chapter 10 viruses viruses contain dna or rna and a protein coat called a the capsid is composed of...

Chapter 10 Viruses• Viruses contain DNA or RNA

• And a protein coat called a

• The capsid is composed of individual

• Some are enclosed by an envelope

• May have spikes protruding from envelope

• Most viruses infect only specific types of cells in one host

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

Viruses

Polyhedral Viruses

Helical Viruses

Causes systemic infection often with hemorrhagic fever

Complex Viruses

Viral Taxonomy• Family names end in -viridae

• Genus names end in -virus

• A group of viruses sharing the same genes and host. Common names are used for species

• Subspecies are designated by a number

Viral Taxonomy

• Family : Herpesviridae

• Genus: Simplexvirus

• Species/Subspecies: Human herpes virus 1, HHV 2

• Family: Retroviridae

• Genus: Lentivirus

• Species/Subspecies: Human Immunodeficiency Virus 1, HIV 2

Growing Animal Viruses• Animal viruses may

be grown

• in living animals

• In embryonated eggs

•

• or in cell culture.

Growing Viruses

• Animal (and plant) viruses may be grown in cell culture.

• For research purposes

• For clinical identification of virus type

• By observing

• Some cytopathic effects (not comfirmatory)

• Inclusion bodies - rabies virus

• Syncytia formation – measles, common cold

• Transformation – human papillomavirus

Virus Identification

Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Virus Identification• Serological tests

• Detect antibodies against viruses in a patient

• Use antibodies to identify viruses

• Western blot

• Viral nucleic acids: RFLPs (i.e. DNA Fingerprinting), PCR tests

Virus Identification – a cytopathic effect

• Viruses require host cell “machinery” for replication

•

• Some viruses have some of their own enzymes: mainly for viral nucleic acid replication

• Most, if not all, enzymes come from host cell

• These enzymes are involved in building new virions

Viral Multiplication

• Phage causes lysis and death of host cell

• Attachment Phage attaches by tail fibers to host cell

• Penetration Phage lysozyme opens cell wall;

DNA is then “injected” into cell

• Biosynthesis Production of phage DNA and proteins

• Maturation Assembly of phage particles

• Release Phage lysozyme breaks cell wall

Multiplication of Bacteriophages (Lytic Cycle)

Attachment:Phage attaches to host cell.

Penetration:Phage penetrates host cell and injects its DNA.

Biosynthesis: phage DNA directs synthesis of viral components by host cell.

1

2

3

Bacterial cell wall

Bacterial chromosome

Capsid DNA

Capsid

Sheath

Tail fiber

Base platePin

Cell wall

Tail

Plasma membrane

Sheath contracted

Tail core

4 Maturation:Viral components are assembled into virions.

Tail

5 Release:Host cell lyses and new virions are released.

DNA

Capsid

Tail fibers

The Lysogenic Cycle: Phage DNA incorporated into host DNA

Specialized Transduction

Prophage exists in galactose-using host (containing the gal gene).

Phage genome excises, carrying with it the adjacent gal gene from the host.

Phage matures and cell lyses, releasing phage carrying gal gene.

1

2

3

Prophage

gal gene

gal gene Bacterial DNA

Galactose-positive donor cell gal gene

Phage infects a cell that cannot utilize galactose (lacking gal gene).

4

Galactose-negative recipient cell

Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA.

5

Lysogenic cell can now metabolize galactose.

6

Galactose-positive recombinant cell

• 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 (non-enveloped

viruses)

Multiplication of Animal viruses

Attachment, Penetration, and Uncoating

Release of an enveloped virus by budding

Nonenveloped DNA virus •

• Can cause cancer (cervical cancer)

• Hand warts

• Genital warts

• Commonly sexually transmitted

• Prevention: vaccine for HPV

•

• Respiratory infections in humans

• Usually sudden onset and short duration

• Tumors in animals

Enveloped DNA virus•

• Can cause chronic liver disease

• Can lead to liver cancer

• Body fluid transmisson

• Prevention: vaccine

• Human Herpes Virus 1 and HHV 2 – “cold sores” and genital herpes

• Sexually transmitted

• Other direct contact transmission

• Prevention: condoms (not 100%), abstinence


Enveloped DNA virus• Varicella-Zoster virus (HHV 3) –

• Chicken pox is initial infection

• Shingles may occur later

• Contracted by inhaling virus


• Epstein-Barr virus (HHV 4) – Infectious Mononucleosis

• Saliva transmission

• Burkitt’s lymphoma in African children

• Prevention: avoid infected body fluids (mainly saliva)

Multiplication of a Retrovirus

Retrovirus penetrates host cell.

Its RNA is uncoated; reverse transcription takes place.

The new viral DNA is tranported into the host cell’s nucleus and integrated as a provirus. The provirus may divide indefinitely with the host cell DNA.

1

2

3

Envelope

Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins.

4

Mature retrovirus leaves host cell, acquiring an envelope as it buds out.

5

CapsidReverse transcriptase

Virus Two identical strands of RNA

DNA of one of the host cell’s chromosomes

Provirus

Host cell

Reverse transcriptase

Viral RNA

RNA

Viral proteins

Identical strands of RNA

• Use reverse transcriptase to produce DNA from RNA viral genome

• HIV - AIDS

• Oncogenic viruses

• Some retroviruses can cause cancer

Retroviruses – enveloped RNA viruses

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

• “transform” normal cells into cancerous cells.

• An oncovirus can promote oncogene expression

• may contain oncogenes

Cancer

•

• Virus remains in host cell for long periods, asymptomatically

• Subsequent activation may lead to: i.e. cold sores, shingles

• Persistent Viral Infections

• Disease progresses over a long period, generally fatal

• Subacute sclerosing panencephalitis, SSPE (measles virus)

• AIDS dementia complex (HIV)

Viral Infections

• Infectious proteins

• Inherited, and transmissible by ingestion, transplant, & surgical instruments

• Spongiform encephalopathies (fatal): Sheep Scrapie, Creutzfeldt-Jakob disease, mad cow disease

• Cause:

• normal cellular prion protein on cell surface,

• converts to

• scrapie protein, accumulates in brain cells forming plaques

Prions

Prions

PrPc

PrPSc

1 2 3 4

5 6 7 8

Endosome

Lysosome

• Plant Viruses

• enter through wounds or via insects

• are infectious RNA

• One causes potato spindle tuber disease

Plant viruses and viroids

Potato Spindle Tuber Viroids

RNA virus, nonenveloped• Poliovirus

• May be paralytic in ~1% of cases

• Transmission:


• Human

• Number one cause of the common cold

•

• Attacks liver, kidneys, spleen

• Transmission: fecal-oral route

• Usually not fatal

• Prevention: vaccine (long-term), immune globulin (short-term)

• Influenza viruses A, B, and C

• Type A have caused

• Both type A and B cause seasonal flu

• Subtypes differ based on H and N spike variation

RNA virus, enveloped


• Influenza viruses continued:

• Mutation of H and N spike can lead to epidemics or pandemics

• Contracted by inhaling virus

• Prevention: vaccine may prevent


RNA virus, enveloped

•

• Infects birds and humans

• Transmitted by mosquitos

• First appeared in U.S. in 1999 in NYC area; now coast to coast

• Symptoms: Usually “flu-like;” less than 1% get West Nile Encephalitis

• Prevention: avoid mosquito bites

chapter 10 viruses viruses contain dna or rna and a protein coat called a the capsid is composed of...

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