virus

• Virus

  Infectious particle that can reproduce only by infecting a host cell (plants, animals, or humans)

  
• Obligatory intracellular parasite


  Viruses that infect bacteria

  
• Bacteriophage


  Infectious particle that can reproduce only by infecting a host cell (a total parasite)
• Viruses are not considered "living"

  
• Viruses do not have cells
• Virus size

  Smaller than cells (20 – 300 nanometers)
• Virus reproduction

  Infecting a host cell and using its machinery to make more viruses ("reprogramming host cells to become virus-making factories")

  
• Viruses are very diverse
• Virus characteristics

  • Different shapes & structures
  • Different kinds of genomes
  • Infect different hosts

  
• Virus composition

  Made up of DNA or RNA genome inside a protein shell (CAPSID)

  
• Virus envelope

  Lipid membrane surrounding the entire capsid, helps viral particles bind to host cells

  
• Virus genomes

  • Genetic material made up of nucleic acid (RNA or DNA, double- or single-stranded)

  
• Capsomer


  Protein molecules that make up the virus capsid

  
• Virus capsid shapes

  • Icosahedral – 20-sided shape
  • Filamentous / Helical – thin, linear, thread-like, rod or helical
  • Head-tail / Complex – combined icosahedral & filamentous

  
• Virus life cycle

  1. Attachment

  2. Entry

  3. Genome replication & Gene expression

  4. Assembly

  5. Release

  
• Virus attachment

  Recognition & binding to a host cell via a receptor molecule on the cell surface

  
• Virus entry

  Through fusion with the membrane (enveloped viruses) or endocytosis (non-enveloped viruses)

  
• Virus genome replication & gene expression

  Viral genome is copied & its genes are expressed to make viral proteins

  
• Virus assembly

  New viral particles are assembled from the genome copies & viral proteins

  
• Virus release

  • Lysis (cell burst)
  • Exocytosis
  • Budding

  
• Virus classification criteria

  • Nucleic Acid: DNA / RNA
  • Capsid shape: Icosahedral / Helical / Complex
  • Envelope: Naked / Enveloped
  • Genome architecture: single-stranded (ss) / double-stranded (ds)

  
• Circovirus

  Small, nonenveloped DNA viruses with a single-stranded, covalently closed genome, the smallest viruses capable of autonomous replication in eukaryotic cells

  
• Circoviruses are part of the Circoviridae family, divided into two genera: Gyrovirus and Circovirus
• Circoviruses


  • Viral genome encodes two to three major open reading frames for proteins involved in viral replication and capsid formation
  • Responsible for diseases of veterinary and economic importance, including postweaning multisystemic wasting syndrome in pigs and beak and feather disease in birds

  
• Circoviruses are associated with lymphoid depletion and immunosuppressive conditions in infected animals leading to systemic illness
• Porcine circovirus 2 (PCV2) was the first circovirus capsid structure solved at atomic resolution using X-ray crystallography

  
• Diseases caused by circoviruses

  • Chicken anemia virus
  • Psittacine Beak and Feather Disease (PBFD)
  • Young Pigeon Disease Syndrome (YPDS)
  • 'Black Spot' in Canary Birds
  • Post-Weaning Multisystemic Wreaking Syndrome (PMWS)
• Parvovirus


  Small DNA viruses that infect various animal species and humans, require a simultaneous helper virus or rapidly dividing cells for replication

  
• Parvovirus groups

  • Dependovirus (multiply only in cells coinfected with a helper adenovirus)
  • Parvovirus (multiply only in cells replicating their own DNA)

  
• Parvovirus structure


  • Nonenveloped, icosahedral virus with 18-26 nm diameter, capsid composed of two structural proteins: VP-l and VP-2

  
• Parvovirus DNA genome is 5.5 kilobases long, packages plus and minus DNA strands into separate virions

  
• Parvoviruses are hardy and resistant to ether, chloroform, DNase, and RNase treatment
• Parvovirus B19


  Only infects humans, not animals
• Parvovirus B19 symptoms

  • Fifth disease or Erythema Infectiosum (distinctive facial rash)
  • Joint soreness in adults
  • Fever, upset stomach, headache, runny nose
• Treatment for parvovirus B19 includes IV fluids, management of electrolytes, antibiotics to prevent secondary infections, and medications to help relieve vomiting, nausea and pain
• Polyomavirus


  Small, nonenveloped viruses that are widely distributed, first human polyomaviruses isolated in 1971 from immunocompromised patients

  
• Two new human polyomaviruses, KI virus and WU virus, were detected in respiratory tract secretions of children in 2007
• Merkel cell virus (MCV) was recently detected, showing its viral genome integrated within the genome of cells of a rare human skin cancer, Merkel cell carcinoma

  
• Polyomavirus structure


  • Virions are about 44 nm in diameter, composed of 72 capsomeres, and have icosahedral symmetry, genome organized into three functional regions: the early and late coding regions and a regulatory region

  
• Polyomavirus infection

  Commonly infect humans without causing noticeable symptoms, but can lead to diseases in certain situations depending on the specific viral strain and individual immune status