Chapter 13: Viruses, Viroids, and Prions

Describe Viruses

• Obligatory intracellular parasites

  • require hosts to multiply

• Contain DNA or RNA

• Protein coat

• NO ribosomes

• NO ATP-generating mechanism

Compare Viruses and Bacteria

What is the host range?

The spectrum of host cells a virus can infect

Describe viruses’ host range.

Most viruses infect only specific types of cells in one host

• determined by specific host attachment sites and cellular factors

What are bacteriophages?

viruses that infect bacteria

What is the length of host ranges?

from 20nm to 1000nm in length

What are virons?

complete, fully developed viral particle

Describe virons.

• Nucleic acid- DNA or RNA can be single- or double- stranded; linear or circular

• Capsid-protein coat made of capsomeres (subunits)

• Envelope-lipid, protein, and carbohydrate coating on some viruses

  • Spikes-projections from outer surface

What is the general morphology of viruses?

• Helical viruses - hollow, cylindrical

• Polyhedral viruses - many-sided

• Enveloped or non-enveloped

• Complex viruses - complicated structures

What is the morphology of complex viruses

There is a capsid head and DNA inside the head, then lower is a sheath, and at the bottom the tail fiber, pin, and base plate

What is the taxonomy of viruses?

• genus name end in -virus

• family names end in -viridae

  • order names end in -ales

What are viral species?

A group of viruses sharing the game genetic information and ecological niche (host)

What are subspecies designated by?

numbers

Where do viruses grow?

in living cells

Where are bacreriophages grown?

• in bacteria

• bacteriophages form plaques, which are clearings on a lawn on bacteria on the surface of agar

  • each plaque corresponds to a single virus; can be expressed as plaque-forming units (PFU)

How do you grow animal viruses?

• in living animals

• in embryonated eggs

  • virus injected into the egg

  • viral growth is signaled by changes or death of the embryo

Describe growing animals viruses in the lab.

• In cell cultures

  • tissues are treated with enzymes to separate cells

  • virally infected cells are detected via their deterioration, known as cytopathic effect (CPE)

  • continuous cell lines are used

How do you identify viruses?

• Viruses can’t be seen without the use of an electron microscope

• Cytopathic effects

• Serological tests - most common

  • Western blotting-reaction of the virus with antibodies from host

• Nucleic acids

  • RFLPs: Restrictive Fragment Length Polymorphism

  • PCR: Polymerase Chain Reaction

How do viruses reproduce?

• it must invade a host cell

• it must take over the host’s metabolic machinery

  • virus contains limited info: genes for virion’s structural components & genes for enzymes used in viral life cycle

• One-step growth curve

What is the Lytic Cycle?

• Virulent phage

• causes lysis and death of the host cell

Attachment → Penetration → Biosynthesis → Maturation → Release (lysis)

What is the Lysogenic Cycle?

• host cell survive - temperate phage

• Phage DNA integrates into host chromosomes= prophage

• Replicates with host; repressors prevent virion production

• Phage conversion; prophage DNA can encode new toxins

What is transduction?

bacterial DNA can be packaged in the capsid

What is generalized transduction?

random bacterial DNA packaged during the lytic cycle

What is specialized transduction?

Specific genes transferred to another bacterium via a phage. Genes adjacent to where the prophage is located

How do animal viruses reproduce?

• viruses attach to the cell membrane

• entry by receptor-mediated endocytosis or fusion

• uncoating by viral or host enzymes

• production of nucleic acid and proteins

• nucleic acid and capsid proteins assemble

• release by budding (enveloped viruses) or rupture

What is biosynthesis in animal viruses?

production of nucleic acid and proteins

What is maturation in animal viruses?

nucleic acid and capsid proteins assemble

What is the biosynthesis of DNA viruses?

• DNA viruses replicate their DNA in the nucleus of the host using viral enzymes (DNA Polymerase)

• Synthesize capsid in the cytoplasm using host cell enzymes

Describe Adenoviridae.

• Double stranded DNA, non-enveloped

  • respiratory infections in humans

  • tumors in animals

• Cellular enzymes transcribe viral DNA in nucleus

Describe Poxviridae.

• Double-stranded DNA, enveloped

• Cause skin lesions

  • vaccinia and smallpox viruses (Orthopoxvirus)

• Viral enzymes transcribe viral DNA in nucleus

Describe Herpesviridae

• Double-stranded DNA enveloped

  • HHV-1 and HHV-2 Simplexivurs; cause cold sores

  • HHV-3-Varicellovirus; causes chickenpox

  • HHV-4-Lymphocrypotovirus;causes mononucleosis

  • HHV-5-Cytomegalovirus

  • HHV-6 and HHV-7-Roseolovirus

  • HHV-8-Rhadinovirus; causes Kaposi’s sarcoma

• Cellular enzymes transcribe viral DNA in nucleus

Describe Papovarviridae

• Double-stranded DNA, non-enveloped

  • Papillomarvirus

    • causes warts

    • can transform cells and cause cancer

Describe Hepadnaviridae

• Partially double-stranded DNA, enveloped

  • Hepititus B virus

  • Use reverse transcriptase to make viral DNA from mRNA

How do viruses multiply in a host’s cytoplasm?

using RNA-dependant RNA polymerase

Describe +ssRNA

• + (sense) strand

• viral RNA serves as mRNA for protein synthesis

Describe -ssRNA

• -(antisense) strand

• Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis

What is dsRNA?

Double-stranded RNA

Describe Picomaviridae

• Single-stranded RNA, + strand, non-enveloped

  • Enterovirus

    • poliovirus and coxsackievirus

  • Rhinovirus

    • common cold

  • Hepatitis A virus

Describe Togaviridae

• Single-stranded RNA, + strand, enveloped

  • Alphavirus

    • transmitted by arthropods; includes chikungunya

  • Rubivirus

    • Rubella

Describe Rhabdoviridae

• Single-stranded RNA, - strand, one RNA strand

  • Lyssavirus

    • rabies

  • Numerous animal diseases

Describe Reoviridae

• Double-stranded RNA, non-enveloped

  • Reovirus (respiratory enteric orphan)

  • Rotavirus (milk respiratory infections and gastroenteritis)

How do RNA produce DNA

• Single-stranded RNA produce DNA

• Use reverse transcriptase to produce DNA from the viral genome

  • viral DNA integrates into the host chromosome as a provirus

Describe Retroviridae

• Dentivirus (HIV)

• Oncovirus

  • Viral RNA is transcribed to DNA (using reverse transcriptase), which can integrate into host DNA

  • HTLV-1 and HTLV-2 cause adult T cell leukemia and lymphoma

How do viruses connect to cancer?

• Several types of cancer are caused by viruses

  • may develop long after a viral infection

  • cancers caused by viruses are not contagious

What is sarcoma?

cancer of connective tissue

What is Adenocarcinomas?

cancers of glandular epithelial tissue

What are Oncogenes?

transform normal cells into cancerous cells

What are oncogenic viruses?

viruses that are integrated into the host cell’s DNA and induce tumors - retrovirus

• A transformed cell harbors a tumor-specific transplantation antigen (TSTA) on the surface and a T antigen in the nucleus

What are Oncolytic viruses?

infect tumor cells and lyse (kill) them

What are latent viruses?

remain in asymptomatic host cell for long periods

• may reactivate due to changes in immunity

  • cold sores, shingles

• a persistent viral infection occurs gradually over a long period; generally fatal

  • subacute sclerosing panencephalitis (measles virus)

What is Varicellovirus?

Shingles

What is Lentivirus?

HIV/AIDS

Describe plant viruses.

• mostly +ssRNA viruses

• enter through wounds or via insects

• plants are generally protected from disease by an impermeable cell wall

What are viroids?

short pieces of cirular naked RNA self-replicating

• cause potato spindle tuber disease

What are Virusoids?

viroids enclosed in a protein coat

• unable to complete self-replication

• only cause disease when plant cell is coinfected with a helper-virus

What are prions?

• Proteinaceous infectious particles

How are prions inherited and transmissable?

• by ingestion, transplant, and surgical instruments

Describe Spongiform encephalopathies

• “mad cow disease”

• Creutzfeldt-Jakob disease (CJD)

• Gerstmann-Sträussler-Scherinker Syndrome

• Fatal familial insomnia

• Sheep scrapie

What are PrPC?

normal cellular prion protein, on the cell surface

What are PrPSc?

Scrapie protein; accumulates in brain cells, forming plaques