VIRUS

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Virus Notes
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IMPORTANT TERMS

1. HOST

AN ORGANISM THAT SHELTERS AND NOURISHES SOMETHING.
2. RETROVIRUS -
A VIRUS THAT REPLICATES BY FIRST TRANSCRIBING
(copying) RNA INTO DNA instead of DNA to RNA .
Example HIV
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3. VACCINE -
A substance produced from killed or weakened
pathogens or inactivated toxins. Vaccines can
help prevent viruses. Antibiotics DO NOT destroy
viruses.
Vaccines work by preventing the virus from
attaching to cells in your body or by attacking
and destroying specific viral enzymes

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4 CAPSID -
THE PROTEIN COAT OF A VIRUS.
5. PROPHAGE -
A VIRAL DNA SEGMENT INSERTED INTO A BACTERIAL
CELLs DNA
6. LYSIS -
THE BREAKING APART OF THE CELL TO RELEASE NEW
VIRUSES.
7. BACTERIOPHAGE- A VIRUS THAT ATTACKS A
BACTERIA.
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Discovery of Viruses
Near the end of the 19th century, scientists were
trying to find the cause of tobacco mosaic
disease. In 1897, scientists filtered tiny
particles from the sap of infected plants and
found that the filtered sap could cause
uninfected plants to become infectedScientists
named the tiny particles in the sap viruses,
after the Latin word for poison.
In 1935, the American biochemist Wendell Stanley
purified the tobacco mosaic virus (TMV) and
crystallized it. Living organisms dont
crystallize so Stanley concluded that TMV was
non-living and a chemical.
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CHARACTERISTICS OF VIRUSES

1. They are non-living particles.
2. They are segments of nucleic acids surrounded by
   a protein coat.
3. Viruses are NOT cells and are smaller than
   prokaryotes
4. Viruses are considered pathogens agents causing
   disease

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• Debate on living vs. nonliving
• Viruses are NOT normally classified (not in a
  Domain, Kingdom, etc no genus species name)
• Considered NONLIVING because they
• can only reproduce inside of living cells
• do not have a metabolism
• do not maintain homeostasis
• do not grow
• Viruses are similar to living cells in that they
• Have genetic material (DNA or RNA)
• Have proteins
• Have receptor/marker molecules on the outside for
  communication/recognition
• Change over time (mutate/evolve)

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Viral Structure
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STRUCTURE OF A VIRUS

• 2 Parts
• CORE OF NUCLEIC ACID on the inside --- the
  nucleic acid is either DNA OR RNA
• PROTEIN COAT
• called the CAPSID which surrounds the
  nucleic acid on the outside

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FIVE BASIC SHAPES OF VIRUSES

• helical
• spherical
• polyhedral
• binal/bacteriophage
• filovirus

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HELICAL

• Rod-like in appearance with capsid proteins
  winding around a core of either DNA or RNA
• Ex tobacco mosaic virus (TMV)

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SPHERICAL

• Typically studded with receptors which help the
  virus enter the cell
• may be enveloped (membrane surrounding the
  capsid)
• Ex Influenza (flu virus)

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POLYHEDRAL VIRUS

• Has many sides and is geometric in appearance
• Ex adenovirus (causes the common cold)

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BINAL

• Polyhedral head containing either DNA or RNA
• Helical tail fibers
• Ex Bacteriophage
• (virus that infects
• bacteria)

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FILOVIRUS

• Ex. Ebola flu-like symptoms leading to internal
  bleeding
• Deadly viruses many are emergent viruses
  viruses that humans have suddenly contracted
• Ex) West Nile Virus first appeared in the United
  States in 1999 (brought from overseas to America
  in an infected bird)
• Ex) hantavirus spread in rodent droppings and
  can cause lethal illness in humans (38 of its
  victims die)

• Long rods that form filaments and loops with no
  distinct shape

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Surface Marker/Receptor Proteins

• Virus surface proteins are very specific - attach
  to cell surface proteins much like pieces of a
  puzzle fit together.
• This facilitates their entry across the cell
  membrane.

influenza
cell
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Viral Diseases Cold (adenovirus) Influenza (flu)
AIDS Small pox Measles Chick
en pox (varicella) Warts West
Nile Mumps Polio Mononucleosis Ebola
Hepatitis (ABC) Yellow fever
SARS Rabies Rotavirus Rubella (German
measles) HPV Routinely immunized against NO
Viral diseases are treatable with ANTIBIOTICS!!!
Vaccines can be used to prevent getting some
viruses
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WARTS
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Viral Replication

• Viruses replicate inside living cells
• They enter a cell by injecting their genetic
  material into the cell
• tears in the plant cell wall
• binds to molecules on the surface of a specific
  cell and this triggers endocytosis.

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RETROVIRUS

• Viruses that get their genetic material copied
  backwards
• From RNA to DNA (instead of DNA to RNA)
• Ex HIV (human immunodeficiency virus) which
  causes the disease AIDS
• contain RNA inject it into the host cell
• When the virus infects the cell, they produce a
  DNA copy of their RNA. The retrovirus may remain
  dormant for varying lengths of time before
  becoming active, then producing new viruses which
  eventually causes the death of the host cell

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2 cycles for Viral Reproduction

• 1. Lytic Cycle
• 2. Lysogenic Cycle

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Lytic Cycle

• Characterized by viral infection, replication,
  and cell destruction
• Virus enters specific cell, often just the
  genetic material is injected.
• Example Bacteriophage T4 has DNA core inside
  a protein capsid that binds to the surface of a
  host cell. Virus injects its DNA into the cell
  and the cell begins to make mRNA from the viral
  genes. The viral mRNA is translated into viral
  proteins that act like a molecular wrecking
  crew, chopping up the cells DNA.
• Under the control of the viral genes (genome),
  host cell now makes copies of viral nucleic acid
  (DNA or RNA) and capsid proteins
• Viral nucleic acid is assembled into new virus
  particles
• Soon the host cell is full of new viruses that
  burst (lyse) from the cell spreading the virus
  particles and disease
• Host cell now destroyed

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LYTIC CYCLE
http//biology.about.com/library/blvirusanim.htm

1. VIRUS ATTACHES TO HOST
2. VIRUS ENTERS CELL (NUCLEIC ACID IS INJECTED)
3. REPLICATES IMMEDIATELY
4. PRODUCES MANY NEW VIRUSES
5. VIRUSES BURST FROM CELL TO INVADE OTHER CELLS
   (This is the step that harms or kills the cell)

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Lysogenic Cycle

• The viral genome (DNA) replicates without
  destroying the host cell
• Virus inserts its DNA into its host DNA and it
  hides there - becomes a prophage
• Host cell reproduces as usual (making more viral
  DNA without knowing it!!!). The host cell is not
  harmed
• Later it will begin the lytic cycle to spread the
  virus. There is usually a change in the
  environment (heat or chemicals) that stimulates
  the provirus to begin the lytic cycle. This
  results in the destruction of the host cell

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LYSOGENIC CYCLE

1. VIRUS INVADES CELL
2. DOES NOT IMMEDIATELY KILL HOST CELL
3. BECOMES PART OF HOSTS DNA
4. HOST CELL REPRODUCES AS USUAL (MAKING MORE VIRAL
   DNA WITHOUT KNOWING IT)
5. EVENTUALLY GOES INTO LYTIC CYCLE TO SPREAD THE
   VIRUS

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VIRAL REPLICATION CYCLES
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HUMAN IMMUNODEFICIENCY VIRUSa retrovirus

• HIV, the virus that can result in the disease
  AIDS, can hide for years in the nucleus of
  T-cells (white blood cells that help fight
  infection) in the lysogenic cycle. People can
  have no symptoms, yet be spreading the disease to
  many partners.

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HIV then begins the lytic cycle and T-cells begin
to die. HIV infection leads to the destruction
of the bodys T cells and weakens the immune
system. This can lead to AIDS (when T cell count
is below 200 cells/ml). The patient eventually
dies from another disease (opportunistic
infection) that the immune system cant control
because there are not enough T-cells to help
fight infection.
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Structure of HIV

• Exterior Viral envelope studded with
  glycoproteins which precisely fits a human cell
  surface receptor
• Inside is the capsid with 2 molecules of RNA

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Viruses and Cancer Many cancers can result from
viral infections HOW? Because when a virus
infects a cell it can cause genes in the cell to
turn on or off. This can interrupt the cell
cycle, causing the cell to divide uncontrollably.
Remember that this is cancer! Ex HPV this
virus can infect women and result in cervical
cancer vaccine for it (guys are NOT immune to
this virus!) Ex hepatitis can often lead to
liver cancer Ex Epstein-Barr virus can lead to
some lymphomas (cancer of the lymph nodes)