Viruses

Related Subjects: |Herpes Varicella-Zoster (Shingles) Infection |Chickenpox Varicella Infection |Varicella Cerebral Vasculopathy |Herpes Viruses |Herpes Zoster Ophthalmicus (HZO) Shingles |MonkeyPox |Mumps |Measles |Rubella (German Measles) |Epstein-Barr Virus infection

About

• Viruses are obligate intracellular organisms composed of RNA or DNA
• DNA/RNA surrounded by various proteins
• They cannot be grown outside a host cell
• May have an envelope studded with viral glycoproteins

Behaviour

• Very small carrying little so use the host cell machinery
• Use host cell enzymes, ribosomes, ATP
• All negative RNA viruses, retroviruses, Hepatitis B all have their own polymerase
• Polymerase has poor error checking and so the mutation rate is high

Viral Genomes

• Genetic Material - DNA or RNA
• DNA - single stranded or double-stranded
• HIV is diploid with two identical chromosomes
• Influenza genome is divided into 8 segments
• RNA can be negative or positive

Structure

• Naked Icosahedral - DNA/RNA surrounded by proteins. No lipid layer. Not easily damaged e.g. solvents. Often all released killing the host cell.
• Enveloped Icosahedral - envelope of a lipid bilayer with glycoproteins. The bilayer coming from the host cell from which they have budded. Chronic slow release of the virus. Host cell viable. HIV is cytotoxic. Cell membrane inserted proteins to stimulate a cell-mediated immune response.
• Enveloped Helical - Glycoprotein studded lipid bilayer usually RNA with polymerase

Cell entry

• Fusion with specific surface cell member receptors and viral surface
• Process of fusion of the membrane with viral envelope or (HIV, Herpes) or pinocytosis with naked virus
• Specific binding - HIV-1 and CD4 receptors

Uncoating

• Once inside virus uncoats exposing the RNA/DNA
• Process of replication can commence

Replication

requires the production of mRNA
• DNA virus ? Host nucleus ? Host enzymes produce mRNA which produces a DNA polymerase used for viral replication
• RNA virus ? uses a virion-associated RNA polymerase to replicate a positive-sense RNA and make mRNA
• Retroviruses ? Use a virally encoded reverse transcriptase to make a dsDNA copy of RNA which is incorporated into host DNA to produce mRNA
• Finally mRNA ? to make proteins

Viral assembly

• Viral proteins made from mRNA
• Assembles to form new viral copies

Viral release

• Cytopathic virus released by cell lysis
• Release by budding over a period of time

Pathogenesis

• Integrate into host DNA and become dormant
• Replicate and destroy host cell releasing contents
• Production of benign and malignant tumours
• Make changes to the host cell which reduce function

  Human DNA viruses
  Herpesviridiae	HSV, VZV, EBV, CMV, HHV-6
  Adenoviridae	Adenovirus
  Hepadnaviridae	Hepatitis B
  Parvoviridae	Parvovirus B19
  Papovaviridae	Papilloma, Polyoma (PML)
  Poxviridae	Molluscum Contagiosum

  Human RNA viruses
  Picornaviridiae	Rhinovirus, Hepatitis A, Coxsackievirus, Echo virus, poliovirus. ss(+)RNA Icosahedral structure, multiplies in cytoplasm, No envelope
  Calicivirus	Norwalk agent Hepatitis E, ss(+)RNA No envelope, Icosahedral
  Orthomyxoviridiae	Influenza A and B. ss(-) Polymerase, envelope, Helical structure, Replicates cytoplasm and nucleus
  Paramyxoviridiae	Parainfluenza, RSV, Mumps, Measles. ss(-) RNA. Polymerase, envelope, Helical spikes with HN and F glycoprotein. Multiplies in cytoplasm.
  Retroviridiae	HIV, HTLV. Diploid, ss(-) RNA RNA dependent DNA polymerase. Replictaes in nucleus. Icosahedral or truncated conical
  Reoviridiae	Rotavirus
  Togaviridiae	Rubella, Alphavirus
  Flaviviridiae	ss(+) RNA, Envelope, Icosahedral. Replicates in cytoplasm. Dengue fever, Yellow fever, St Louis encephalitis virus, Hepatitis C
  Bunyaviridiae	California encephalitis virus, Hanta, LaCrosse. Polymerase, Envelope. Replicates in cytoplasm.
  Arenaviridiae	Lassa fever virus
  Filoviridiae	Ebola virus
  Coronaviridiae	Corona virus ss(+) RNA Envelope. Replicates in cytoplasm.
  Rhabdoviridiae	Rabies