Viruses
Chapter 18 Pg 489-499
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What are Viruses
A virus is a non-cellular particle made up of
genetic material and protein that can invade
living cells.
 Non-living= doesn’t carry out respiration,
grow, develop, or reproduce on its own
 Living cells used in replication = host cells
Naming Viruses
 Viruses are not named like living organisms, i.e.
using binomial nomenclature.
 Named after:
 the disease it causes (ex. Rabies, polio)
named after the person(s) who discovered them
after tissue they infect (ex. Adenovirus infects adenoid
tissue…common cold)
Some given a genus and species name or code
number
T4 Bacteriophage
Bacteriophage a virus that infects bacteria.
The Structure Of a Virus
 a core of nucleic acid
(DNA or RNA)
 Nucleic acid core is
surrounded by a
protein coat (capsid)
 Some viruses have
additional envelope
(made of cell
membrane parts)
ex. Human flu
THE END
Viruses come in many shapes and sizes
•Protein arrangement determines shape
• Shape determines cells that can be infected (page
491)
The Influenza Virus!
1918 Influenza or
Spanish flu Protein.
Common flu structure
Spanish Flu continued
http://www.pbs.org/wgbh/nova/sciencenow
/video/3318/q02-220.html
What does all this mean…?
Viruses use Host Cell to Replicate
Virus recognizes
and attaches to
host cell
Each virus has
specifically shaped
proteins that
interlock with
receptor sites on
host cell
Viruses are mostly
species specific or
cell type specific
• Smallpox is said to
be eradicated
because it is
human specific
• Flu is not human
specific
Viral Replication
Virus inject nucleic acid into cell
A virus can have two different cycles of
replication: lytic and lysogenic
Viral Replication Cycles: Lytic Cycle
Lytic Cycle
Analogy
A. Virus attaches to cell
B. Nucleic Acid is injected
into cell
C. Host cell makes viral
nucleic acid and proteins
D. New virus particles are
assembled
E. Host cell breaks open to
release new virus
particles
1. An army tank (virus) filled with
enemy troops (nucleic acid)
2. Crashes through the wall of an
automobile factory (host cell)
3. Troops take over factory’s
machinery (nucleus) and adapt
them to make new tanks
(viruses) instead of cars (cell
parts)
Viral Replication Cycles 2: Lysogenic
Lysogenic Cycle
A. Virus attachment and entry into cell
B. Viral nucleic acid becomes part of host’s
chromosomes (provirus - viral DNA that
is part of the host cell’s chromosomes)
C. Viral nucleic acids are replicated during
chromosome replication
At some point it will enter lytic cycle
Lytic vs. Lysogenic Examples
Lytic = Measles, cold viruses
Lysogenic = shingles, warts, HIV
Disease Symptoms of provirus's
Cold sores – herpes simplex I virus
- virus remains in cells as provirus,
when enters lytic cycle, new cold sore
erupts
• Possible triggers for activating lytic cycle =
physical stress (ex. Sunburn), emotional
stress (ex. Anxiety)
• Chicken pox…later becomes shingles
Release of viruses
Lysis – bursting of cell
Exocytosis
Cycle of Lytic and Lysogenic
Flu viruses are spread mainly from
person to person through:
coughing, sneezing or talking by people
with influenza.
Sometimes people may become infected
by touching something – such as a surface
or object – with flu viruses on it and then
touching their mouth or nose.
Vaccines
Viruses grown on chicken embryos are
attenuated vaccines
Another type of vaccine is made by heat
“killing” the virus
These non-harmful forms of virus are
injected into body so that the body can
learn how to defend itself
Retrovirus
 Nucleic Acid is
RNA (not DNA)
 Once RNA is
injected into host
cell, it will make
viral DNA with
help from an
enyzme (reverse
transcriptase)
 Viral DNA
becomes a
provirus (page
495)
HIV = infection of white blood cells
Infected host cells function normally –
provirus
No symptoms – can still transmit virus in
body fluids
White blood cells will be lost when provirus
enters lytic cycle
Immune system is compromised
HIV Info
 HIV becomes AIDS when the number of immune
cells drops below a predetermined number
 No one dies from HIV or AIDS; people die from
secondary infections (ranging from the common
cold to cancer)
 More than 3 million people (size of Chicago) die
each year
There are approx. 14,000 new cases of
HIV worldwide every day
Viruses and Cancer
Retroviruses may cause cancer - Normal
cells become tumor cells
DNA viruses, papilloma virus (HPV) and
hepatitis B virus may cause cancer