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VIRUSES
Structure, Function, and
Reproduction
WHAT IS A VIRUS?
 A non-cellular particle made up
of genetic material and a
protein coat that can invade
living cells.
 Viral structure is very simple.
 Nucleic acid (DNA or RNA)
 Surrounded by a protein coat
(capsid)
 Viruses are much smaller than
even the smallest bacteria
 10-400 micrometers
 Complex
 Cylindrical
 Polyhedral
VIRUS SHAPES
Tobacco Mosaic
Virus
T4 Bacteriophage
Head
DNA
Influenza
Virus
RNA
Capsid
proteins
Capsid
RNA
Tail
sheath
Tail
fiber
Surface
proteins
Membrane
envelope
VIRAL
REPRODUCTION
 Viruses cannot reproduce on
their own
 They must invade, or infect, a
living host cell
 There are two different
reproductive cycles:
 Lytic cycle
 Lysogenic cycle
LYTIC CYCLE
 Attachment to the host cell
 Penetration
 Injects DNA/RNA into host cell
 Take over of the host cell activity
 Construction of new virus parts (What
are the new parts?)
 Assembly of new viruses
 Release of new viruses
 Viruses break out of (lyse) the host cell
 Host cell dies
 Ie. bacteriophage
THE LYTIC CYCLE
Bacteriophage
protein coat
Bacteriophage DNA
Bacterial
chromosome
Bacteriophage attaches to
bacterium’s cell wall
Bacteriophage enzyme lyses the
bacterium’s cell wall, releasing
new bacteriophage particles that
can attack other cells.
Lytic Cycle
Bacteriophage injects DNA
into bacterium
Bacteriophage proteins and
nucleic acids assemble into
complete bacteriophage
particles
Bacteriophage takes over
bacterium’s metabolism, causing
synthesis of new bacteriophage
proteins and nucleic acids
Bacteriophage
Bacteriophage DNA
Bacteriophage protein
LYSOGENIC CYCLE
 The virus does not reproduce and lyse its
host cell… at least not right away.
 Attachment to the host cell
 Penetration
 Viral DNA is inserted into the host cell’s
DNA
 Once inserted into the host cell’s DNA, the viral
DNA is known as a prophage
 Every time the host cell divides, it makes
more viral nucleic acid
 Does not harm the host cell
 May live in the host cell for many generations
 Eventually, the DNA of the prophage will
become active
 Prophage removes itself from host DNA
THE LYSOGENIC
CYCLE
Bacterial
chromosome
Bacteriophage
DNA
Bacteriophage injects
DNA into bacterium
Bacteriophage DNA
(prophage) can exit the
bacterial chromosome
Lytic
Cycle
Bacteriophage enzyme lyses the
bacterium’s cell wall, releasing
new bacteriophage particles that
can attack other cells
Lysogenic
Cycle
Bacteriophage DNA
(prophage) may replicate
with bacterium for many
generations
Bacteriophage DNA
forms a circle
Bacteriophage proteins and
nucleic acids assemble into
complete bacteriophage particles
Prophage
Bacteriophage DNA
inserts itself into
bacterial chromosome
VIRUSES
Beneficial and Harmful
Aspects
BENEFICIAL ASPECTS
 The presence of a prophage can
sometimes be beneficial
 Can block the entry of other viruses into the
cell
 Can add useful DNA to the host cells DNA
 Viruses are extremely valuable in genetic
research
 Gene therapy
 Introduce corrected genes into a victim of a
genetic defect (ie. Cystic fibrosis)
 It’s possible that modified viruses may one day
be used as medical tools
 Genetic engineering
 put gene that codes for big, red tomato plant in
inactive virus and inject virus into plant
BENEFICIAL ASPECTS
continued
 Viruses can be used to make
vaccines
 Substances that contain the
weakened or inactive diseasecausing virus
 When injected into the body,
vaccines provide immunity to the
disease
 They can only provide protection
if they are used before an
HARMFUL ASPECTS
 Viruses are parasites
 Depend entirely on the host cell for reproduction
and energy supply
 They harm the host cell
 Causes disease
 Only a small number of viruses are capable of
causing diseases in humans
 However, these pathogens (disease producing
agents) are responsible for a large amount of
human suffering
 Can infect:
 Plants (causing destruction of crops)
 Animals (including humans; influenza viruses
mutate rapidly and can spread from animals to
people)
 Bacteria (even the very smallest of single-celled
organisms are susceptible to viral infection)
COMMON DISEASES
CAUSED BY VIRUSES
Type of Virus
Nucleic Acid
Disease
Oncogenic viruses
DNA
cancer
Retroviruses
RNA
cancer, AIDS
Adenoviruses
DNA
respiratory infections
Herpesviruses
DNA
chickenpox
Poxviruses
DNA
smallpox
HOW ARE VIRUSES
TRANSMITTED?
 Air
 Chicken pox, measles, flu
 Contaminated food or water
 Hepatitis
 Mosquitoes
 Yellow fever, West Nile
 Sexual contact
 Herpes, AIDS
VIRAL SPECIFICITY
 Viruses have very specific host
requirements
 This is seen at the attachment step during
viral reproduction
 How does a virus recognize and attach to a
host cell?
 Viral capsid or envelope fits into a receptor on
the surface of the host cell like a lock and key
 If the lock and key don’t fit, then the virus cannot
enter the cell
ARE VIRUSES ALIVE?
 NO!!! They do not have cellular
characteristics
 They are not made of cells
 They cannot reproduce on their
own
 When did viruses evolve?
 Probably evolved from living cells
since they cannot “survive” without
a living cell