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Viruses
Essential question: Describe how the structure of viruses and
cells are similar? Different?
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
1. Bacteriophage—viruses that infect bacteria
Capsid (protein coat)
– inside contains either
RNA or DNA
2. Flu (influenza), HIV
DNA or RNA
Surface
Marker
Capsid (protein coat)
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.
Example: The rabies virus only attacks brain or
nervous cells.
Surface
Markers
Viru
s
Receptor Sites
It’s like the pieces of a puzzle.
The ends have to match up so
only certain pieces fit.
Cell
A virus recognizes cells it can infect by matching
its surface marker with a receptor site on a cell.
Virus
Surface
Markers
Receptor
Sites
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
There are 5
steps in the
Lytic Cycle
ATTACHMENT
RELEASE (LYSIS)
Lytic
Cycle
1.
ATTACHMENT
Attachment – the virus attaches
to receptors on the host
Lytic
Cycle
2.
Penetration – the nucleic acid
moves across the cell membrane
into the host cell
Lytic
Cycle
3.
Replication and Synthesis – the
virus degrades the host nucleic
acid and uses the host to make
new viral components.
Lytic
Cycle
4.
Assembly – viral components are
assembled into new viruses.
Lytic
Cycle
RELEASE / LYSIS
5.
Release / Lysis – fully assembled
viruses are released when the host
cell bursts
ATTACHMENT
REPLICATION
NNNNNNNNNNN
NNNNNNNNNNN
NNNNNNNNNNN
NNNNNNNNNNN
N
Lysogenic
Cycle
PENETRATION
nnnn
nnnn
nnnn
nnnn
nddd
d
INTEGRATION
There are 4 steps in the Lysogenic Cycle
ATTACHMENT
NNNNNNNNNNN
NNNNNNNNNNN
NNNNNNNNNNN
NNNNNNNNNNN
N
Lysogenic
Cycle
PENETRATION
nnnn
nnnn
nnnn
nnnn
nddd
d
Steps 1 and 2 are identical to the first two
steps in the lytic cycle.
Lysogenic
Cycle
3.
INTEGRATION
Integration – Phage DNA
integrates into bacterial DNA
Lysogenic
Cycle
INTEGRATION
When it becomes integrated into the
host DNA, the viral genome is
referred to as a PROPHAGE.
Lysogenic
Cycle
4.
REPLICATION
Replication – The integrated prophage
replicates when bacterial DNA replicates
Cycle of Lytic and Lysogenic
Essential question
What is the difference between the lytic and lysogenic cycles?
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
This is why it does NOT belong to any
kingdom.
vs
Structure
Reproduction
RNA or DNA core
(center), protein coat
(capsid)
Copies itself only inside
host cell--REPLICATION
Cell membrane, cytoplasm,
genetic material,
organelles
Asexual or Sexual
DNA and RNA
Genetic Material
DNA or RNA
Growth and
Development
NO
YES—Multicellular Organisms
NO
YES
NO
YES
YES
YES
Obtain and
Use Energy
Response to
Environment
Change over time
Structure
Reproduction
Genetic Material
Growth and
Development
Obtain and
Use Energy
Response to
Environment
Change over time