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characteristics of viruses
- smaller than any living cell
- can pass though filters
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how big are viruses
- size ranges from 20-400 nm
- only seen under electron microscopes
- infect humans, animals, plants & bacteria
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are viruses alive
- no, they can only replicate in living cells (can't reproduce on its own, must invade a host cell)
- employ the host cell's replicative and metobolic pathways
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what are viral disease
- animals: rabies, foot and mouth disease
- humans: hepatitas, polio, influenza, AIDS and SARS
- others: smallpox, yellowfever, chicken pox, measles and mumps, HIV, west nile, ebola, etc
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how do viruses take over a host cell
- protein synthesis is how cells make proteins
- 1. living cells carry all the genetic info (DNA) to make copies of themselves stored in chromosomes
- 2. DNA molecules are copied into mRNA molecules to make proteins for the structure of the cell
- 3. when cells divide they replicate their DNA to give two identical copies of the cell's DNA
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define viral replication
- viruses carry DNA, they use it to make a few special proteins which 'hijack' the host cell and forces it to make new viruses
- all viruses have a protein that recognizes the proper host cell
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what are the 2 types of cycles
- lytic cycle
- lysogenic cycle
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define lytic cycle
- virus reproduces itself using the host cell's chemical machinery
- virus injects its genetic material and 'tricks' the cell into replication which releases many viruses
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define lysogenic cycle
- has a dormant phase
- inside the host cell, some viruses do not reproduce right away
- they mix their genetic instructions into the host cell's DNA, host cell reproduces, viral info does too
- environmental or genetic signal will stir the 'sleeping' viral instructions
- lysogenic cycle reverts back to the lytic cycle
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can viruses be cured
- immunizations: work by pre-infecting the body with a weak or dead form of the virus so the body builds up antibodies to it
- antibiotics have no affect
- vaccines only work against viruses with non-varying proteins; if surface proteins mutate often, new changes aren't recognized
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what are the factors of how a virus is spread
- Factors:
- size of the population
- number of days contagious
- number of people came in contact with affected person
- probability of contracting the virus
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what ways are viruses spread
- airbourne
- water
- blood/bodily fluid contact
- surfaces
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define epidemic
- a widespread outbreak of an infectious disease
- many people are infected at the same time
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define pandemic
- an epidemic that is geographically widespread
- occurring throughout a region or even throughout the world
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define endemic
confined to a specific area
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