Ch 13

Characteristics of viruses

• Obligate intracellular parasites

  • require living host cells to multiply

• Acellular

• DNA or RNA (never both)

• Protein coat

• no ribosomes

• no ATP generating mechanism

Viral morphology

• Helical (hollow, cylindrical capsid)

• Polyhedral (many-sided/icosahedral)

• Enveloped virus

• Complex viruses. (bacteriophage, complex structure)

host range vs tissue tropism

• host range: host cells a virus can infect

  • determined by host attachment sites

• tissue tropism: specific cell types virus can infect

Viral particle components

(Viron)

• Nucleic acid (DNA or RNA)

• Capsid (protein coat)

• Envelope (lipid, protein, carbohydrate coating on some)

• Spikes (projections from outer surface)

bacteriophages

• virus that infect bacteria

• grown in living cells (bacteria)

• form plagues - circular clearings on a lawn of bacteria on agar surface

• 1 plaque = 1 virus: plaque-forming units (PFU)

Growing animal viruses in laboratory

• in cell cultures

• Tissue treated with an enzyme to separate cells

• cytopathic effect (CPE) - detection of infected cells using deterioration

• continuos cell lines are used

viral multiplication

• must invade a host cell

• must take over host’s metabolic machinery

• one step growth curve

one step growth curve

1. eclipse period: no viral particles, hiding in host cells

2. Virions released from host cell; dectable

3. acute infections: person experience viral infection

4. virus slowly dies out

Lytic vs lysogenic (mutiplication of bacterial)

• Lytic = immediate replication → cell dies

• Lysogenic = dormant integration → later becomes lytic

Lytic cycle

1. Attatchment: Phage attaches to host cell.

2. Penetration: Phage lysozyme opens cell wall; injects its DNA

3. Biosynthesis: production of phage DNA and proteins

4. Maturation: assemble phage particles

5. Release: phage lysozyme breaks cell wall, cells lyse and release

Lysogenic cycle

• Integrates viral DNA into host chromosome as a latent prophage (Lysogeny)

• Replicates chromosome, replicates DNA prophage = results in phage conversion: host cell exhibts new properties

• can later enter lytic cylce

• specialized transduction (change properties)

multiplication of animal virus

1. attachment: virus attach to cell membrane

2. Entry: by receptor-mediated endocytosis or fusion

3. uncoating: by viral or host enzymes

4. Biosynthesis: production of nucleic acid and proteins

5. Maturation: nucleic acid and capsid proteins assemble

6. release by budding (enveloped virus) or rupture

adenoviridae

dsDNA, nonenveloped; causes respiratory infections in humans

Poxviridae

• double stranded DNA, enveloped

• causes skin lesions; smallpox virus

Herpesviridae

• double stranded DNA, enveloped

  • HHV1-2 Simplexvirus; cold sores

  • HVV-3 Varicellovirus; chickenpox

  • HHV-4 Lympcryptovirus; mononucleosis

  • HH-5 Cytomegalovirus

  • HHV-8 Rhadinovirus; Kaposi’s sarcoma

Papovaviridae

• Double stranded DNA, noneveloped

  • Papillomavirus

    • causes warts & cancer

RNA-dependent RNA polymerase vs. Reverse transcriptase

• RNA-dep: Copies viral RNA to RNA.

• Reverse transcriptase: Copies viral RNA into DNA (used by retroviruses like HIV).

+ ssRNA vs. − ssRNA

+ (sense) strand: serves as mRNA for protein synthesis; direct immediate translation

- (antisense) strand: transcribed to a + strand first

dsRNA

double-stranded RNA

Rhinovirus

+ ssRNA, nonenveloped, causes common cold

rhabdoviridae

• − ssRNA

• Lyssavirus: causes rabies and animal disease

Retroviridae; Lentivirus

HIV

Prions

• infectious proteins without nucleic acids

• cause spongiform encephalopathies

  • Mad cow disease

  • creutzfeldt-Jakob disease (CJD)