chapter 19 viruses and simple infectious agents. 1) 1892, use porcelain filter to filterize tobacco...
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Chapter 19
VIRUSES AND SIMPLE INFECTIOUS AGENTS
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1) 1892, use porcelain filter to filterize tobacco leaves extratfilterable agent
2) 1898, Martinus Beijerinck called it virus, which means “poison” in Latin.
3) In 1939, electron microscope to capture the first images of tobacco mosaic virus.
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1) Plaques
2) Gene therapy
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Tiny, infectious agents that: have a nucleic acid core (RNA or
DNA) have a protein coat (capsid)
composed of capsomers may have an envelope, which is
derived from the host cell membrane
HIV (enveloped virus)
Adenovirus (naked virus)
Viral structure
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various shape…
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Basic structural unit is the virion (a single viral particle).
Viruses are NOT cells.[no nucleus, organelles, or cytoplasm]
Viruses are NOT considered to be living.[do not metabolize, respond or reproduce on their own]
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Viruses must infect cells (prokaryote or eukaryote) to replicate.
Step 1: Binding to host cellVirus attaches to specific receptors on cell surface.
Viral Infection & Replication
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Step 2: Entry into host cell
Bacteriophages inject nucleic acid.
Animal viruses enter by receptor-mediated endocytosis & then uncoat.
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Plant viruses enter through mechanical damage (insect bite, mechanical damage by farm equipment, wind, or nibbling animals).
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plasmodesma
cell membrane
cell wall
Within a plant, virions easily spread from cell to cell through the plasmodesma
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Step 3: Replication
BindingViral entry
Virus directs host cell to produce viral nucleic acids & proteins
Progeny virions are assembled & released as host cell ruptures
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Viruses follow two major strategies to replicate
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Lytic lifestyle (most viruses)Virus invades host cell, replicates, and
progeny virions are released.
Lysogenic lifestyle (some bacteriophages)Virus invades host cell and integrates it’s
DNA into host DNA.Virus “hides” in host cell for indefinite
period of time.Virus reverts to lytic lifestyle.
Viral Lifestyles
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Replication of HIV
(human immunodeficiency virus)
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- viruses that enter a period of dormancy after invading a host cell.
Ex. Herpesvirus that causes cold sores Epstein-Barr virus is so efficient at
remaining undetected that more than 80% of the human population carries it. Because latent viruses persist by signaling their host cells to divide continuously, they may cause cancer.
Latent Viruses
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Viral Pathogenicity
Pathogenicity depends upon the ability of the virus to infect its host, and the condition of host defenses.
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Epidemiology
Host ranges
Ex. rabies virus infects many mammals; humans, skunks (臭鼬 ), and bats
Ex. small pox virus infects humans only: can be controlled by vaccination
Ex. Ebola virus causes hemorrhagic fever.
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Certain hosts develop illness.
Certain hosts show no symptoms - function as reservoirsEx. pigs & ducks serve as reservoirs for influenza (flu) viruses.Influenza can jump from either animal to humans (zoonosis).
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Animal cells employ an immune system: Antibodies coat viral particles so they can’t
adhere to target cells. Certain cells rupture virus-infected cells before
progeny viruses can be released. Virus-infected cells release chemicals that
protect surrounding uninfected cells.
Antiviral drugs like AZT & ddC have been developed to slow viral replication.
Defense against Viral Infection
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Other Infectious Agents
. Viroids (naked RNA) - infectious RNAs that affect plants.
Ex. Avocado sunblotch, Tomato bunchy top, Chrysanthemum stunt disease
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prion protein (PrP)
Prions (proteinaceous infectious particle) - infectious proteins associated with slow-virus spongiform encephalopathy.
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A prion disease: bovine spongiform encephalopathy (BSE)= mad cow disease