What is a virus?
Viruses are particles that are
composed of two macromolecules
Nucleic acid (DNA or RNA)
Protein
All are pathogens- an agent that causes illness or
disease in an organism
The capsid is made of protein and functions to protect the
nucleic acid for the external environment
The capsid can vary in shape
Capsid (protein coat)
– inside contains either RNA or DNA
Surface Marker
DNA or RNA
Capsid (protein coat)
1. Bacteriophage—viruses that infect bacteria
2. Flu (influenza), HIV
Are viruses organisms?
No, viruses are not organisms for the following reasons:
viruses cannot reproduce on their own
viruses cannot grow on their own
Viruses can only replicate (produce more of themselves)
in a host cell
What organisms do viruses infect?
Viruses can infect all organisms: animals, plants,
bacteria
Viruses that specifically infect bacteria are called
bacteriophages
How do viruses enter an
organism?
Through an open wound: cut or scrape
Mouth
Nose
Eyes
Ears
Genital area
Do viruses infect all cells?
No, virures attack specific cells based on structure
The virus can only recognized the host cell by it
receptor molecules
The virus then attaches its surface proteins into the
host cells receptor molecules (shape specific)
Viruses cannot infect cells that do not have receptor
molecules it can recognize
Certain viruses can only attack
certain cell types. They are said to
be specific.
It’s like the pieces of a puzzle.
The ends have to match up so
only certain pieces fit.
Surface
Markers
Receptor Sites
Example: The rabies virus only attacks brain or
nervous cells.
Viru
s
Cell
Surface Markers
Receptor Sites
Virus
A virus recognizes cells it can infect by matching its surface marker with a receptor site on a cell.
Cell
Once a viruses attaches to a cell,
what happens?
After a virus attaches to a host cell, it can infect a cell
in one of two ways
Lytic cycle
Lysogenic cycle
Both cycle cause infections, but have two different pathways
with different steps
Lytic
Cycle
3. Replication and Synthesis – the
virus degrades the host nucleic
acid and uses the host to make
new viral components.
Lytic
Cycle
RELEASE / LYSIS
5. Release / Lysis – fully assembled
viruses are released when the host
cell bursts
Lysogenic
Cycle
There are 4 steps in the Lysogenic Cycle
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N
nnnn
nnnn
nnnn
nnnn
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d
ATTACHMENT
PENETRATION
INTEGRATION
REPLICATION
Lysogenic
Cycle
Steps 1 and 2 are identical to the first two
steps in the lytic cycle.
NNNNNNNNNNN
NNNNNNNNNNN
NNNNNNNNNNN
NNNNNNNNNNN
N
nnnn
nnnn
nnnn
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nddd
d
ATTACHMENT
PENETRATION
Lysogenic
Cycle INTEGRATION
When it becomes integrated into the
host DNA, the viral genome is
referred to as a PROPHAGE.
Lysogenic
Cycle REPLICATION
4. Replication – The integrated prophage
replicates when bacterial DNA replicates
How do humans defend
themselves against viruses
Skin
Immune system- white blood cells
Vaccines- a substance the stimulates the body’s own
immune response against invasion by foreign microbes
or viruses
Chickenpox, flu, measles, mumps, herpes
Viruses can be prevented with vaccines, but NOT treated
with antibiotics.
A virus is an infectious agent made up of
nucleic acid (DNA or RNA) wrapped in a
protein coat called a capsid.
Viruses have no nucleus, no organelles, no
cytoplasm or cell membrane—Non-cellular
vs
This is why it does NOT belong to any
kingdom.
RNA or DNA core
(center), protein coat
(capsid)
Copies itself only inside
host cell--REPLICATION
DNA or RNA
NO
NO
NO
YES
Cell membrane, cytoplasm,
genetic material,
organelles
Asexual or Sexual
DNA and RNA
YES—Multicellular Organisms
YES
YES
YES
Structure
Reproduction
Genetic Material
Growth and
Development
Response to
Environment
Change over time
Obtain and
Use Energy