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Viruses
The Smallest Organisms?
What are viruses?

Very small particles

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
Too small to see with a light microscope
Can be “seen” with an electron microscope
Invade living cells

Experts think that it is probable that all
living cells are subject to infection by one
or more viruses
Discovery


Scientists suspected the existence of
viruses by the end of the 1800s.
Question: How could scientists find
something that they could not see?
Discovery

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The first virus that
scientists discovered
was the tobacco
mosaic virus
(TMV).
TMV is a virus that
infects a variety of
plants including
tobacco, tomatoes
and peppers.
Photo of a tobacco leaf with symptoms
of tobacco mosaic virus
Public Domain (from USDA Forest Service)
http://commons.wikimedia.org/wiki/File:Tobacco_mosaic_vir
us_symptoms_tobacco.jpg
Discovery


1883 – Showed tobacco mosaic
disease was infectious – if you took
plant juice from a diseased plant and
placed it on healthy plants the plants
developed the disease.
1889 – Filtered the juice to remove all
particles large enough to see with a
light microscope and the juice still
caused the disease.
Discovery


Next experiment proved that the
disease could reproduce in the plant –
this is a characteristic of living things
Unlike other living things the germ
could not be grown outside the host
plant
Discovery

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1935 – Used chemical techniques to
isolate the germ from the juice. Ended
up with a crystalline substance that did
not grow, breathe, eat, reproduce or
perform any other life function – but
caused the disease in plants
Called this germ a virus
Are viruses living?
Tobacco Mosaic Virus

Public Domain
http://en.wikipedia.org/wiki/File:
TMV.jpg
So What Is a Virus?
Three things to remember:
1.
Tiny particles that can invade living
cells
2.
Do not perform any of the life function
of cells
3.
Can reproduce only within the host
cell
Host vs. Parasite
Host = living thing that provides
a home and/or food for a
parasite
Parasite = an organism that
survives by living on or in
another organism and harms
the host in the process
Host vs. Parasite
ALL VIRUSES ARE PARASITES
 Each virus can infect only a few
specific kinds of cells, both
specific species and specific
cells within the host organism.

What do viruses look like?
Swine Flu virus electron micrograph
Public Domain: CDC http://commons.wikimedia.org/wiki/File:B00528Swine-flu.png
Diagram of a Flu Virus
Public Domain
http://en.wikipedia.org/wiki/File:Flu_und_legende_color_c.jpg
What do viruses look like?
Electron micrograph of the poliovirus
Public Domain: CDC
http://en.wikipedia.org/wiki/File:Polio_EM_PHIL_1875_lores.PNG
Computer model of Adenovirus
Public Domain: National Cancer Institute
http://commons.wikimedia.org/wiki/File:Adenovirus.jpg
What do viruses look like?
Vesiculovirus, a member of the
Rhabdoviridae similar in
morphology to the rabies virus.
Public Domain: This image is a work of the Centers for Disease
Control and Prevention, part of the United States Department of
Health and Human Services
http://en.wikipedia.org/wiki/File:Vesicular_stomatitis_virus_(VSV
)_EM_18_lores.jpg
HIV-1. Transmission electron
micrograph
Public Domain: CDC
http://commons.wikimedia.org/wiki/File:HIV1_Transmission_electron_micrograph_AIDS02bbb_lores.jpg
What do viruses look like?
Ebola, Transmission Electron Micrograph
Public Domain: CDC http://en.wikipedia.org/wiki/File:Ebola_virus_em.jpg
Virus particles of G. indiensis polydnavirus
that infects a parasitoid wasp. The virions are
formed by 5 to 10 nucleocapsids enclosed by
only one viral envelope.
Public Domain: USDA http://commons.wikimedia.org/wiki/File:GiBV-em.jpg
Structure of Viruses
Two Basic Parts
1.
A core of hereditary material


DNA or RNA
Controls the reproduction of the virus
An outer protein coat
2.

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Protects the virus
Allows the virus to identify and attach to
the host cell
Bacteriophages

Viruses that infect bacteria
Photo Credit: Dr. Jan Mast, CODA
http://commons.wikimedia.org/wiki
/File:Bacteriofaag.gif
Reproduction of Viruses
Three basic steps
1.
The virus gets its hereditary material
(DNA or RNA) into the host cell
2.
The host cell makes new viruses
3.
The new virus particles leave the host
cell and infect new host cells
Reproduction of Viruses
Points to note:
The entire virus may enter the cell or it might
inject the hereditary material leaving the
protein coat outside the cell.
When the new virus particles leave the cell they
may kill the cell by bursting it or they may
not kill the cell in which case the cell may
go on producing more and more virus
particles.
Viruses and Humans

Viruses cause
diseases including:
 Cold sores
 Warts
 Colds
 AIDS
 Measles

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Mumps
Hepatitis
Chicken pox
Small pox
Influenza
Viruses and Humans

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Vaccines – weakened or killed diseasecausing organisms used to stimulate an
immune response (antibodies) allowing the
body to react immediately to exposure to the
disease-causing organism and prevent it from
causing disease
Most, but not all, vaccines are against
viruses. Includes mumps, measles, polio,
hepatitis, chicken pox, and rubella.
Viruses and Humans
Uses for viruses
 Control other “pests” for example
rabbits in Australia. Remember that
most viruses are very specific as to
what they can infect.
 As possible transmitters of
“replacement genes” to cure genetic
diseases such as cystic fibrosis.
The End