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NO T ES :
2 0 .1 - V IRU S ES
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Vocabulary
– 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 non-
cellular 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
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
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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: rod-
shaped; tadpole-shaped; helical;
cubelike (see pg 575)
T-4 bacteriophageHuman Influenza
virus particles
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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
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
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