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Viruses
What are Viruses?
• Virus – particles of nucleic acid, protein,
and in some cases lipids
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Not alive
Do NOT exhibit characteristics of life
Can only reproduce when inside a host cell
Cannot maintain homeostasis
• Very simply a virus is genetic
information enclosed in a protein coat
Viral Structure
• Viruses contain DNA or RNA but never
both
• DNA or RNA is surrounded by a protein
coat call a capsid
– Proteins in the capsid give the virus
different shapes
– Shape of the virus causes the virus to be
cell specific (can only infect a certain type
of cell)
Viral Structure
• Some viruses are
enclosed by a
protective protein
envelope
• Some viruses have
spikes to help attach
to the host cell
CAPSID
DNA
ENVELOPE
SPIKES
Virus vs Cell
Characteristic
Virus
Cell
Structure
DNA or RNA core,
capsid
Cell membrane, cytoplasm;
eukaryotes also contain
nucleus & organelles
Reproduction
Only within host
Independent cell division
either asexually or sexually
Genetic Code
DNA or RNA
DNA
Growth &
Development
No
Yes, in multicellular
organisms, cells increase in
number & differenetiate
Obtain & Use Energy
No
Yes
Response to
Environment
No
Yes
Change Over Time
Yes
Yes
Viruses & Disease
**Remember ALL viruses are parasitic
which means they require a living host
and they cause harm to that host**
Bacteriophages
• A virus that infects
bacteria is known as a
bacteriophage or
phage
• Bacteriophages are
used for research
tools in genetics
– Ex: to “program”
bacteria to produce
insulin for the
treatment of diabetes
Retroviruses
• Group of viruses that contain RNA as
their genetic material
• Retroviruses have a complicated
reproductive cycle because they must
carry out reverse transcription
• Contain a special enzyme call reverse
transcriptase
– This enzyme helps a cell transcribe DNA
from RNA
• Examples: HIV and feline leukemia
ENZYME
Fighting Viral Infections
• Antiviral drugs work
by interfering with
the genetic material
or the formation of
the capsid
– This will prevent the
virus from replicating
in the host cell
• Examples: tamiflu,
acyclovir
Benefits of Viruses
• Not all viruses are bad!
• Example:
– Vaccines for viral diseases are made from
weaker strains of the virus
– A weaker virus is capable of stimulating an
immune response and creating immunity,
but not causing illness
Viral Hosts
• Remember that a host is necessary for
viral replication
• Broad host range = infect several species
– Ex: Swine flu and rabies
• Narrow host range = infect only one
species
• Very narrow host range = infect specific
tissue types of one species
– Ex: Human cold virus attack cells of the
respiratory system
Viral Cycles
There are three initial steps that are
common to all viral infections:
1. Virus attach to the cell membrane of the
host cell
2. Trick cell into allowing it inside
3. Virus releases its genetic information (DNA
or RNA) into the host cell
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Replication – Viral DNA is copied
Transcription – Viral DNA is converted into RNA
Translated – Viral proteins are made from the
RNA
Viral Cycles
Once inside the host cell, there are two
ways that a virus can take over and
reproduce:
1. Lytic Infection
2. Lysogenic Infection
Lytic Infection
1. Attachment to the cell
2. Penetration (injection) of viral DNA or
RNA
3. Virus reprograms host to copy viral DNA
and make viral proteins
4. New viruses assemble and mature
5. Cell lyses (bursts) and releases the new
viruses to attack other cells
6. Results in death of the host cell
Lysogenic Infection
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Virus attaches to host and injects DNA
Viral DNA joins host DNA forming a
prophage
Viral DNA replicates along with host DNA
Remain inactive for a period (generations)
Eventually viral DNA exits host cell and
enters lytic cycle