Download What are Viruses?

Virus,
Lytic Cycle and
Lysogenic Cycle
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Are Viruses Living or
Non-living?
Viruses are non living
They have some properties of life
but not others
They can’t maintain a constant
internal state (homeostasis) or
reproduce without a host cell.
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What are Viruses?
A virus is a noncellular particle
made up of a
nucleic acid and
protein coat
(capsid) that can
invade living cells.
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Smallpox
Edward Jenner
(1796) developed a
smallpox vaccine using
milder cowpox viruses
Deadly viruses are
said to be virulent
Smallpox has been
eradicated in the
world today
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Viewing Viruses
Viruses are
smaller than the
smallest cell
Viruses couldn’t
be seen until the
electron microscope
was invented in the
20th century
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What is a Nanometer?
A nanometer is a unit of
measure. Just like inches, feet
and miles. By definition a
nanometer is one-billionth of a
meter. A meter is about 39 inches
long. That is a big number and
when you divide a meter into one
billion pieces, well that is very
small.
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Size of Viruses
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Viral
Structure
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Characteristics
Non living
Noncellular
Contain:
1. a protein coat called the
capsid
2. nucleic acid core containing
DNA or RNA
Capable of reproducing only when
inside a HOST cell
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Characteristics
Some viruses are
DNA
enclosed in an
protective envelope
Some viruses may
have spikes to help
attach to the host cell
Most viruses infect
only SPECIFIC host
ENVELOPE
cells
CAPSID
SPIKES
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Characteristics
Outside of host cells,
viruses are inactive
Lack ribosomes and
enzymes needed for
metabolism
To be able to
reproduce they use the
raw materials and
enzymes of the host.
EBOLA VIRUS
HIV VIRUS
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Characteristics
Some viruses cause
disease
Smallpox, measles,
mononucleosis, influenza,
colds, warts, AIDS, Ebola
Some viruses may cause
some cancers like leukemia
MEASLES
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Viral Shapes
Viruses come in a variety
of shapes
Some may be helical shape
like the Ebola virus
Some may be polyhedral
shapes like the influenza virus
Others have more complex
shapes like bacteriophages
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Helical Viruses
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Polyhedral Viruses
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Complex Viruses
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Herpes Virus
SIMPLEX I and II
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Adenovirus
COMMON COLD
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Influenza Virus
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Chickenpox Virus
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Papillomavirus – Warts!
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Bacteriophages
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Phages
Viruses that
attack bacteria
are called
bacteriophage or
just phage
http://highered.mcgrawhill.com/sites/0072556781/student_view0/chapter17/anim
ation_quiz_2.html
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Bacteriophage
They infect E. coli ,
an intestinal bacteria
Six small spikes at the
base of a contractile tail
are used to attach to
the host cell
Viral DNA is injected
into the host cell
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Escherichia coli
Bacterium
T - EVEN PHAGES ATTACK THIS BACTERIUM
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Bacteriophages
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Retrovirus
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Characteristics of
Retroviruses/Latent
Contain RNA, not DNA
Contain enzyme called
Reverse Transcriptase
When a retrovirus infects a
cell, it injects its RNA and
reverse transcriptase enzyme into
the cytoplasm of that cell and it
is able to make DNA
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Retroviruses
HIV, the
AIDS virus
and human T
cell leukemia
virus are
retroviruses
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Viral Replication
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Viral Attack
Viruses are very specific as to
which species they attack
HOST specific
Humans rarely share viral
diseases with other animals
Eukaryotic viruses usually have
protective envelopes made from
the host cell membrane
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Bacteriophage Replication
Bacteriophage
inject their
nucleic acid
They lysis
(break open) the
bacterial cell
when replication
is finished
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Lytic Cycle Review
Attachment
Phage attaches by tail fibers to
host cell
Entry
Tail sheath contracts to
force tail core and DNA into
cell
Replication
Production of phage DNA
and proteins
Assembly
Assembly of phage particles
Lysis/Release Phage breaks cell wall and
releases new viruses
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Viral Latency
Some viruses have the ability to
become dormant inside the cell
Called latent viruses
They may remain inactive for
long periods of time (years)
Later, they activate to produce
new viruses in response to some
external signal
HIV and Herpes viruses are
examples
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Lysogenic Cycle
Phage DNA or
genome injected into
host cell
Viral DNA joins
host DNA forming a
provirus
When an activation
signal occurs, the
phage DNA starts
replicating
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Lysogenic Cycle
Viral DNA (part of
provirus) may stay
inactive in host cell for
long periods of time
Provirus Replicates
during each binary fission
in bacteria and Mitosis in
Eukaryotic Cells
Over time, many cells
result which contain the
provirus
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Lytic and Lysogenic Cycle
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Viral Latency
Once a provirus cell is activated, host cell enters
the lytic cycle
New viruses form and the cell lyses (bursts)
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Latency in Eukaryotes
Some eukaryotic
viruses remain dormant
for many years in the
nervous system tissues
Chickenpox (caused
by the virus Varicella
zoster) is a childhood
infection
It can reappear later
in life as shingles, a
painful itching rash
limited to small areas
of the body
SHINGLES
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Latency in Eukaryotes
Herpes viruses also
become latent in the
nervous system
A herpes infection
lasts for a person’s
lifetime
Genital herpes (Herpes
Simplex 2)
Cold sores or fever
blisters (Herpes
Simplex1)
SKIN TO SKIN CONTACT
PASSED AT BIRTH TO
BABY
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Interferon are
naturally occurring
proteins made by
cells to fight
viruses
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The End
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