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Lesson 7-3
What are Viruses?
Characteristics of Viruses
• A virus is a strand of DNA surrounded
by a layer of protein that can infect and
replicate in a host cell.
Characteristics of Viruses (cont.)
• A virus:
is smaller than the smallest bacteria!
Viruses contain proteins,
nucleic acids & can
Characteristics of Viruses (cont.)
• Viruses
– Don’t eat, grow or breathe
– Do not have any organelles, or a cell membrane.
– They are NOT cells!
• Based on this info, do you think viruses are living organisms????
• No!
• A virus is not considered alive because it does not have all the
characteristics of a living organism.
Characteristics of Viruses (cont.)
• Viruses can
have different
shapes, such as
crystal, cylinder,
sphere, and
Characteristics of Viruses (cont.)
• Bacteriophage viruses only infect bacteria
Characteristics of Viruses (cont.)
• Viruses must use organisms to carry
on the processes that we usually
associate with a living cell.
• Viruses must be inside a cell, called a
host cell, to replicate.
Lytic Cycle
• Even though viruses are not truly a living
organism, they can reproduce.
• How?
• The Lytic Cycle
– A 4-step process
Characteristics of Viruses (cont.)
• When a virus enters a cell, it can either
be active or latent.
• Once it becomes active, a virus takes
control of the host cell and replicates.
Lytic Cycle
Step 1- attachment
Step 2 - injection
Step 3- Replication
Step 4 - Escape
Lytic Cycle
1. Virus attaches to a host cell
2. Virus injects DNA into the host cell
3. Host cell is then directed to make more viral
DNA, and new protein coats (replication)
4. The host cell is often destroyed and the
viruses escape and move on to infect other
Characteristics of Viruses (cont.)
• As viruses replicate, their DNA
frequently mutates, enabling them to
adjust to changes in their host cell.
• Viruses cause many human diseases,
such as chicken pox, influenza, HIV,
and the common cold. Viruses can
also affect other animals and plants.
Treating and Preventing Viral Diseases
• Since viruses are constantly changing,
viral diseases can be difficult to treat.
• Antibiotics work only against bacteria,
not viruses.
• One of the best ways to prevent a viral
infection is to limit contact with an
infected human or animal.
Treating and Preventing Viral Diseases
• When a virus infects a person, their body begins to
make antibodies.
• An antibody is a protein that prevents an infection in
your body.
• Antibodies bind to viruses and other pathogens and
prevent them from attaching to a host cell.
Treating and Preventing Viral Diseases
• Natural immunity develops when a
mother passes antibodies on to her
unborn baby.
Treating and Preventing Viral Diseases
• A vaccine is a mixture containing
material from one or more deactivated
pathogens, such as viruses.
• When an organism is given a vaccine
for a viral disease, the vaccine triggers
the production of antibodies.
• After being vaccinated against a
particular pathogen, the organism will
not get as sick if exposed to the same
pathogen again.
Research with Viruses
• Scientists are researching new ways to
treat and prevent viral diseases in
humans, animals, and plants.
• Viruses have been used by some
scientists to treat genetic disorders and
cancer using gene transfer.