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Chapter 19: Viruses (Viral Genetic Material (Class I. (Double-stranded DNA…
Chapter 19: Viruses
Viral Genetic Material
Double- or single-stranded DNA, double- or single-stranded RNA.
Most plant viruses RNA, variety of RNA genomes found among viruses that affect animals.
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Evolution
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Virus found that affect every form of life, not just plants and animals.
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Hypothesis: Viruses originated from naked bits of cellular nucleic acids that moved from one cell to another.
Plasmids and Transposons
Plasmids are small, circular DNA molecules found in bacteria and yeasts.
Plasmids replicate independently, occasionally transferred between cells.
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Virus Replication
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Only affect cells within limited number of host species, host range.
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Virus Replicative Cycle
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May be taken up by endocytosis, fusion of viral envelope with host's plasma membrane.
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Hosts provides enzymes, nucleic acids, ATP, amino acids, etc. to make viral proteins.
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Phages
Lytic Cycle
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Infect healthy cell, can destroy entire bacterial population in a few hours.
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Lysogenic Cycle
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Break both circular DNA molecules and join them together, prophage.
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Bacterial Defenses
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Natural selection favors bacterial mutants with surface proteins that phages no longer recognize as receptors.
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Viral Diseases
Animals
Vaccines are harmless derivative of a pathogen that simulates the immune system to defend against the pathogen.
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Emerging Viruses
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West Nile Virus
Appeared in North America in 1999, spread to all 48 continuous states.
Over 40,000 cases and 2,000 deaths.
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Ebola Virus
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Cause Hemorrhagic fever that is often fatal. Fever, vomiting, massive bleeding, and circulatory system collapse.
Outbreak in 2014, 27,000 illnesses and 11,000 deaths.
Chikungunya
Acute illness; fever, rashes, and persistent joint pain.
Zika Virus
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Infection of pregnant women correlated with increase in babies with abnormally small brains, microcephaly.
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Infleunza
2009, widespread epidemic appeared in Mexico and United States.
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Declared global epidemic, or pandemic, shortly thereafter.
207 countries, infected 600,000 people and killed almost 8,000.
Three Types
B and C infect only humans, never cause an epidemic.
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Three Processes
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Dissemination of viral disease from small, isolated human population.
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Plants
More than 20,000 types known, annual loss of $15 billion.
Bleached, brown spots, stunted growth, damaged flowers and/or roots.
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Horizontal Transmission
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More susceptible is damaged by wind, injury, or herbivores.
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Plasodesmata, cytoplasmic connections that pentrate the walls between adjacent plant cells, can spread viral genomes.
Prions
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Scrapie, mad cow disease, etc.
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Act very slowly, ten years before symptoms develop.
Virtually indestructible, no known cure to date.
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Overlook of Viruses
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Structure
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Largest known is 1,500 nanometers, barely visible under light microscope.
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Revealed it is infectious particle consisting of nucleic acid in protein coat, some surrounded by membranous envelope.
Viral Genomes
Genomes have double-stranded DNA, single-stranded DNA, double-stranded RNA, and single-stranded RNA.
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Some contain multiple molecules of nucleic acid, smallest have 3, largest have several hundred to 2,000.
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