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

Virus Identification Some cytopathic effects (not comfirmatory) Inclusion bodies - rabies virus Syncytia formation – measles, common cold Transformation – human papillomavirus

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

Viral Multiplication 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

Multiplication of Bacteriophages (Lytic Cycle) 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

1 2 3 Bacterial cell wall Bacterial chromosome Capsid DNA Capsid Sheath Tail fiber 1 Attachment: Phage attaches to host cell. Tail Base plate Pin Cell wall Plasma membrane 2 Penetration: Phage penetrates host cell and injects its DNA. Sheath contracted Tail core 3 Biosynthesis: phage DNA directs synthesis of viral components by host cell.

4 5 Tail DNA Maturation: Viral components are assembled into virions. Capsid 5 Release: Host cell lyses and new virions are released. Tail fibers

The Lysogenic Cycle: Phage DNA incorporated into host DNA

Specialized Transduction gal gene Prophage Bacterial DNA 1 Prophage exists in galactose-using host (containing the gal gene). Galactose-positive donor cell gal gene 2 Phage genome excises, carrying with it the adjacent gal gene from the host. 3 Phage matures and cell lyses, releasing phage carrying gal gene. gal gene 4 Phage infects a cell that cannot utilize galactose (lacking gal gene). Galactose-negative recipient cell 5 Along with the prophage, the bacterial gal gene becomes integrated into the new host’s DNA. 6 Lysogenic cell can now metabolize galactose. Galactose-positive recombinant cell

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 (non-enveloped 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 Prevention: vaccine 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 Reverse transcriptase Capsid Envelope Virus Two identical strands of RNA 1 Retrovirus penetrates host cell. Host cell DNA of one of the host cell’s chromosomes 5 Mature retrovirus leaves host cell, acquiring an envelope as it buds out. Reverse transcriptase 2 Its RNA is uncoated; reverse transcription takes place. Identical strands of RNA Viral RNA 4 Transcription of the provirus may also occur, producing RNA for new retrovirus genomes and RNA that codes for the retrovirus capsid and envelope proteins. Viral proteins RNA 3 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. Provirus

Retroviruses – enveloped RNA viruses Use reverse transcriptase to produce DNA from RNA viral genome HIV - AIDS Oncogenic viruses Some retroviruses can cause cancer

Cancer 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

Viral Infections 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)

Prions 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 PrPSc PrPc 1 2 3 4 Lysosome Endosome 5 6 7 8

Plant viruses and viroids enter through wounds or via insects are infectious RNA One causes potato spindle tuber disease Potato Spindle Tuber Viroids

RNA virus, nonenveloped Poliovirus May be paralytic in ~1% of cases Transmission: Prevention: vaccine 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)

RNA virus, enveloped 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

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