Transcript VIRUSES

Microbiology Pretest
1. Are viruses alive?
2. Vaccines are effective against:
(a) viral diseases (b) bacterial diseases
(c) both of these
3. Antibiotics are effective against:
(a) viral diseases (b) bacterial diseases
(c) both of these
4. Name a disease caused by a prion.
5. Viruses contain:
(a) DNA (b) RNA (c) either DNA or RNA
(d) Both DNA and RNA
6. Approximately what percent of bacteria
are harmful to humans?
(a) 1% (b) 25% (c) 50% (d) 75%
7. Name 2 useful functions of bacteria.
8. Name a viral disease.
VIRUSES
“A virus is a piece of bad news
wrapped in a protein.”
-Sir Peter Medawar
Nobel Laureate
Why are viruses considered
non-living?
They lack the ability to reproduce on
their own.
They do not transform their own energy,
or demonstrate homeostasis.
The lack cell membranes, cytoplasm
and cell organelles.
They are dormant unless in contact with
a host cell.
I. Early Studies
• Tobacco
mosaic virus
which causes
discoloration of
tobacco
leaves, was
the first
identified virus
• TMV can form crystals.
• The ability to form crystals is a property
of chemicals, not cells.
Viral Vaccine History
• In 1796, Edward Jenner experimented with
cowpox and smallpox
• noticed that milkmaids got cowpox, but
never acquired smallpox
• injected a young boy with cowpox, then
smallpox
• boy survived; first successful vaccine for
smallpox
II. Composition and shape
A. Protein coat known as capsid.
B. Inside the coat, the virus contains
either DNA or RNA but never both.
C. Many have an envelope
surrounding the capsid.
D. Envelope contains proteins
derived from host, which help
the virus gain entry to cells.
Shapes include:
• Polyhedral
(adenovirus)
• Helical
(TMV and rabies)
• enveloped
(often appears
spherical – example
is the influenza virus)
• filovirus
(long and threadlike –
example is Ebola)
Viruses can infect plants, bacteria
and animals

Bacteriophages
These are
viruses that
infect bacteria.
•
The bacteriophage injects its genetic
material into a bacterium. The protein coat
remains outside the cell.
capsid
DNA
tail sheath
tail fiber
Viruses of eukaryotes often fuse with cell
membrane
III. Replication inside living cells
http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap11.html
The Lytic Cycle
Causes death of the host cell (lysis)
1.
2.
3.
4.
5.
Attachment or Docking
Entry
Biosynthesis of the viral components
Assembly
Release of completed virus from host cell
In 24 minutes, 1000 viruses can be produced
Lysogenic cycle
• Virus’s nucleic acid is inserted into the
host cell’s chromosome and is known as a
prophage.
• Prophage is replicated along with host
cell’s chromosome
• At any time, prophage can be activated
and enter the lytic cycle.
Lytic or Lysogenic?
• Common cold
lytic
• HIV
lysogenic
• Herpes
lysogenic
• Influenza
lytic
Two Main Groups of Viruses
A. DNA Viruses
• herpes virus, poxviruses
B. RNA Viruses
• cause diseases such as the common
cold, rabies, measles
Retrovirus: a type of RNA virus
• Retroviruses have a
core of RNA.
• Use enzyme called
reverse transcriptase to
form DNA.
• Viral DNA is inserted
into the host DNA.
• HIV is an example of a
retrovirus.
• Other retroviruses can
cause some cancers.
Emerging Viruses
• An existing virus can spread from one type
of host to another. Humans can get
viruses from other species.
• An existing virus can spread from a small
population to become more widespread.
Hantavirus can spread from a small
mouse population to the human
population.
• Other emerging viruses are Ebola, HIV,
SARS, and bird flu.
PRIONS and MAD COW
DISEASE
What are prions?
• discovered in 1981
• "proteinaceous infectious particle"
• an abnormal (misfolded) protein, with
no nucleic acid
• present in the brain and neurons
• pathogenic variants of proteins
• agents that cause a group of fatal and
degenerative brain diseases
Why are prions
so deadly?
• No immune response
• Long incubation period
• Mutated proteins multiply in the brain
• When a normal protein (PrPc) comes
in contact with an infectious prion
(PrPsc), it’s shape is changed and it is
converted into an infectious prion.
Prion Diseases
• affect the brain, disrupting or destroying
neurons in large numbers (all are lethal)
• Creutzfeldt Jakob Disease (CJD) in humans
• Bovine Spongiform
Encephalopathy (BSE)
or ”mad cow disease"
How could prions infect me?
• dairy products are “safe”
• muscle tissue is “safe”
• you can only get mad cow
disease if you ingest the
abnormal prion
• these are present in nervous
tissue (spinal cord, brain)
Viroids
• discovered in 1971
• smaller than viruses
• single circular strand of nucleic
acid (RNA) with no protein coat
• serious metabolic disruption and
stunting in plants
1 nanometer (nm) = one
billionth of a meter
100 nm
eukaryotics cells
10,000-100,000 nm
viruses
50-200 nm
prokaryotics cells
200-10,000 nm
viroids
5-150 nm
prion
2-10 nm