Lesson 2
Viruses and Prions
Focus Question
How are viruses classified? Are they alive?
New Vocabulary
virus
capsid
lytic cycle
lysogenic cycle
retrovirus
prion
Review Vocabulary
protein: large, complex polymer composed of carbon,
hydrogen, oxygen, nitrogen, and sometimes sulfur
Viruses
A virus is a nonliving strand of genetic material
within a protein coat. Scientists do not consider
viruses alive because they:
• Have no organelles to take in nutrients or use
energy
• Cannot make proteins
• Cannot move
• Cannot replicate on their own
Viruses
• Viruses are some of the smallest disease-causing
structures.
• Viruses range from 5-300 nanometers.
• They most likely came from parts of cells.
• Viral DNA is similar to cellular genes.
Viruses
• The outside layer of all viruses, which is made
of proteins, is called the capsid.
• Inside the capsid is the genetic material,
either DNA or RNA (never both).
Viruses
Viruses
• In order to replicate, a virus must enter a host
cell.
• The virus attaches to the host cell using specific
receptors on the plasma membrane.
• After the genetic material is inside the host cell,
the virus uses the cell to replicate by either the
lytic or lysogenic cycle.
Viruses
Viruses
• In the lytic cycle, the host cell makes many copies
of the viral RNA or DNA.
• The viral genes instruct the host cells to make
more viral protein capsids and enzymes.
• Viruses leave the cell by exocytosis or by causing
the cell to burst.
Viruses
• In the lysogenic cycle, viral DNA integrates
into a chromosome in a host cell.
• Viral genes may remain dormant for some
time.
• Activated viral genes result in the lytic cycle.
Retroviruses
• Retroviruses have RNA instead of DNA for
their genetic material.
• They have a lipid envelope surrounding the
capsid obtained from the plasma membrane
of a host cell.
• In the host cytoplasm, reverse transcriptase
synthesizes DNA from RNA template; DNA
integrates into the host chromosome until
activated.
Retroviruses
Prions
• A protein that can cause infection or disease
is called a proteinaceous infectious particle,
or prion.
• Prions normally exist in cells, but their
function is poorly understood.
• Normally spiral shaped
• Mutations cause incorrect folding,
mutated prions cause diseases
Prions
• Examples of diseases caused by prions are:
• Mad cow disease (cattle)
• Creutzfeldt-Jacob disease, also called CJD
(humans) encephalopathy, porous brain
tissue gradual loss of muscular
coordination
• Scrapie (sheep)characterized by
twitching, intense itching excessive thirst,
weakness and finally paralysis
• Chronic wasting disease (deer and elk)
Prions
• In CJD, prions infect nerve
cells in the brain, causing
them to burst.
• This results in spaces in
the brain, called
spongiform
encephalopathy (spongelike brain disease).
• Some scientists think that
prions can be transmitted
across species.
Quiz
1. Viruses contain which substances?
A
genetic material and a capsid CORRECT
B
a nucleus, genetic material, and a capsid
C
a nucleus, genetic material, a capsid, and
ribosomes
D
a nucleus, genetic material, a capsid,
ribosomes, and a plasma membrane
Quiz
2. Imagine that a patient in a hospital has died mysteriously.
A doctor suspects that the cause of death is CreutzfeldtJakob disease. How could this diagnosis be confirmed?
A
by examining the blood
to see if there is a high
viral count
C
by examining the brain
to see if there are a lot
of spaces in the tissue
CORRECT
B
by asking the patient’s family
and friends if the patient
consumed a lot of meat
D
by examining nerve
cells to see if they have
been affected by a
bacterial neurotoxin
Quiz
3. Which statement about prions is true?
A
Prions are renegade
pieces of RNA that
infect cells.
C
Prion-based diseases
affect only cows.
B
Prions are infectious
proteins.
D
Prions are a newly
discovered type of
genetic material.
CORRECT
Quiz
4. HIV is a retrovirus. What does this mean?
A
Viral RNA is used to
make DNA.
C
Protein is made
directly from viral RNA.
D
Protein is made
directly from viral DNA.
CORRECT
B
Viral DNA is used to
make RNA.
Quiz
5. Which labeled structure represents
the genetic material of a virus?
A
A
C
C
D
D
CORRECT
B
B
Quiz
6. Which structure represents the
capsid of a virus?
A
A
C
C
B
B
D
D
CORRECT