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Acellular Biological Entities: Viruses, Viroids, & Prions (Outline)
• Acellular entities as disease agents • Compare and contrast viruses and living cells • Parasitic nature of viruses:
– Host-cell specificity: relate to common human diseases – Insertion of some viral genomes into host DNA
• Life cycle of a retrovirus such as HIV • Role of ancient and current retroviruses in shaping animal
genomes. • Role of ancient and current viral infections as selective
pressure affecting human genetic variability
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Acellular Disease-causing biological entities (Health Connection)
Simple infectious agents Virus- genetic material in transit from one host cell to the next
Viroid- Circular naked RNA
Prion- misfolded infectious proteins
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Acellular Disease-causing biological entities (Health Connection)
Virus • Childhood diseases • Age-independent disease • Emerging diseases Viroid
• Plant diseases
Prion • Scarpie • Mad Cow disease • Kuru- Fore tribe of Papua New Guinea
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Viroids and Prions: The Simplest Infectious Agents
• Viroids are circular RNA molecules that infect plants and disrupt their growth- Cadang-Cadang of coconut trees
• Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
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Prion diseases • Caused by mis-folded infectious proteins, lack
nucleic acids, cause molecules of the normal protein to mis-fold
• Diseases caused by prions affect the nervous system
Scrapie in sheep Mad cow disease Kuru in humans Creutzfeldt-Jakob Disease (CJD) in humans Fatal Familial Insomnia (FFI)
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Viruses are genes packaged in protein – Biological non-living entities – Have no cytoplasm – Cannot self-replicate – Cannot metabolize – Genetic material either DNA or RNA never both
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Viruses
• To replicate they need to infect a living cell • Every living cells has one or more viruses that
can infect it, specifically.
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RNA
Capsomere
Capsomere of capsid
DNA
Glycoprotein 18 250 nm 70–90 nm (diameter)
Glycoproteins
80–200 nm (diameter) 80 225 nm
Membranous envelope RNA
Capsid
Head DNA
Tail sheath
Tail fiber
50 nm 50 nm 50 nm 20 nm (a) Tobacco mosaic virus
(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
Naked Enveloped Complex
Viral Shapes
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Viral Structures
• Virion: an individual viral particle • Capsid: protein coat surrounding the nucleic acid • Nucleocapsid- nucleic acid and protein capsid
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Basis of Host-Range of Viruses
• Host range is determined by “lock-and-key” fit between virus surface and cellular receptors on host cell
• Most viruses infect only specific types of cells in one host Narrow host range with tissue specificity – cold viruses: upper respiratory tract cells. – HIV, AIDS virus: a certain white blood cell. • Some have a broad host-range infecting multiple
species – rabies
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Membranous envelope
RNA
Protein coat
Glycoprotein spike
Viral genomes are made of either DNA or RNA – Flu viruses are RNA – Genital warts virus (HPV) and Herpes virus are DNA
viruses http://www.cdc.gov/STD/HPV/STDFact-HPV.htm
HPV Influenza
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Plant viruses are serious agricultural pests – Most plant viruses have RNA genomes
Enter their hosts via wounds in the plant’s outer layers
Protein RNA
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Emerging viruses threaten human health
Colo
rized
TEM
50,
000×
Colo
rized
TEM
370
,000×
Figure 10.20A, B
Ebola Virus (RNA) SARS Virus (RNA)
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Phage DNA
Phage
The phage injects its DNA.
Bacterial chromosome
Phage DNA circularizes.
Daughter cell with prophage
Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.
Cell divisions produce population of bacteria infected with the prophage.
The cell lyses, releasing phages.
Lytic cycle
Lytic cycle is induced
or Lysogenic cycle is entered
Lysogenic cycle
Prophage
The bacterium reproduces, copying the prophage and transmitting it to daughter cells.
Phage DNA integrates into the bacterial chromosome, becoming a prophage.
New phage DNA and proteins are synthesized and assembled into phages.
Bacteriophage Life Cycles
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• One of many reasons for why bacteria cause disease is the production of a toxic protein.
• Some proviral genes code for toxic proteins. • Examples:
o Diphtheria o Botulism o Scarlet fever o E. coli food poisoning
Prophage-mediated Diseases
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Animal Virus Life Cycle • Attachment • Entry • Uncoating of virion separate protein from NA • Replication of nucleic acid and synthesis of viral
proteins • Maturation of virions (assembly of NA and proteins) • Release: cell lysis or budding
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The AIDS virus – HIV is an RNA retrovirus – It makes DNA using RNA template – Inside a cell, HIV uses its RNA as a template to make a DNA
copy of itself, which integrates into the host genome. http://www.susanahalpine.com/anim/KubyHTML/HIV.htm
Envelope
Glycoprotein
Protein coat
RNA (two identical strands)
Reverse transcriptase
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About 8% of our genome is derived from RNA viruses called retroviruses
- This is evidence of past infection - Sequences tend to increase over time
Viral DNA
Figure 11.11
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Roles of endogenous retroviruses
Genes of endogenous retroviruses code for important proteins for:
• Placenta formation • Large Brain development • Powerful emotions
Explorer: The virus hunters (DVD)
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Role for Viruses in shaping the Human Genome
• Past retroviral infection in a primate ancestor – Insertion of sequences – New proteins or protein domains for host that
maybe beneficial and influence behavior
• Present retroviral infections add to the genetic variability of the human populations