Viruses
Section 21.1
Learning Targets

Discuss the difficulties in classifying a virus.
 Describe, identify, and label the parts of a T4
Bacteriophage virus.
 Define:Virion, Lysis, Prophage, Bacteriophage, Host cell,
Vector, Interferon, Virulence
 Discuss the characteristics in which a virus is classified.
 Describe how viruses multiply: lytic and lysogenic cycles
(most likely will not get to this one)
 Discuss why viruses might be considered parasitic.
 Know the name of the person and the history of the first
virus that we identified
Are Viruses Alive?
 Do
they go through metabolism?
 Do
they grow and develop?
 Do
they reproduce?
 On
own?
Viral Characteristics

Smaller than the
smallest known cell
 Carry on no life
functions on own
(must have host)
 Replicate ( reproduce)
only if in a living cell –
parasite)
 Found in air, soil, and
water
What is the structure of a
virus? ( 2 main parts)

NUCLEIC ACID CORE



DNA or RNA
contains instructions only
for making copies of the
virus
PROTEIN COAT

Capsid to protect DNA/RNA
ENVELOPE (additional coat)
outside the capsid – some
Viruses, not all.
Viral envelope

surrounds the
capsid of some
large viruses
 consists primarily of
phospholipids
 influenza and HIV
Because of the UNIQUE shape of the protein
coat and/or the envelope, viruses are usually
very SPECIFIC as to what they attack.
-some viruses are limited to attacking a
SINGLE species of organism (1 type of host cell)
Ex: SMALL POX is found only in humans
Ex: POLIO only attacks the nerve cell
Virus Classification
 SHAPE
Type of NUCLEIC ACID (DNA / RNA)
Whether they have an ENVELOPE or not?
Type of HOST they infect
How they are transmitted (i.e. = vectors)
How they replicate
lytic or lysogenic
Viral shape (4 types)

determines
what cell
can be
infected &
how the
virus
infects the
cell
Polyhedral viruses – many
sided

many are polygons
with 20 sides –polio
Other Examples:
 Herpes (cold sores)
 Chix pox
 Cold virus
Spherical (i.e. HIV, some flu)
Rod Shaped (TMV, herpes, flu)
Polyhedral with a tail bacteriophage
Body Defenses Against Viruses
 WHITE BLOOD cells
 ANTIBODIES (specific, commpounds that tie up
specific viruses)
- can remain in the body and “REMEMBER”
viruses that have attacked before
 INTERFERON – protein that prevents viral replication
 VACCINES – harmless form of a virus that causes our
body to produce antibodies
History of Viruses
Tobacco mosaic virus

first virus to be discovered by
Dimetri Ivanoski in 1880’s

has a cylindrical shape

infects plants, especially tobacco

shows characteristic patterns
(mottling and discoloration) on the
leaves

TMV crystallized in 1935 ( living
organisms cannot be crystallized)
http:/babylon.com
Vector
 Organism
or object that transmits a
virus
Ex’s:
Rats
insects
birds
Bacteriophage

virus that infects
bacteria
 has a polyhedral
head containing
DNA, a protein tail,
and protein tail
fibers
 T4 virus
Virulence
 Degree
in which a virus causes a
disease
 Video:
Flu Attack (love this video)
– Rknot – reproductive rate
how efficient the virus is at spreading
- # people infected by one person
 R0
Viral Lecture part II
Label the bacteriophage!!
Viral Replication
A virus needs the Nucleic acids
and proteins found in living
cells to replicate ( reproduce)
2 Types of Reproduction…
Lytic Cycle:
the virus destroys the host
cell during reproduction
What causes cold symptoms?
1. Adsorption/ Attachment

every virus has a
specifically shaped
attachment protein
 each virus can only
attach to a few kinds
of cells
 like 2 pieces of a
jigsaw puzzle fitting
together
1. Virus attaches to cell with
tail fibers, to adhere to
receptors on the host cell.
http://www.studyblue.com/notes/note/n/phage-lyticcycle/deck/2958162
Bacteriophages attaching to bacterium
engineering.curiouscatblog.net
2. Entry and Invasion
1.
Virus can inject its nucleic acid
into the host cell
 Protein capsid stays attached to
the outside of the host cell
3. Replication
 The
viral genes take over the
host cell and force the host cell
to become a virus factory (coats
and nucleic acids are made)
4. Assembly
 New
viruses are assembled from
the protein coats and nucleic acid
made during replication
 Virions:
newly replicated viruses
5. Lysis and Release
 Enzymes
made by the invading virus
causes lysis – bursting of the host cell
 The
virions are released to go through
this cycle in hundreds of more of the
host’s cell
HIV(green dots) emerging from white
blood cell (littleenigma.tumblr.com)
http://www.mcatzone.com/t2.ph
Adsorption and Entry
Lytic cycle video clip
Lytic Cycle Examples
Cold (rhinovirus, cornavirus, adenovirus)
Flu (influenza)
Herpes Simplex Virus
Epstein Barr Virus ( cancer?)
Lysogenic cycle –
not immediate cell destruction

Provirus = viral DNA
that has been integrated
into host cell’s
chromosome



does NOT interfere with
normal functioning of host
cell
is replicated every time
host cell reproduces
can at any time pop out
of host cell’s
chromosome & enter a
lytic cycle
What are some diseases
caused by proviruses?

herpes simplex I
virus causes cold
sores
 hepatitis B
 chicken pox virus
can cause shingles
later in life
Lysogenic Cycle Stages
1.
2.
3.
Attachment and Entry – the viral DNA enters
the host cell similar to lytic cycle
Prophage formation ( DNA becomes part of
host cell DNA)
Eventual Lysis of infected host cells: some
time in future the infected host cells enter the
lytic phase and will cause lysis to release
thousands of viruses and symptoms appear
What is a retrovirus?

RNA is its only nucleic
acid
 contains reverse
transcriptase



enzyme that changes
RNA into DNA
viral DNA is integrated
into host cell’s
chromosome &
becomes a provirus
evidence for infection
by a retrovirus
What is HIV ( HUMAN
IMMUNODEFICIENCY VIRUS)?

retrovirus that causes AIDS

new viruses are produced slowly
because the viral genetic
material is a provirus ( lysogenic)
Transmission: through body fluid
-sexual contact
-needles and blood
-mother to fetus

infects white blood cells

function until proviruses enter a
lytic cycle & kill their host cells
HIV
 Symptoms:
swollen glands, loss of appetite and
weight, night sweats, fatigue.
Most people die of pneumonia or other
disease.
Facts on AIDS ( acquired immunodeficiency disease)
 CDC
in Atlanta survey says: (updated 2012)

25% of teens report having 4 different sex partners
in teen yrs
 35% of teens consistently use birth control.
 50% of teens are sexually active!
 1 in 50 teenagers have injected drugs

93-98% of teens are aware AIDS is transmitted by
sexual contact and I.V. drug use!


40,000-80,000 new cases each year
4,000+ from teenagers in high school
For those who received HIV Today…
 Every 9.5 minutes someone new is infected
with HIV
 10% of you will see no symptoms of AIDS
for 5 yrs.
 50% will see no symptoms for 10 years.
 40% will see no symptoms for 15 years.
 All will die within 10 years of the symptoms!
 AIDS
is a DEATH SENTENCE!
What are prions?

particles that
behave like viruses
& cause infectious
diseases


made up of proteins
but have no genetic
material
cause other proteins
to malfunction
 responsible for mad
cow disease
What are viroids?

particles that
behave like viruses
& infect plants


single circular strand
of RNA with no
protein coat
enter & infect host
plant through
wounds or insect
bites
 cause beautiful
patterns of color
Viruses that live in animals without causing harm, but
cause disease when they infect other animals or
humans are a real threat to society.- Emergent
Viruses
– They are difficult to study because it is not always
obvious where the source of the virus is, nor the means of
transmission…..An example of this is Ebola….movie contagion
WHAT IS THE CDC?