Viruses

Viruses

Acellular Agents Viruses – protein and nucleic acid Viroids – only RNA Satellites – only nucleic acids Prions – proteins only
Viruses
1. Major cause of disease
2. Important member of aquatic world
3. Important in evolution
4. Important model system in molecular biology
Virion
1. complete viruses particle
2. consists of ≥1 molecule of DNA or RNA enclosed in coat of protein
3. may have additional layers & cannot reproduce independent of living cells
4. Virions Infect All Cell Types
Structure of Viruses - 10–400 nm in diameter, contain a nucleocapsid composed of nucleic acid (DNA or RNA) and a protein coat (capsid)
1. Capsids - Large macromolecular protein coat of virus, protect viral genetic material and aids in its transfer between host cells , made of protein subunits called protomers
  1. Helicals capsids - hollow tubes shapes with protein walls
  2. Icosahedral capsids – ring or knob-shaped units made of 5 or 6 protomers, pentamers (pentons) – 5 subunit capsomers and hexamers (hexons) – 6 subunit capsomers 1
  3. Complex capsids - poxviruses – largest animal virus or large bacteriophages – binal symmetry
Viral envelope and enzymes - viruses are bound by an outer, flexible, membranous layer called the envelope
1. Envelope proteins - viral encoded, may project from the envelope surface as spikes or peplomers
2. variety of virions have enzymes – some are associated with the envelope or capsid but most are within the capsid
3. Viral genome - A virus may have single or double stranded DNA or RNA and genomes can be segmented or circular
Viral Multiplication
1. 1. Attachment (adsorption)
  1. Specific receptor attachment & receptor determines host preference
2. 2. Viral entry and uncoating
  1. Three methods used i. fusion of the viral envelope with host membrane; nucleocapsid enters ii. endocytosis in vesicle; endosome aids in viral uncoating iii. injection of nucleic acid
3. 3. Synthesis stage
  1. Genome detect the events, DNA viruses may use enzymes that is similar to the host enzymes while RNA viruses must carry in or synthesize the proteins necessary to complete synthesis
4. 4. Assembly
  1. Late proteins are important in assembly, it is complicated but varies
5. 5. Virion release
  1. Nonenveloped viruses lyse the host cell – viral proteins may attack peptidoglycan or membrane, Enveloped viruses use budding
Types of Viral Infection
1. Infections in Bacteria and Archaea
  1. Virulent phage of one reproductive choice . It multiplies immediately upon entry and lyses bacterial host cell
    1. Lysogenic Conversion
2. Infections in eukaryotic cells
  1. Cytocidal infection results in cell death through lysis , Persistent infections may last years
3. Viruses and cancer
4. Carcinogenesis
Cultivation of Viruses
1. Hosts for Bacterial and Archael Viruses - Usually cultivated in broth or agar cultures of suitable, young, actively growing bacteria
2. Hosts for Animal Viruses - Tissue (cell) cultures, embryonated eggs
3. Hosts for Plant Viruses - Plant tissue cultures, plant protoplast cultures, suitable whole plants
4. Quantification of Virus - Direct counting by count viral particles, Indirect counting by an observable of the virus – hemagglutination assay & plaque assays
Viroids and satellites
1. Viroids - infectious agents composed of closed, circular ssRNA, requires host cell DNA-dependent RNA polymerase to replicate
2. Satellites - Infectious nucleic acids (DNA or RNA), satellites viruses encode their own capsid proteins when helped by a helper virus but Satellite RNAs/DNAs do NOT encode their own capsid proteins
Prions
1. Proteinaceous Infectious Particle - Cause a variety of degenerative diseases in humans and animals
2. All prion caused diseases – have no effective treatment – result in progressive degeneration of the brain and eventual death

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