Viro

• Attachment

  1. Viruses attach to the host cell through their ligands

  2. Once attached, a virus must now enter the cell, which is covered by a phospholipid bilayer, a cell's natural barrier to the outside world
• + sense and - sense strands are always complementary to each other, with different nucleotide sequences; only + sense strands have the right codons for translation
• Outcomes of viral replication

  • Lytic Pathway
  • Lysogenic Pathway
• Biosynthesis

  DNA VIRUSES: Nuclear ds DNA viruses, Cytoplasmic ds DNA viruses
• Viruses can only multiply on living cells because the cells provide the necessary energy and machinery for the synthesis of viral nucleic acids and proteins
• Lysogenic Pathway

  1. The cells will not die right after infection, instead they are used as factories for production of viral genes

  2. DNA is replicated first many times and at a certain point, mass lysis will occur
• Uncoating

  1. The viral capsid will be degraded by enzymes, allowing the viral nucleic acid to be released into the cytoplasm

  2. Viral genome priorities: Create viral proteins, Create more copies of itself

  3. Transcription - the viral genome is copied into mRNA, Translation - the viral genome is copied into mRNA
• Viruses are unique and cannot multiply by binary fission like other microorganisms; instead, new viral particles are synthesized from the host cell
• During replication, some viruses may utilize enzymes: DdDp, DdRp, RdRp, RdDp
• General Steps in Viral Replication Cycle

  • Attachment
  • Penetration
  • Uncoating
  • Biosynthesis
  • Assembly
  • Release
• Penetration


  Viruses generally have two methods: Membrane fusion or Hemifusion, Endocytosis
• Messenger RNAs (mRNA) are always +sense RNAs, making them readily translated by ribosomes in the cell
• Lytic Pathway

  1. The viruses will damage most of the host's membrane, making the contents of the cytoplasm leak out

  2. The cell will die rapidly and release the viruses
• RELEASE

  Non-enveloped viruses are released through cell lysis, while enveloped viruses can be released through budding/exocytosis
• Nuclear ds DNA viruses

  • Adeno-, Herpes- & Papova-virus
• ss + sense RNA viruses

  • Picornavirus, Coronavirus, Calicivirus & Flavivirus
• BIOSYNTHESIS

  1. DNA VIRUSES

  2. Nuclear ds DNA viruses utilize DdRp present in the host cell to transcribe mRNA

  3. Cytoplasmic ds DNA viruses carry their own DdRp to transcribe mRNA

  4. Most DNA viruses replicate solely in the nucleus, except for Pox-, Asfar- & Irido- virus

  5. Iridovirus replication involves both nuclear and cytoplasmic compartments

  6. ASFV DNA replication starts in the nucleus and then diverts into the cytoplasmic compartment

  7. Ss DNA viruses are transcribed by the cell DdRp to produce mRNA

  8. ds RNA Viruses use their - sense strands to create mRNA using the RdRp they possess

  9. ss + sense RNA viruses bind directly to ribosomes without needing transcription first

  10. ss – sense RNA viruses use RdRp carried by the virus

  11. Most RNA viruses replicate in the cytoplasm, except for Orthomyxovirus & Retrovirus

  12. ss + sense RNA viruses use the + sense RNA strand to create a - sense DNA strand

  13. ds DNA viruses are converted into covalently closed circular (CCC) DNA used as a template for transcription of viral RNAs
• Cultivation of Viruses allows detection of multiple viruses that may not have been suspected initially and provides a viable isolate for further study
• Cytoplasmic ds DNA viruses

  • Pox-, Asfar- & Irido- virus
• Methods of viral diagnosis include direct examination and indirect examination
• ss – sense RNA viruses

  • Arena-, Bunya, & Orthomyxo- virus
• ds RNA Viruses


  • Reovirus & Birnavirus
• Primary Cell Cultures are normal cells obtained from freshly killed adult animals, often using monkey kidney cell lines
• Primary Cell Culture is used for isolation
• ASSEMBLY

  Viruses are assembled and packaged into a whole virion once translated into amino acids/proteins
• Indirect examination methods

  • Cell culture
  • Serology
  • Animals
• Ss DNA viruses

  • Parvovirus & Circovirus
• Organ Cultures involve maintaining small bits of organ from humans or animals in tissue culture media to accurately model organ functions
• Virus Detection methods

  • Antigen detection
  • Serology (immunofluorescence, ELISA, etc.)
  • Electron microscopy
  • Hybridization with specific nucleic acid probes
  • Polymerase chain reaction (PCR)
• ds DNA viruses


  • Not specified
• Primary Cell Culture

  1. Used for isolation of respiratory and enteroviruses

  2. Growth of viruses in cell culture is usually detected by visualizing morphological changes in the cells, known as cytopathic effect

  3. The characteristics of the cytopathic effects can give an idea to what virus is the one isolated
• Organ Cultures

  1. Small bits of organ from human or animal are maintained in tissue culture media

  2. This technique can accurately model functions of an organ in various states and conditions
• Embryonated Egg Culture

  1. Embryonated eggs of 7-12 days are used

  2. Virus can be cultured in different parts of the embryonated egg depending on the type of virus
• Semi-Continuous Cell Culture


  1. Usually made up of embryonic kidney and skin fibroblasts

  2. Can be more viable for longer periods than primary cell lines (usually up to 50 subcultures)
• Antigen/Antibody Detection

  • Fluorescent antibody (FA) staining
  • Immunoperoxidase staining
  • Enzyme-linked Immunosorbent Assay (ELISA)
• Animal Inoculation


  Some viral diseases can be confirmed through inoculating laboratory animals with the infective agent
• Continuous Cell Culture

  1. One popular example is HeLa Cells (discovered in 1951)

  2. Tumor cells collected from Henrietta Lacks

  3. Exact mechanisms are still unknown, but HeLa cells prevent apoptosis

  4. HeLa cells are key in creating vaccines against diseases like polio, measles, mumps, HIV, and Ebola
• Main advantage of ELISA
• Serology


  This technique is used for detecting antibodies in a patient's serum
• Antigen/Antibody Detection