiotics used to kill bacteria

Virus

Not a living cell
(genes packaged in protein shell)




Intracellular parasite
1/1000 size of bacteria
Vaccines used to prevent
viral infection
Antiviral treatment
General Characteristics of Viruses
Very small (<ribosomes)
 Components = nucleic acid + capsid
◦ Nucleic acid: DNA or RNA
(double or single-stranded)
◦ Capsid: protein shell
◦ Some viruses also have
viral envelopes that surround capsid
 Limited host range
 Reproduce within host cells

Viruses

Limited host range
◦ Entry = attach to host cell membrane receptors
through capsid proteins or glycoproteins
on viral envelope (animal)
◦ Ex: human cold virus (rhinovirus)  upper respiratory
tract (mouth & nose)
Reproduce quickly within host cells
 Can mutate easily

◦ RNA viruses: no error-checking
mechanisms
Simplified viral replicative cycle
Bacteriophage

Virus that infects bacterial cells
Video
Lytic Cycle of T4 Phage
Bacteriophage Reproduction
Lytic Cycle:
◦ Use host machinery to make copies of virus
◦ Death of host cell by rupturing it (lysis)
◦ Virulent phages replicate by this method
 Lysogenic Cycle:
◦ Phage DNA incorporated into host DNA
and replicated along with it
◦ Phage DNA = prophage
 Temperate Phage: uses both methods of replication

Lytic Cycle vs. Lysogenic Cycle
Animal viruses have a membranous envelope

Host membrane
forms around
exiting virus

Difficult for host
immune system
to detect virus
HIV =
Retrovirus
HIV
◦ Infects white blood cells
◦ HIV+: provirus
(DNA inserted)
◦ AIDS: active viral
reproduction
Other Human Viruses

Herpes virus

Smallpox
Herpes Simplex Virus 1 (HSV-1)
Herpes Simplex Virus 2 (HSV-2)
Eradicated in 1979 due to
worldwide vaccination campaigns
Ebola
Transmission: contact with contaminated blood
or bodily fluids
 Symptoms: fatigue, fever, severe headache, vomiting,
diarrhea, rash, bleeding

◦ May appear 2-21 days after exposure
Ebola

Treatment:
◦ Intravenous fluids, balance electrolytes
◦ Experimental: antiviral drugs, plasma transfusions
from survivors, antibodies (Zmapp)
◦ No vaccine (yet)

Statistics:
◦ 2014 Ebola Outbreak (worldwide): 21,382 cases,
8474 deaths (*as of 1/19/15)
◦ Seasonal Influenza: estimated 36,000 deaths in U.S.
each year (2015: mutated H3N2 strain)
2014 Ebola Outbreak
Cumulative Ebola Cases
Vaccines

Weakened virus or part of pathogen that
triggers immune system response
Emerging viruses = mutation of existing
viruses
Current Outbreaks

Zika Virus
◦ Spread by Aedes mosquitoes (Aedes aegypti)
◦ Major outbreak in Brazil and Latin America, now in US
◦ Linked to birth defects (microcephaly)

Dengue Fever
VIDEO
Zika Virus (as of Feb. 2016)
Drugs for Prevention/Treatment

Vaccine: weakened virus or part of pathogen that
triggers immune system response to
prevent infection
◦ Ex. HPV, MMR, HepA, Flu shot

Antiviral Drugs: block viral replication
after infection
◦ Ex. Tamiflu (influenza), AZT (HIV)
Viroids
Small, circular RNA
molecules that infect plants
 Cause errors in regulatory
systems that control plant
growth
 Eg. coconut palms in
Philippines

Prions

Misfolded, infectious proteins that cause
misfolding of normal proteins

Ex: mad cow disease (BSE), scrapie (sheep)
Diseases caused by prions
Prions act slowly – incubation period of at least
10 years before symptoms develop
 Prions are virtually indestructible (cannot be
denatured by heating)
 No known cure for prion diseases

Prion Neurodegenerative Diseases
Alzheimer’s Disease
 Parkinson’s Disease
