MDMICRO (2)

• Virus


  A genetic element that can multiply only inside a living cell, called the host cell. Not considered living entities and not found on the tree of life. Possess their own nucleic acid genomes and are independent of the host’s genome. Obligate intracellular parasites
• Virus replication


  • Enzymes involved: DNA polymerase, helicase, gyrase, SSBPs. Both strands are templates for new DNA synthesis. DNA to DNA
• Virus transcription


  Enzymes involved: RNA polymerase. Dark green strand is template for RNA synthesis. DNA to RNA, specifically mRNA. DNA and RNA are made up of nucleotides
• Virus translation


  Messenger RNA is template for protein synthesis. Ribosome is responsible for translation. Amino acids are the building blocks of protein
• Most viruses do not have enzymes for replication, transcription, and translation processes
• Viruses are obligate intracellular parasites


  Depend on the host and use the host's enzymes for replication, transcription, and translation
• The genome of viruses encodes for viral proteins
• Viruses have a structurally intricate extracellular form called the virion
• Virions protect viruses and allow transmission of infections from one host to another
• Virions of different viruses

  • Zika virus: human
  • Pepino mosaic virus: tomato
  • Mimivirus: Acanthamoeba
  • His1: Haloarcula hispanica
  • Lactococcus phage 1358 (bacteriophage)
• Viral components


  • Capsid - protein shell
  • Outer layer (in animal viruses)
  • Envelope (if composed of a phospholipid bilayer from host cell membrane)
• Capsid


  • Composed of capsomeres, protects the genome, aids in infecting other cells and multiplying
• Outer layer of viruses

  • Composed of a phospholipid bilayer with glycoprotein
• Envelope of viruses


  • Portion from the host membrane surrounding the nucleocapsid, aids in faster infection, merges with other cells, releases nucleocapsid inside the cell, aids in attachment to other cells
• Viral structure


  The capsid is composed of capsomeres arranged around the nucleic acid. Capsomeres form a capsid. Some viruses can form a helical structure on the nucleocapsid
• Formation of capsid


  Self assembly
• Viral Symmetry


  • Most virions are highly symmetric structures
  • Two kinds of symmetry: Rod-shaped viruses have helical symmetry while spherical viruses have icosahedral symmetry
  • Pandoraviruses are asymmetrical
• Enveloped Viruses


  • Ebola virus
  • Influenza virus
• Envelope is basically the cell membrane of the host
• Enzymes inside virions


  1. Lysozyme-like enzyme used to make a small hole in the peptidoglycan layer of the bacterial cell

  2. Neuraminidases destroy glycoproteins and glycolipids of animal cell connective tissue to break down the cell membrane of the host

  3. Viruses with RNA genomes carry RNA-dependent RNA polymerases called RNA replicases in their virion to replicate and express the viral RNA genome

  4. Reverse transcriptase converts RNA genome to a DNA intermediate

  5. Stabilizing the RNA genome by making it double stranded
• Size and structure of viral genomes

  • DNA viruses exist along a gradient from tiny circovirus to Pandoravirus
  • DNA viruses usually smaller than the genome of their host
• ICTV Classification is an older classification system for viruses
• The Baltimore Scheme was developed by American virologist David Baltimore
• The goal of all viruses is to produce mRNA (+) strand as the template for translation of viral proteins
• Classes of viruses in the Baltimore Scheme


  • Class I: dsDNA (±) virus, Ex. Phage T4
  • Class VII: dsDNA (±) virus, Ex. Hepatitis B
  • Class II: ssRNA (+) virus, Ex. ɸX174, Parvovirus
  • Class III: dsRNA (±) virus, Ex. Phage phi6, Rotavirus
  • Class IV: ssRNA (+) virus, Ex. Phage MS2, Poliovirus
• Class I & VII viruses: Transcription of the minus strand → mRNA (+)
• Class II viruses: Synthesis of the minus strand → mRNA (+) → dsDNA intermediate (replicative form) → mRNA (+)
• Class III viruses: Transcription of the minus strand → mRNA (+)
• Class IV viruses: Used directly as mRNA
• Synthesis of the minus strand

  mRNA
• Transcription of the minus strand

  mRNA (+)
• Transcription followed by reverse transcription

  Classical semiconservative, discard (-) strand
• Reverse transcription


  dsDNA intermediate (for stability)→ transcription of the minus strand → mRNA (+)
• Viral diversity and viral morphology
• Coronavirus is different from the flu virus
• Not all viruses have negative effects on their hosts, and in some cases, viruses are even beneficial to their hosts
• Useful for genetic engineering
• When Arabidopsis plants are infected with plum pox virus, the plant’s tolerance to drought increases due to virally induced production of the plant growth factor salicyclic acid
• Bacterial viruses are called bacteriophages (or simply phage for short) and have been intensively studied as model symptoms for the molecular biology and genetics of virus replication
• Electron microscopes are used to visualize bacteriophages