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VIRUSES
CH 20
Viruses are ...
• biological particles.
• made of nucleic acid & a
protein coat.
• not living.
Viruses have...
• No growth
• No homeostasis
• No metabolism
virus means “poison”
• They do mutate!
• They have DNA or RNA
• Reproduction occurs only
within a HOST.
Viruses have
structure
• They come in
many shapes.
Viruses are grouped into families
• based on their nucleic acid
type,
• capsid structure,
• presence or absence of an
“envelope”.
Viruses cause diseases
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smallpox
hepatitis B
AIDS
shingles, chicken pox
Cancer
Ebola
VIRUS REPRODUCTION
• Lytic Cycle
• Lysogenic Cycle
Lytic Cycle
Absorption (Attachment): Virus attaches
itself to the cell.
• Entry: Enzymes weaken cell
wall; nucleic acid is injected
into cell, leaving the empty
capsid outside the cell. Many
viruses actually enter the
host cell intact.
• Replication: Viral
DNA takes
control of cell
activity.
Assembly: All metabolic activity of the cell
is directed to assemble new viruses.
• Release: Enzymes disintegrate
the cell in a process called
lysis, releasing the new
viruses.
Lysogenic Cycle
• The virus attaches itself and
injects its DNA into the cell
• The viral DNA incorporates itself
to the host DNA, becoming a new
set of cell genes called a
prophage.
• When the host cell divides, this
new gene is replicated and passed
to new cells.
• This causes no harm to the cell,
but may alter its traits.
Now there are two possibilities:
• The prophage survives as a
permanent part of the DNA
of the host organism
• Some external stimuli can cause the
prophage to become active, using
the cell to produce new viruses