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Chapter 13 – Characterizing and Classifying Viruses, Viroids, and Prions
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 Viruses have single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), or singlestranded RNA (ssRNA)
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 Viruses infect a particular host with precision by viral proteins to complementary proteins
on the host cell
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Some viruses are generalists, which is they infect many cells in many hosts
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Compare and contrast viruses of fungi, plants, animals, and bacteria
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 Viruses of plants are introduced via abrasions of the cell or by plant parasites (like
nematodes and aphids)
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 Capsids are protein coats composed of capsomeres
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 The envelope is composed of a phospholipid bilayer and proteins
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 An envelope’s proteins plays a role only in recognition of host cells
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List three characteristics by which viruses are classified Viral
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); presence of an envelope; shape and
size
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 Example used is a T4 bacteriophage infecting an E
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 Attachment
o Attachment occurs via the tail fibers (spider-legs) and the complementary receptor
proteins on E
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 Entry
o T4 releases lysozyme and weakens the peptidoglycan of the cell wall
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 Synthesis
o Viral enzymes degrade E
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Viral DNA and E
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 Assembly
o Capsomeres, tails, etc
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Afterwards, enzymes pump viral genome (sometimes leftover E
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Release
o All the while, lysozyme has been hard at work on the cell wall and newly assemble
virions burst out
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Compare and contrast the lysogenic replication cycle of bacteriophages with the lytic cycle
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 The host DNA is not destroyed
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So, all daughter
cells are infected
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 After the cell is fully grown, during Induction, the inactive viral DNA is cut out by recombination
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 Attachment
o Animal viruses lack both tails and tail fibers
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 Entry {3 Methods}
o Direct Penetration involves a capsid sinking into the membrane, and slipping its genome
in hole
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o Endocytosis involves the entire enveloped or naked virus being engulfed by the cell
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RNA viruses are replicated in the cytoplasm
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o Naked animal viruses may be released in one of two ways: exocytosis or causing the cell
to lyse
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Compare the replication and synthesis of DNA viruses and RNA viruses
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 Cells do not use ssDNA, so cell enzymes need to make a complementary DNA strand for the
virus
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There are four types of RNA:
o Each type below basically creates a template for further replication of starting material
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RNA polymerase transcribes a negative
strand
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Retroviruses (a kind of +ssRNA) uses a DNA intermediary transcribed with
reverse transcriptase
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o dsRNA can split into +ssRNA and –ssRNA
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Budding: As viruses extrude through the cell’s membrane, each virus acquires a part of the
membrane that covers it like a blanket
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Lysis: Disruption of the cell membrane causes the cell’s death and leakage of its contents,
including the viruses
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Compare and contrast latency in animal viruses with phage lysogeny
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AKA, proviruses
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 However, when a provirus is incorporated, it is permanent
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 Media consisting of mature organisms (bacteria, plants, or animals)
o After incubation, lysed bacteria are shown by plaques, clear zones in the lawn of
bacteria
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 Cell tissue culture could consist of diploids, animals, plants, human, or cancer cells
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Explain the use of a plaque assay in estimating virus numbers
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 Use of lab animals are difficult, expensive, and ethically questionable
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Define and describe prions, including their replication process
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 All mammals make a membrane protein called PrP
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o Just like Trump’s bad influence on a crowd of idiots to misbehave
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Compare and contrast prions and viruses
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 Only viruses contain nucleic acids, a cytoplasmic membrane, and show responsiveness
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o Bovine spongiform encephalopathies (mad cow disease)
o Scrapie; Chronic wasting disease; and Variant Creutzfeldt-Jakob disease



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