VIRUS DEFINITION Non-Living particle that causes disease Pathogen: any agent that causes disease
Human papillomavirus: cause of sexually transmitted infections like genital wartssmallpox
HISTORY OF VIROLOGYSmallpox epidemics
Reached epidemic size before 1800 Thousands died or survivors were disfigured Survivors were thereafter immune Started inoculating with a mild form Problem: risk of dying!
HISTORY OF VIROLOGYMilkmaids and Cowpox
Milkmaids seemed immune to smallpox Milkmaids often got cowpox (harmless) Use cowpox to safely immunize
HISTORY OF VIROLOGY1796 Edward Jenner
Took matter from cowpox sore on milkmaid Injected cowpox matter into a boy Inoculated boy with smallpox
Boy remained healthy!
VIRUSES SEEM ALIVE
Contain protein and DNA or RNA Reproduce Evolve
HIV has an amazing knack for evolving
VIRUSES DON’T SEEM ALIVE!
Obligate parasites: must have host cell Cannot move: spread by wind, water, food, secretions Smaller than bacteria: 20 to 250 nanometers
100 flu viruses = length of 1 bacterium!
VIRUSES ARE NOT CELLS
NOTE: No cell membrane, nucleus or organelles!
CAPSID (protein coat)
CORE (DNA or RNA)
SPIKES (chemical recognition)
ENVELOPE (surrounds capsid)
How do viruses spread?
Virus is non-living outside the host cell Spread by air, water, food, body fluids
HOW DO VIRUSES RECOGNIZE A HOST?
Glycoproteins: spikes of carbohydrate + protein Embedded in viral envelope
HOW DO VIRUSES RECOGNIZE A HOST?
Viral glycoproteins can recognize receptor proteins on the host cell
How do viruses find a host cell?
Virus needs to recognize host cell before infection Envelope protein “fits” host receptor molecule
lock
key
envelope protein
host receptor molecule
How are viruses classified?
Not classified in any kingdom of life Classification is based on 3 characteristics
1 capsid helical, icosahedral,
spherical, head and tail, or filamentous?
2 envelope present or absent?
3 nucleic acid DNA or RNA?
What is a helical capsid? In some viruses the capsid or protein coat is a
corkscrew shape
RNA
capsid
What is an icosahedral capsid?
capsid
DNA
In some viruses, like the polio virus, the capsid or protein coat, is a geometric shape
What is a spherical capsid?
capsidRNA
In some viruses, like the influenza virus, the capsid or protein coat, is a ball shape
What is a filamentous capsid?
In some viruses, like the Marburg virus or the ebola virus, the capsid or protein coat, is a stringy shape
What is a Head and Tail capsid?
In some viruses, like a bacteriophage, the capsid or protein coat, is a stringy shape
HOW DO VIRUSES REPLICATE?The Lytic Cycle
(DNA) + TRANSLATION (CAPSIDS)
: DNA + CAPSIDS
LYSIS!
HOW DO VIRUSES REPLICATE?The Lysogenic Cycle
lysis
AIDS (Acquired Immune Deficiency Syndrome)
Caused by Human Immunodeficiency Virus (HIV) Symptoms: immune system failure Transmission
AIDS (Acquired Immune Deficiency Syndrome)
Structure and size: enveloped icosahedron, 125 nm Specificity: single host, human Perhaps 2 million Americans infected Much worse in Africa and Caribbean
HIV Replication
AIDS (Acquired Immune Deficiency Syndrome)
Prevention means lowering transmission Potential cures
Interfere with
RNA transcriptase Interfere with
translation of protein
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