Viruses

The Smallest Organisms?

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.

The End