NO T ES :
2 0 .1 - V IRU S ES
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 Vocabulary
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Virus
Capsid
Lytic cycle
Lysogenic cycle
Retrovirus
 Key Concepts
– What is a virus?
– How do viruses
reproduce?
– How do viruses
make us sick?
– Importance of
viruses
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What is a virus?
 viruses are noncellular particles
made up of genetic
material (DNA or
RNA… not both) and
protein that invade
living cells
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Are Viruses Alive?
 like living things, viruses have genetic material
and they can pass this on to future generations
 BUT, unlike living things, viruses are NOT made
of cells
 outside of a host cell, they don't reproduce,
feed, or grow
 therefore, most biologists consider viruses
NONLIVING
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What do viruses do?
 viruses can invade the cells of plants,
animals, and bacteria
 viruses cause illnesses like colds, flu,
chicken pox, herpes, AIDS, polio, rabies,
measles, mumps…
Chicken pox
How are viruses named?
● since viruses are non-living they are not
named in the same way that living things are
named
● many viruses are named for the disease that
they cause or the organ/tissue they infect
● EX: rabies virus; poliovirus
adenovirus
(infects adenoid tonsils)
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What do viruses look like?
 a virus is made of a core of genetic material
surrounded by a protein coat (CAPSID)
 viruses are SMALL!! (20-400 nanometers)
Bar = 100nm
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 viruses come in a variety shapes: rodshaped; tadpole-shaped; helical;
cubelike (see pg 575)
Human Influenza
virus particles
T-4 bacteriophage
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How does a virus get into a host cell?
● the proteins on the outer coat (capsid) of
the virus attach to a receptor site on the
host cell’s plasma membrane
● Analogy: the 2 proteins fit together like 2
puzzle pieces
● this attachment process is SPECIFIC!
 each virus can usually only attach to a
few kinds of cells
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How do viruses reproduce?
1) LYTIC CYCLE:
 viruses insert their genetic material into a
host cell
 once inside, the viral genes take over the
cell’s machinery (ribosomes, etc.)
 the “hijacked” cell transcribes the viral genes
-uses the host cells own enzymes (e.g. DNA
polymerase) & raw materials to make
thousands of new viruses
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1) LYTIC CYCLE
 the cells LYSE (burst) when the concentration
of new viral particles is high
 the lytic cycle has its name because the
host cell lyses and allows viral particles to
escape
 the new viruses can then infect other host
cells
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2) LYSOGENIC CYCLE:
● begins like the lytic cycle (virus injects its
DNA into host cell);
● the viral DNA is integrated into the host
cell’s chromosome – now called a
PROVIRUS.
● the provirus may remain dormant, but every
time the host cell reproduces, the provirus
is also replicated and passed along to the
new daughter cells.
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2) LYSOGENIC CYCLE:
● the lysogenic phase may continue for many
years;
● at any time, the provirus may be activated
and enter a LYTIC CYCLE.
EX: cold sores caused by herpes simplex I
virus (the provirus remains in your cells;
when it enters a lytic cycle, a cold sore
forms)…WHY does this happen?
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Examples of Lysogenic Viruses:
● herpes simplex I and II
● hepatitis B virus
● chicken pox (may erupt later in life as
shingles)
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RETROVIRUSES:
 contain RNA as their genetic material
 retroviruses infect a cell and produce a DNA copy of
their RNA
-(retro = backward; RNA is copied in to DNA)
 this DNA is inserted into the host cell's DNA
 example of a retrovirus:
HIV
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Retrovirus example: HIV
● HIV = human immunodeficiency virus
● infects white blood cells (helper T cells)
● the viral genetic material becomes a
provirus – infected person may not know
it, but can still transmit the virus
● eventually, white blood cell count drops
too low and AIDS develops
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How Can You Protect Yourself
From Viral Infections?
• The best way: use of
VACCINES
-preparation of weakened or
killed virus or viral proteins
-when injected, the vaccine
stimulates the immune
system
-sometimes produces
permanent immunity
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How Can You Protect Yourself
From Viral Infections?
• Protect yourself
-stay away from known sick people
-wash your hands often
-safe practices (i.e. coughing /
sneezing into a tissue or your
sleeve; don’t share food or drink
with sick people)
Viral Diseases:
● common cold
● influenza
● AIDS (HIV)
● chicken pox
● hepatitis B
● West Nile Virus
● human papillomavirus
(HPV) – linked to forms
of cancer