What are viruses?
Very small particles
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
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_virus_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
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?
Diagram of a Flu Virus Public Domain
http://en.wikipedia.org/wiki/File:Flu_und_legende_color_c.jpg
Swine Flu virus electron micrograph Public Domain: CDC http://commons.wikimedia.org/wiki/File:B00528-Swine-flu.png
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:HIV-1_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
2. An outer protein coat 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
Mumps
Hepatitis
Chicken pox
Small pox
Influenza
Viruses and Humans
Vaccines – weakened or killed disease-causing 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.
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