Chapter 6: Viruses and Prions

Virus

submicroscopic (20-400 nm), acellular, nonliving obligate intracellular pathogen

Virion

a single complete, infectious viral particle

Capsid

protein shell made of capsomeres

Helical capsid

hollow tube shape

Icosahedral capsid

3D, 20 sided polygon

Complex capsid

polyhedral head plus sheath/tail structures e

Envelope

lipid coat stolen from the host membrane during budding

Naked virus

a virus that lacks an envelope; exits cells via lysis

Spiked

glycoprotein extensions used for host attachment

What forms can a viral genome take?

DNA or RNA (never both); single or double stranded; linear, circular, or segmented

Why are mutation rates high in RNA viruses?

RNA polymerase lack proofreading

Antigenic drift

minor point mutations in spikes over time

Antigenic shift

major genetic reassortment from coinfection with two different strains

Which is the minor change and which is the major change, drift or shift?

drift = minor

shift = major

Host range

the species a virus can infect

Tropism

the specific cells or tissue a virus targets, determined by surface factor binding

What is the highest rank in viral taxonomy?

phylum; viruses have no domain or kingdom

What are the endings for viral order, family, and genus?

order: - virales

family: -viridae

genus: virus

What are the steps of bacteriophage replication, in order?

attachment

penetration

replication

assembly

release

What happens during phage attachment?

the phage binds to the bacterial cell

What happens during phage penetration?

the phage injects its genetic material into the cell

What happens during phage replication?

the phage commandeers host cell factors to transcribe and translate viral genes

What happens during phage assembly?

the genome is packed into the capsid and phage structures are assembled

How are new phages released?

the bacterial cell lyses

What the steps of animal virus replication?

attachment

penetration

uncoating

replication

assembly

release

How does an animal virus attach?

spiked or capsid bind host membrane receptors

How do enveloped vs. naked animal viruses enter cells?

enveloped: fusion or endocytosis

naked” endocytosis

Uncoating

enzymes break down the capsid to release the genome

How do enveloped vs. naked animal viruses exit cells?

enveloped: budding

naked: cell lysis

Which replication step is unique to animal viruses and not in the phage list?

uncoating

Acute infection

rapid virion production, cleared by host immunity

Chronic persistent infection

continuous, slow release of virions over time

Latent persistent infeciton

intermittent flare-ups separated by long dormant periods

Oncogenic viruses

viruses that cause ~10-15% of human cancers by triggering uncontrolled cell division

What cancers does HPV cause?

cervical and oropharyngeal cancers

What cancers fo EBV and HHV-8 cause?

lymphomas and kaposi sarcoma

How do HBV and HCV cause cancer?

liver cancer via chronic inflammation

Plaque assay

Phages grown on bacterial lawns produce clear spots (plaques), measured in PFUs

What doe PFU stand for?

plaque-forming units

What is required to culture animal viruses?

tissue culture, live animals, or embryonated eggs

Agglutination

antibodies linked to latex beads clump when bound to a viral target

ELISA

uses enzyme-linked antibodies to produce color changes that indicate antigen/antibody presence

Molecular tests

PCR and probes that detect viral DNA/RNA directly

Which antiviral target entry/uncoating?

docosanol (cold sores) and amantadine (influenza)

How do nucleoside analogs work?

they mimic normal bases to halt nucleic acid chain elongation (they target replication)

Interferon

host signaling proteins that prompt uninfected cells to set up antiviral defenses

How do Tamiflu and Relenza work?

they block neuraminidase spikes to stop flu budding

Prion

an infectious misfolded protein containing no DNA or RNA

How do prions cause disease?

they force normal brain proteins to misfold, causing transmissible spongiform encephalopathies

Do prions replicate like viruses?

no; they have no genoe, and instead spread by converting normal proteins

Which diseases share a prion-like protein misfolding mechanism?

alzheimer’s, parkinson’s, and ALS