acellular biological entities: viruses, viroids , & prionsfaculty.sdmiramar.edu/bhaidar/bio 210a...
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Acellular Biological Entities: Viruses, Viroids, & Prions (Outline)
• Acellular entities as infectious agents of animal and plant diseases • Structure and functional properties. • Viral structural components: genome (DNA or RNA), capsid, nucleocapsid, and
envelope. Naked and enveloped viruses • Parasitic nature of viruses
– Life cycle of bacteriophages and relationship to human disease – Host-cell specificity: common human diseases – Life cycle of animal viruses and association with susceptibility to infection. – Human genetic variability in susceptibility to viral infections
• Life cycle of the Ebola virus • Life cycle of a retrovirus such as HIV • Role of ancient and current retroviruses:
– In shaping the human genome; addition of genetic variability – as selective pressure for human genetic variability
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
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
Prion diseases • Prions are mis-folded infectious proteins, lack nucleic
acids • Prions cause disease by converting normal proteins
into the prion version
• 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) The Truth Will Out: Is vCJD Caused by BSE?
http://ffh.films.com/PreviewClip.aspx?id=5955 (5 mins)
Acellular Disease-causing biological entities (Health Connection)
Virus • Childhood diseases • Age-independent disease • Emerging diseases – Discovery Channel Video Clip (Textbook site)
Viroid • Plant diseases
Prion • Scarpie • Mad Cow disease • Kuru- Fore tribe of Papua New Guinea
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
Viruses
• To replicate they need to infect a living cell • Every living cells has one or more viruses that
can infect it, specifically.
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
Viral Structures
• Virion: an individual viral particle • Capsid: protein coat surrounding the nucleic acid Capsid made of individual protein subunits
(capsomeres) gives the virion its shape • Nucleocapsid- nucleic acid and protein capsid
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
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
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
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)
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
Lytic Cycle
• Attachment • Entry of DNA • Degradation of host DNA • Replication of nucleic acid and synthesis of
viral proteins • Packaging/Assembly of
phage particles (DNA and proteins)
• Release by cell lysis
Bacteriophage Life Cycles Lysogenic Cycle
• Attachment • Entry of DNA • No degradation of host
DNA • Integration of nucleic into
bacterial genome • Prophage: an integrated
phage • Exit to lytic cycle induced
by chemicals and high energy radiation
• 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
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 http://www.youtube.com/watch?v=cE
0qdqoBFa8
The Ebola virus – An enveloped RNA (- strand) Entry and exit – http://www.youtube.com/watch?v=57P6MuM3_F8 Pathogenesis – http://www.openculture.com/2014/11/the-ebola-virus-
explained-with-animated-video.html
Transmission Links http://www.cdc.gov/vhf/ebola/transmission/index.html?s_cid=
cs_284 http://www.who.int/mediacentre/factsheets/fs103/en/
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
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 in the Human Genome
Figure 11.11
Roles of human endogenous retroviruses
Endogenous retroviruses • Placenta formation • Large Brain development • Powerful emotions
Explorer: The virus hunters (DVD) Natgeotv.vom http://channel.nationalgeographic.com/series/expl
orer/3828/Overview#tab-Videos/06253_00
The Virus Hunters (DVD)
• Retroviruses in human genomes (first 12:30 mins)
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
The Virus Hunters (DVD)
• Role of ancient retroviruses in major evolutionary steps
(12:30- 15:30 mins)
The Virus Hunters (DVD)
• Role of integrated viruses in complex emotions and behavior (~40:30 to 46:20 mins)
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