CHARACTERISTICS OF VIRUSES
Pre-Quiz (True or False)
1.
2.
3.
4.
5.
6.
Many common illnesses are caused by viruses.
Viruses are easily killed by drugs such as antibiotics.
Once you have contracted a specific virus, you will not get
sick from this virus again because of the antibodies produced
in your body.
Many diseases caused by viruses can be avoided through
vaccination.
Viruses are very fragile and cannot live on surfaces outside fo
the human body.
Viruses have the ability to change themselves, or mutate, over
time.
Pre-Quiz Continued
7. Viral illnesses cannot be transferred from animals to people.
8. The only way to transmit viruses is through close physical
contact with an infected person.
9. Viruses can become resistant to drugs used to treat them.
10. One of the best ways to prevent the spread of viral diseases is
frequent, thorough hand washing.
Are Viruses Living or Non-living?


Biologists consider viruses to be non-living because:
– Are not cells
– Do not use energy to grow or respond to their
surroundings
– Cannot make food, take in food, or produce wastes
– Viruses do not respond to stimuli in the environment
such as light, temperature, chemicals, sound,
gravity, heat, water and pressure.
They can only multiply like other organisms but only
inside a living cell
What are Viruses?
Definition


Viruses are noncellular
particles made up of
genetic material and protein
that can invade living cells.
Small, non-living, invades
and reproduces inside a
living cell
1 virus- can produce
thousands of new viruses
Discovery of Viruses
Beijerinck
(1897)
coined the Latin name
“virus” meaning poison
He
studied filtered
plant juices & found
they caused healthy
plants to become sick
6
Tobacco Mosaic Virus
Wendell
Stanley (1935)
crystallized sap from
sick tobacco plants
He
discovered viruses
were made of nucleic
acid and protein
7
Smallpox
Edward
Jenner (1796)
developed a smallpox
vaccine using milder
cowpox viruses
Deadly viruses are said
to be virulent
Smallpox has been
eradicated in the world
8 today
How Big is a Virus?

Viruses are very small – smaller than the
smallest cell.
–
They may vary in size from 20-400 nm.
 Measured in nanometers (nm = one billionth of
a meter)
 one millionth of an inch
 1000x smaller than bacteria
 Bacteria = much smaller than a human cell
Characteristics
Non living structures
 Non-cellular
 Contain a protein coat called the capsid
 Have a nucleic acid core containing DNA
or RNA (one or the other - not both)
 Capable of reproducing only when inside
a
HOST
cell
10

Characteristics
CAPSID
Some viruses are
enclosed in an protective
envelope
 Some viruses may have
spikes to help attach to
the host cell
 Most viruses infect only
SPECIFIC host cells ENVELOPE

11
DNA
SPIKES
Characteristics
Outside
of host cells,
viruses are inactive
EBOLA VIRUS
Lack ribosomes and
enzymes needed for
metabolism
Use the raw materials
and enzymes of the host
cell to be able to
HIV VIRUS
12 reproduce
Characteristics
Viruses
diseases
cause many common illnesses
Smallpox,
measles, mononucleosis,
influenza, colds, warts, AIDS, Ebola,
the “flu,” chicken pox, measles, polio,
and hepatitis
Some
13
viruses may cause some
cancers like leukemia
Virus-free cells are rare
MEASLES
What do Viruses look like?


Viruses are unusual and different from other things in
nature.
Viruses come in a variety of shapes
–
Some are rod-shaped, round-shaped, tadpole-shaped, manysided, helical, cube-like and so on.
Some
may be helical shape like the Ebola virus
Some may be polyhedral shapes like the influenza virus
Others have more complex shapes like bacteriophages
THEY JUST DON’T LOOK LIKE OTHER THINGS!
Types of Viruses:
Helical Viruses
15
Polyhedral Viruses
16
Complex Viruses
17
Viral Taxonomy
Family names end in -viridae
 Genus names end in -virus
 Viral species: A group of viruses sharing
the same genetic information and
ecological niche (host).
 Common names are used for species
 Subspecies are designated by a number

18
Viral Taxonomy
Examples






19
Herpesviridae
Herpesvirus
Human herpes virus 1, HHV 2, HHV 3
Retroviridae
Lentivirus
Human Immunodeficiency Virus 1, HIV 2
Used for Virus
Identification
 RNA
or DNA Virus
 Do or do NOT have an envelope
 Capsid shape
 HOST they infect
20
Herpes Virus
SIMPLEX I and II
21
Adenovirus
22
COMMON COLD
Influenza Virus
23
Chickenpox Virus
24
Papillomavirus – Warts!
25
Bacteriophages
26
Phages
Viruses
27
that attack
bacteria are called
bacteriophage or just
phage
T-phages are a specific
class of bacteriophages
with icosahedral heads,
double-stranded DNA, and
tails
T-phages
The
28
most commonly
studied T-phages are T4
and T7
They infect E. coli , an
intestinal bacteria
Six small spikes at the
base of a contractile tail
are used to attach to the
host cell
Inject viral DNA into cell
Retroviruses
29
Characteristics of
Retroviruses
Contain
RNA, not DNA
Family Retroviridae
Contain enzyme called Reverse
Transcriptase
When a retrovirus infects a cell, it injects
its RNA and reverse transcriptase enzyme
into the cytoplasm of that cell
Examples: HIV, the AIDS virus, Feline
30
Leukemia Virus
Viroids
Small,
circular RNA
molecules without a
protein coat
Infects
Potato
Ireland
31
plants
famine in
Prion Diseases
Prions
RNA
Prions
32
have no DNA or
form insoluble
deposits in the brain
Causes neurons to rapidly
degeneration.
Mad cow disease (bovine
spongiform encephalitis:
BSE) is an example
People in New Guinea
used to suffer from kuru,
which they got from
eating the brains of their
enemies
Viral Attack






33
Viruses are very specific as to which species they
attack
HOST specific
A vector is a host that transfers viruses to another
organism
Example – West Nile Virus is spread through
mosquitoes
Humans rarely share viral diseases with other animals
Eukaryotic viruses usually have protective envelopes
made from the host cell membrane
5 Steps of Lytic
Cycle
1. Attachment to the cell
 2. Penetration (injection) of viral DNA
or RNA
 3. Replication (Biosynthesis) of new viral
proteins and nucleic acids
 4. Assembly (Maturation) of the new
viruses
 5. Release of the new viruses into the
environment (cell lyses)

34
Bacteriophage
Replication
Bacteriophage
inject
their nucleic acid
They lyse (break
open) the bacterial
cell when replication is
finished
35
INFECTION
Viral Latency
Some
viruses have the ability to become
dormant inside the cell
Called latent viruses
They may remain inactive for long periods
of time (years)
Later, they activate to produce new
viruses in response to some external
signal
37 HIV and Herpes viruses are examples
Lysogenic Cycle
Phage
38
DNA injected into
host cell
Viral DNA joins host DNA
forming a prophage
When an activation signal
occurs, the phage DNA starts
replicating
Viral DNA (part of
prophage) may stay inactive in
host cell for long periods of
time
Latency in Eukaryotes
Some
eukaryotic viruses
remain dormant for many
years in the nervous
system tissues
 Chickenpox (caused by
the virus Varicella zoster)
is a childhood infection
It can reappear later in
life as shingles, a painful
itching rash limited to
small areas of the body
39
SHINGLES
Latency in Eukaryotes
SKIN TO SKIN CONTACT
Herpes
viruses also
become latent in the
nervous system
A
herpes infection lasts
for a person’s lifetime
Genital
herpes (Herpes
Simplex 2)
Cold
40
sores or fever
blisters (Herpes Simplex1)
PASSED AT BIRTH TO
BABY
HOST SPECIFICITY





All kingdoms can be infected by viruses
Viruses that infect organisms of one kingdom are
not able to infect organisms of another kingdom
Viruses can infect species of organisms that are
closely related or not closely related (some viral
diseases can be passed from different species)
Viruses can even be species specific (only passed
within the same species)
Spread is specific to the type of virus
-Carrier organisms, the air, direct transfer of body fluids
-Surfaces on which body fluids have dried
PARASITISM

Viruses are parasites.
–
–
–
A parasite is an organism that depends upon
another living organism for its existence in
such a way that it harms that organism.
Viruses must infect living cells in order to
carry out their functions of growth and
reproduction.
Viruses depend upon their hosts for
respiration, nutrition and all the other
functions that occur in living things.
Vaccines




43
An attenuated virus is a weakened, less
vigorous virus
“Attenuate" refers to procedures that weaken
an agent of disease (heating)
A vaccine against a viral disease can be made
from an attenuated, less virulent strain of the
virus
Attenuated virus is capable of stimulating an
immune response and creating immunity, but
not causing illness
Viral illness





Treatment of viral disease include vaccination,
vector control, and drug therapy
Drug therapy does NOT work very well
Vaccination and vector control are the most
successful way to beat viruses
Vaccination works because once you are exposed to
and overcome a virus your immune system creates
antibodies that will immediately attack the virus if it
shows up again
Hand-washing is effective against bacterial
diseases and viral diseases
Other Viral Treatments
Interferon
45
are naturally
occurring proteins made
by cells to fight viruses
Genetic altering of
viruses (attenuated
viruses)
Antiviral drugs (AZT)
Protease inhibitors –
prevent capsid formation
copyright cmassengale
Reducing the Risk

Cover mouth/nose when you sneeze of
cough
Wash hands frequently
Avoid contact with the body fluids

Not foolproof but reduces the risks

