Viruses, Viroids, and Prions

General Characteristics of Viruses

Obligatory intracellular parasites

Contain DNA or RNA

Contain a protein coat

Some are enclosed by an envelope

Some viruses have spikes

Most viruses infect only specific types of cells
in one host

Host range is determined by specific host attachment sites and cellular factors

Virus Sizes

Virion Structure

Nucleic acid

DNA or RNA

Capsid

Capsomeres

Envelope

Spikes

Morphology of a Polyhedral Virus

Polyhedral Viruses

Morphology of an Enveloped Virus

Enveloped Viruses

Morphology of a Helical Virus

Morphology of a Complex Virus

Taxonomy of Viruses

Family names end in -viridae.

Genus names end in -virus.

Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species.

Subspecies are designated by a number.

Taxonomy of Viruses

Herpesviridae

Herpesvirus

Human herpes virus HHV-1, HHV-2, HHV-3

Retroviridae

Lentivirus

Human immunodeficiency virus HIV-1, HIV-2

Growing Viruses

Viruses must be grown in living cells

Bacteriophages form plaques on a lawn of bacteria

Growing Viruses

Animal viruses may be grown in living animals or in embryonated eggs

Growing Viruses

Animal and plant viruses may be grown in cell culture

Continuous cell lines may be maintained indefinitely

Virus Identification

Cytopathic effects

Serological tests

Detect antibodies against viruses in a patient

Use antibodies to identify viruses in neutralization tests, viral hemagglutination, and Western blot

Nucleic acids

RFLPs

PCR

Virus Identification

The Lytic Cycle

Attachment: Phage attaches by tail fibers to host cell

Penetration: Phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell

Biosynthesis: Production of phage DNA and proteins

Maturation: Assembly of phage particles

Release: Phage lysozyme breaks cell wall

A Viral One-Step Growth Curve

Lytic Cycle of a T-Even Bacteriophage

Lytic Cycle of a T-Even Bacteriophage

Results of Multiplication of Bacteriophages

Lytic cycle

Phage causes lysis and death of host cell

Lysogenic cycle

Prophage DNA incorporated in host DNA

Phage conversion

Specialized transduction

The Lysogenic Cycle

Generalized Transduction

Specialized Transduction

Specialized Transduction

Multiplication of Animal Viruses

Attachment: Viruses attach to cell membrane

Penetration by endocytosis or fusion

Uncoating by viral or host enzymes

Biosynthesis: Production of nucleic acid and proteins

Maturation: Nucleic acid and capsid proteins assemble

Release by budding (enveloped viruses) or rupture

Attachment, Penetration, Uncoating

By pinocytosis

Attachment, Penetration, Uncoating

By fusion

Budding of an Enveloped Virus

Budding of an Enveloped Virus

Multiplication of DNA Virus

Sense Strand (+ Strand) RNA Virus

Antisense Strand (– Strand) RNA Virus

Double-Stranded RNA Virus

Multiplication of RNA-Containing Viruses

Multiplication of a Retrovirus

Cancer

Activated oncogenes transform normal cells into cancerous cells

Transformed cells have increased growth, loss of contact inhibition, tumor-specific transplant antigens, and T antigens

The genetic material of oncogenic viruses becomes integrated into the host cell's DNA

Oncogenic Viruses

Oncogenic DNA viruses

Adenoviridae

Herpesviridae

Poxviridae

Papovaviridae

Hepadnaviridae

Oncogenic RNA viruses

Retroviridae

Viral RNA is transcribed to DNA, which can integrate into host DNA

HTLV-1

HTLV-2

Latent and Persistent Viral Infections

Latent Viral Infections

Virus remains in asymptomatic host cell for long periods

Cold sores, shingles

Persistent Viral Infections

Disease processes occurs over a long period; generally is fatal

Subacute sclerosing panencephalitis (measles virus)

Prions

Proteinaceous Infectious particle

Inherited and transmissible by ingestion, transplant, and surgical instruments

Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease

Prions

PrP^C: Normal cellular prion protein, on cell surface

PrP^Sc: Scrapie protein; accumulates in brain cells, forming plaques

How a Protein Can Be Infectious

Plant Viruses and Viroids

Plant viruses: Enter through wounds or via insects

Viroids: Infectious RNA; e.g., potato spindle tuber disease

Parvoviridae

Single-stranded DNA, nonenveloped viruses

Fifth disease

Anemia in immunocompromised patients

Adenoviridae

Double-stranded DNA, nonenveloped viruses

Respiratory infections in humans

Tumors in animals

Papovaviridae

Double-stranded DNA, nonenveloped viruses

Papillomavirus

Human wart virus

Polyomavirus

Cause tumors; some  cause cancer

Poxviridae

Double-stranded DNA, enveloped viruses

Orthopoxvirus (vaccinia                                  and smallpox viruses)

Molluscipoxvirus

Smallpox

Molluscum                                         contagiosum

Cowpox

Herpesviridae

Double-stranded DNA, enveloped viruses

Simplexvirus (HHV-1 and HHV-2)

Varicellovirus (HHV-3)

Lymphocryptovirus (HHV-4)

Cytomegalovirus (HHV-5)

Roseolovirus (HHV-6)

HHV-7

Kaposi's sarcoma (HHV-8)

Some herpesviruses can remain latent in host cells

Hepadnaviridae

Double-stranded DNA, enveloped viruses

Hepatitis B virus

Use reverse transcriptase

Picornaviridae

Single-stranded RNA,  + strand, nonenveloped

Enterovirus

Poliovirus and coxsackievirus

Rhinovirus

Hepatitis A virus

Caliciviridae

Single-stranded RNA, + strand, nonenveloped

Hepatitis E virus

Norovirus causes gastroenteritis

Togaviridae

Single-stranded RNA, + strand, enveloped

Alphavirus

Transmitted by arthropods; includes EEE and WEE

Rubivirus (rubella virus)

Flaviviridae

Single-stranded RNA, + strand, enveloped

Arboviruses can replicate in arthropods; include yellow fever, dengue, SLE, and West Nile viruses

Hepatitis C virus

Coronaviridae

Single-stranded RNA, + strand, enveloped

Upper respiratory infections

Coronavirus

SARS

Rhabdoviridae

Single-stranded  RNA, – strand,    one RNA strand

Vesiculovirus

Lyssavirus           (rabies virus)

Cause numerous animal diseases

Filoviridae

Single-stranded    RNA, – strand,        one RNA strand

Filovirus

Enveloped,           helical viruses

Ebola and Marburg viruses

Paramyxoviridae

Single-stranded RNA, – strand, one RNA strand

Paramyxovirus

Morbillivirus

Parainfluenza

Mumps

Newcastle disease (chickens)

Deltaviridae

Single-stranded RNA,       – strand, one RNA strand

Hepatitis D virus

Depends on coinfection with hepadnavirus

Orthomyxoviridae

Single-stranded RNA,       – strand, multiple RNA strands

Envelope spikes can agglutinate RBCs

Influenzavirus (influenza viruses A and B)

Influenza C virus

Avian Influenza

Bunyaviridae

Single-stranded RNA, – strand, multiple RNA strands

Bunyavirus (CE virus)

Hantavirus

Arenaviridae

Single-stranded RNA,       – strand, multiple RNA strands

Helical capsids contain RNA-containing granules

Lymphocytic choriomeningitis

VEE and Lassa fever

Retroviridae

Single-stranded RNA, 2 RNA strands,
produce DNA

Use reverse transcriptase to produce DNA from viral genome

Lentivirus (HIV)

Oncogenic viruses

Includes all RNA tumor viruses

Reoviridae

Double-stranded RNA, nonenveloped

Reovirus (respiratory  enteric orphan)

Rotavirus (mild respiratory infections and gastroenteritis)

Colorado tick fever