Viruses

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Viruses

• What are they?

QAre they alive? AOnly when inside a living
cell.
McFadden, 862
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Are Viruses Alive?Life Traits in Viruses

• NOT CELLS
• Organized recognizable shape and size
• contain complex molecules nucleic acids,
  proteins
• Replicate - no other life processes
• Change over time (mutate)
• Known since 1800s but too small
• to see until electron microscope
• discovered in 1930s
• Did you know? Viruses can be
• as small as 30 nm.

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Viral structure
Core is nucleic acid - DNA or RNA Capsid -
Protein coat surface markers proteins stick out
on surface
Starr, 317
Some have outer envelope made of host cell
membrane and viral proteins
Schraer,654
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What do they look like?

• Some common virus shapes

Bacteriophage Infects bacteria
Tobacco Mosaic Virus First one discovered
Adenovirus Causes colds and flu
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How are viruses classified?1) type of organism
infected (animal, plant, bacteria) 2) type of
nucleic acid (DNA or RNA)3) shape (geometric,
thread, rod)
Tulip colors caused by harmless virus
Starr,. 317
polyhedron
Ebola - thread-shape virus
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How do viruses reproduce?

• Viruses must
• be INSIDE living
• cells to reproduce.
• They take over the
• cell machinery to
• make more virus
• particles.

HIV virus entering cell
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Lytic Cycle
2. Viral DNA enters host cell
1. Virus attaches to host cell
3. Virus destroys host cell DNA
5. Host cell bursts new viruses infect more
cells
4. Host cell makes new viruses
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Lysogenic Cycle Some viruses have a dormant
stage
1. Viral DNA blends into host DNA
2. When cell reproduces, so does viral DNA
3. Later, a cell message activates lytic cycle
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Enveloped Viruses
Some viruses wrap in a piece of host
cell membrane - viral proteins stick out through
the membrane
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How are RNA Viruses different?

• DNA or RNA controls protein synthesis
• In normal cells DNA contains code
• - code is written on RNA molecule
• - RNA goes to ribosome ? makes protei
• 2) DNA viruses ? enter cell? their code is
  written on RNA ? make proteins
• 3) RNA viruses ? make proteins

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What is a retrovirus?

• A retrovirus is a virus that can make DNA from
  its RNA. This DNA blends into the host cell DNA
  and changes it
• Viral DNA later becomes active
• Did you know? AIDS
• is caused by a retrovirus.

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How a retrovirus changes host cell DNA
Reverse transcriptase enzyme makes DNA from RNA
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How do viruses spread?

• Viruses can be spread by
• air, water, food, insects
• other people (body fluids saliva, blood, semen)
• Some can stay dormant on contaminated surfaces
  for decades!!!

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Bodys Defense against Disease

• 1st line of defense physical barriers
• - skin, mucous membranes, tears
• 2nd line Inflammation
• a. chemical messages
• b. White blood cells - phagocytes
• 3rd line Immune Response
• a. Antigens and antibodies

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Inflammation
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B and T CellsSpecial kinds of white blood cells

• Lymphocytes - Mature in thymus gland and bone
  marrow
• B cells make antibodies
• Proteins, specific for each antigen (molecule on
  invader)
• Attach to antigen, makes destruction easier

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How antibodies work
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Other B and T cells

• Memory B cells - remember invader, so body can
  react fast if this invader comes back again
• Killer T cells destroy invaders
• Helper T cells label invaders for destruction
  alert and coordinate immune response

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How do immune cells fight viruses?
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Killer T cells attack a cancer cell
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Role of Helper T cells
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Why HIV stops immunity

• HIV virus attacks helper T cells
• Without labeling by helper T cells, other immune
  cells do not recognize pathogen as foreign
• No response

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Some Human Disease Viruses
measles
herpes
Polio
Hepatitis
Chicken pox
flu
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Fighting Viral Infections
Immunity resistance to an infection a) active
immunity immune system has responded to an
infection and remembers the invader ex.
Vaccinations, illnesses b) passive immunity -
defense cells and antibodies from another person
to resist a current infection ex.
Mother-to-fetus, serums, injections of
antibodies
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Drugs that slow viral replication

• Antiviral drugs slow reproduction (ex. Tamiflu)
• Interferon protein made by viral-infected cells
• - sent to neighboring cells
• - interferes with viral reproduction
• 3) Antibiotics DO NOT work on a virus
• - virus hides inside a living cell

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Even Bacteria have defenses!

• Bacteria make Restriction Enzymes
• Cells infected with phage virus send distress
  signals to nearby cells
• These cells make enzymes that cut up viral DNA

Restricted cut at a specific sequence
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VaccinesMade from dead or weakened
virusPrimes immune system to recognize virus and
respond quickly 1798 Edward Jenner
smallpox 1884 Louis Pasteur rabies 1954
Jonas Salk polio, injected 1960 Albert Sabin
polio, oral
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Gene therapy using viruses
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SourcesSchraer and Stoltze, Biology, the Study
of Life, 7th ed., Prentice-Hall, 1999.Starr,
Cecie. Biology, concepts and Applications,5th
ed., Wadsworth Group,Belmont CA 2003.McFadden
and Keeton, Biology, An Exploration of Life, W.
W. Norton Co, New York, 1995. Big Picture
Book of Viruses http//www.virology.net/Big_Virolo
gy/BVDiseaseList.html