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Viruses
Virus – Latin for “toxin or poison”
Particles of nucleic acid, protein, and sometimes lipids
Enter living cells and use the cell to produce more viruses
Are viruses living?
To be classified as living one has to:
– Be made of cells– Reproduce independently– Have a genetic code– Obtain and use energy– Respond to environment– Change over time
Do viruses match these criteria:
– No– No– Yes– No– No– Yes
Viruses
Viruses have a core of DNA or RNA surrounded by a protein coat
Protein coat is called a capsid– Enables virus to enter host cell
Can live outside of a host in a crystallized form
Virus Structures: Helical - plant viruses
Tobacco Mosaic Virus – infects tobacco plants
Virus Structures: Icosohedral (spherical) – animal viruses
Influenza virus
Virus Structures: Complex
T4 Bacteriophage - infect bacteria
Virus Structures: Complex
Poxvirus (smallpox)
Viral Infections
Bacteriophages can cause two types of viral infections– Lytic – Virus DNA enters cell, makes new copies of
virus, and causes lysis Lysis = cell death, cell bursts open
– Lysogenic – virus inserts DNA into DNA of host cell so it can replicate with the host DNA
Lytic and Lysogenic Infections
Retroviruses
Virus RNA is translated into DNA The DNA copy is then inserted into the host
cell’s DNA Often remains dormant before directing
production of new viruses Causes cell death retro- means “backward”
Human Immunodeficiency Virus (HIV) - retrovirus
Virus Classification
Viruses are classified according to1. the type of nucleic acid (DNA or RNA)
2. Whether the nucleic acid is single or double stranded
3. Whether the virus has an envelope
4. By the type of infections that occur
There are about 1,550 known virus species and about 30,000 known strains
Modes of transmission
Vertical transmission (mother to child) - the baby is born with the virus. Ex. HIV or Hepatitis B
Horizontal transmission (person to person) - most common– Blood exchange, sexual activity, saliva exchange,
breathing (aerosol affect), insect vectors, or contaminated food or water
Diseases Caused by Viruses
Common Cold Chicken Pox
Diseases Caused by Viruses (cont’d)
Herpes Simplex Hepatitis B (liver)
Diseases Caused by Viruses (cont’d)
Polio (paralysis) Ebola (hemorrhage)
Vaccinations
Help prevent infection Can consist of live or killed viruses Vaccines build up the body’s immune response
for when the actual virus enters the body
Cow pox vaccination 1749
Acquired immunization
Artificial injection of a small amount of virus
Body’s immune response makes antibodies