BIOLEC

• Viruses


  • Are energy less
  • Basic life forms composed of a protein coat called capsid that surrounds the genetic material
  • Do not have organelles or ribosomes
  • Genetic material is either DNA or RNA
  • Replication of genetic material occurs when the virus takes control of the hosts cells' synthetic machinery
  • Contain the genetic information but not the enzyme
• Viral Morphology


  • Nucleic acid - can be single, stranded, double stranded, linear or looped in separate segments or one continuous strand
  • RNA viruses - (+) stranded, (-) stranded, or RNA of retroviruses
  • Capsids - a structure that houses a genome
  • Envelope - a lipid bilayer that covers the nuclear material
• (+) stranded RNA


  RNA is just like mRNA; RNA can immediately be translated by the host's ribosomes into protein
• (-) stranded RNA


  Not able to begin translation immediately; must be transcribed into a (+) strand of RNA (like mRNA); (-) RNA virus must carry capsid with enzyme RNA dependent RNA polymerase, which will carry out transcription of the (-) strand into (+) strand
• Translation


  (+) RNA
• Every DNA virus has both a (+) and (-) strand
• (+) strand - refers to the strand that is read
• (-) strand - is ignored
• (+) strand RNA - used as a template for transcription into mRNA
• Capsomere


  Building block composed of 1 or more polypeptide chains and organize them into a globular protein subunit
• Types of Capsids


  • Icosahedral - 20 triangles together (RNA or DNA)
  • Helical - protein capsomeres are bound to RNA (always RNA because only RNA viruses have helical symmetry) and coiled into a helical nucleoprotein capsid (most of RNA assume a spherical shape except for rhabdovirus which have a bullet shaped capsid)
• Envelope


  A lipid bilayer that covers the nuclear material; viruses acquire this membrane by building thru the host cell nuclear or cytoplasmic membrane and tearing off a piece of the membrane as they leave; with various protein embedded
• Naked viruses


  Viruses that are not enveloped
• Enveloped viruses


  Viruses with a membrane
• Classification of viruses


  • Nucleic acid type: DNA or RNA, double stranded vs single stranded, single or segmented pieces of nucleic acid, (+) or (-) stranded, complexity of genome
  • Capsid: icosahedral or helical
  • Envelope: naked or enveloped
  • Size: diameter of helical capsid, no. of capsomeres- icosahedral capsids
• DNA Viruses


  • Parvoviridae
  • Hepadnaviridae
  • Papovaviridae
  • Adenoviridae
  • Herpesviridae
  • Poxviridae
• Most DNA viruses are double stranded and show icosahedral symmetry; they replicate in the nucleus
• Parvoviridae have a simple single stranded DNA
• Poxviridae have an extremely complex (+) double stranded DNA, do not have icosahedral symmetry, and their DNA is surrounded by complex structural proteins; they replicate in the cytoplasm
• Herpes, Hepadna, & Pox DNA viruses have envelopes
• Papova, Adeno, Parvo (PAP) DNA viruses are naked
• RNA Viruses


  • Togaviridae
  • Coronaviridae
  • Picornaviridae
  • Retroviridae
  • Caliciviridae
  • Reoviridae
  • Orthomyxoviridae
  • Paramyxoviridae
  • Rhabdoviridae
  • Bunyaviridae
  • Arenaviridae
  • Filoviridae
• Most RNA viruses are single stranded (half are + stranded, half - stranded), enveloped, and replicate in the cytoplasm
• Reoviridae are double stranded RNA viruses
• Picorna, Calici, Reoviridae are non-enveloped RNA viruses
• Reo, Picorna, Toga, Flavi, Calici RNA viruses have icosahedral symmetry
• Rhabdo has helical symmetry but has a bullet shaped capsid
• Retro & Orthomyxo RNA viruses undergo replication in the nucleus
• Viral Replication


  1. Adsorption and penetration

  2. Uncoating of the virus

  3. Synthesis and assembly of viral products (as well as inhibits of hosts cell own DNA, RNA & protein)

  4. Release of virions from the host cell (either by lysis or budding)
• RNA Viruses usually undergo transcription, translation & replication in the cytoplasm
• DNA Viruses - transcription & replication usually occur in the nucleus; tend to be more genetically complex than RNA
• Host Cell Outcome


  • Death - cell death
  • Transformation - infection can activate or introduce oncogenes (results in uncontrolled & uninhibited cell growth)
  • Latent infection - virus can survive in a sleeping state, surviving but not producing clinically overt infection
  • Chronic slow infection - some viruses will cause disease only after many years, often decades of indolent infection
• Orthomyxoviridae (Orthomyxo virus)


  Influenza virus - causes flu, pneumonia in at risk group; spherical virions; has 2 types of glycoprotein (cell membrane) - hemaglutinin (HA) and neuramidase (NA)
• Hemaglutinin (HA)


  Can attach to host sialic acid receptors (present on surface of erythrocytes) causing heme-agglutination reaction when mixed with RBC; Host cell sialic acid receptors also exist on upper respiratory tract membranes and HA binding causes fusion to host cell membrane with the virion membrane, dumping of the virion genome to the host cell
• Neuramidase (NA)


  Cleaves neuraminic acid and disrupts the mucin barrier exposing the sialic acid binding sites beneath; Neuraminic acid is an important component of the substance covering mucosal epithelial cells as of upper respiratory defense
• Types of Influenza Virus

  • Type A - infects human, other animals (swine & birds)
  • Type B & C - only isolated to humans
• Antibody to the NA and HA are protective


  But we continually get epidemics of the bothersome flu
• Antigenic drift


  During viral replication, mutation can occur in the HA and NA leading to changes in the antigenic nature of those glycoproteins
• Antigenic shift


  There is a complete change of HA and NA or both, which can also occur with influenza type A because the mechanism involves the trading of RNA segment between animal and humans
• Avian influenza (Infleunza A) - Hongkong antigenic shift was deadly causing fever, severe pneumonia & death in almost half of the infected person; poorly transmissible in human (has been controlled by destroying poultry in HK)