ch 26-27

• Viruses


  Nucleic acid core surrounded by protein, no cytoplasm, not a cell

  
• Viral nucleic acid


  • Can be DNA or RNA, circular or linear, single- or double-stranded, RNA viruses may have segmented genomes

  
• Viral classification


  • RNA viruses
  • DNA viruses
  • Retroviruses

  
• Viral structure

  Protein sheath (capsid) around nucleic acid core, some have envelope derived from host cell membrane

  
• Viral shapes


  • Helical
  • Icosahedral
  • Complex with binal symmetry

  
• Viral hosts


  Obligate intracellular parasites, limited host range, can remain dormant or latent for years

  
• Viral replication


  1. Infect host cell

  2. Hijack host cell machinery

  3. Express viral genes

  4. Assemble and release new viruses

  
• Viruses lack their own ribosomes and enzymes for protein and nucleic acid synthesis
• Bacteriophages

  Viruses that infect bacteria, diverse, include "T" series that infect E. coli

  
• Lytic cycle of bacteriophages

  1. Attachment

  2. Penetration

  3. Synthesis

  4. Assembly

  5. Release

  
• Lysogenic cycle of bacteriophages

  Virus genome integrates into host genome, forms prophage, replicates with host

  
• Viral infections

  Persistent (latent or chronic) vs. Acute (rapid replication, can cause outbreaks)

  
• Influenza is one of the most lethal viruses in human history
• Influenza virus types


  Type A - serious epidemics, Types B and C - mild human infections
• Influenza virus subtypes


  Differ in hemagglutinin (HA) and neuraminidase (NA) proteins

  
• Antigenic drift in influenza


  Accumulation of random mutations in HA and NA proteins

  
• Antigenic shift in influenza

  Genetic recombination between two strains, produces novel HA and NA combinations

  
• Influenza pandemics have occurred in 1918, 1957, and 1968
• HIV

  Causes acquired immune deficiency syndrome (AIDS) by infecting and destroying CD4+ immune cells

  
• HIV infection cycle


  1. Attachment

  2. Entry

  3. Replication

  4. Assembly

  5. Release

  
• Prions


  Proteinaceous infectious particles that cause transmissible spongiform encephalopathies

  
• Prions cause normal prion proteins to misfold and cause disease
• Viroids


  Tiny naked circular RNA molecules that cause diseases in plants
• Prokaryotes are the oldest, structurally simplest, and most abundant forms of life
• Encephalopathy (BSE)


  Bovine spongiform encephalopathy
• Scrapie


  Prion disease in sheep
• Creutzfeldt–Jacob disease (CJD)


  Prion disease in humans
• Host has normal prion proteins (PrPc)


  Normal prion proteins
• Misfolded proteins (PrPsc)


  Causes normal PrPc to misfold, causing disease
• Normal PrPc protein is necessary for productive infection by scrapie infectious particle

  
• Viroids


  Tiny (250 to 400 nucleotides) naked molecules of circular RNA
• Viroids


  • Caused diseases in economically important crops, e.g. recent outbreak killed over 10 million coconut palms in the Philippines
• Viroid replication

  1. Mature viroids travel from cell to cell in plants via plasmodesmata

  2. Use host protein to replicate

  3. Replication intermediates may act as the source of small interfering RNAs (siRNAs) which can interfere with plant growth and development

  
• Prokaryotes are abundant for over a billion years before eukaryotes appeared
• 90-99% of prokaryotes are unknown and undescribed
• Prokaryotic domains

  • Bacteria
  • Archaea

  
• Archaea

  • Many are extremophiles (they live in hotsprings, in hypersaline environments that would dehydrate other cells, and in atmospheres rich in toxic gases such as hydrogen sulfide)
  • Have special modifications in the lipids of cell membranes and components of cell wall
• Prokaryotes

  • Unicellular
  • Cell size varies tremendously, most are less than 1 μm in diameter
  • Have a single circular double-stranded DNA chromosome in the nucleoid region
  • Often have plasmids
  • Divide by binary fission
  • Exchange genetic material extensively through horizontal gene transfer
• Prokaryotes have no membrane-bounded organelles and no internal compartments, but their plasma membrane can be extensively infolded
• Prokaryotic flagella

  • Simple in structure, different from eukaryotic flagella