Viruses: Section 10.17—10.21

Bacteria: Section 8.3, section 16.1– 16.7, and pg. 55

Immune Response: Chapter 24

General Information:

1. “Virus” comes from the Latin word for “poison”.

2. It took approximately 52 years to discover viruses.

3. The following people were involved in the discovery by researching a disease that was attacking their tobacco crop (later called the “Tobacco Mosaic Virus”.

Adolf Mayer (German)

Dmitri Ivanovsky (Russian)

Martinus Beijerinck (Dutch)

W.M. Stanley (American)

What he did…

1. Transferred sap of a sick tobacco plant to a healthy one.

2. The healthy plant got sick.

3. He looked for bacteria in the sap but couldn’t find any

Conclusions he made…

1. The Tobacco Mosaic disease is contagious

2. He assumed it was caused by a very small bacteria

What he did…

1. Ground up diseased leaves to collect sap from plant.

2. Filtered the sap through a bacteria trapping filter.

3. Applied the filtered sap to healthy plant (should have no bacteria in it)

4. The healthy plant caught the disease.

Conclusions he made…

1. A very small bacteria leaked through the filter.

2. The bacteria in the infected plant secretes toxins and it is these that is causing the infection.

What he did…1. He took the sap from infected plants, diluted it and applied it to healthy plants. 2. Then he repeated the procedure from the newly infected plants, repeated the above procedure, so on and on.

What he concluded…1. The newly infected plants were as infected as the original. If it were a toxin, it would have been diluted

thousands of times, and it would have showed signs of weakening infection. 2. He hypothesized that the disease was caused by

something smaller and simpler than bacteria. 3. He called this agent a “virus”.

What he did…1. Tried to grow the “small bacteria” on agar.2. Tried to kill it using alcohol (bacteria would have died)3. Isolated and crystallized the tobacco mosaic virus

What he concluded…1. Because viruses can be crystallized, they are not cells (and therefore not bacteria), but something much simpler. 2. Now that viruses were isolated, they could study the structure and behavior.

II. Structure of a Virus

A. Basic parts:

1. Inner core that is either DNA or RNA (not both)

2. Inner core is surrounded by a “CAPSID”

a. made of protein…also called a protein coat

b. can be complex (head, tail)

c. can be simple (rod)

d. arrangement of proteins in the capsid determines the shape

3. Some, not all, have an “ENVELOPE” that surrounds the capsid.

Ebola virus Marburg virusRabies virus

These are bacteriophages. This is the “head, tail” structure. They attack bacteria.

Adenovirus-African swine fever

Foot and Mouth disease virus

Polio virus

Tobacco Mosaic Virus

B. VIROID

1. A small circular piece of RNA with no capsid.

2. The RNA pairs with itself to make it circular.

3. Contain only enough information to allow an RNA polymerase to copy them but not enough to make a protein message.

4. Their presense causes side effects.

a. may interfere with the plants production of ribosomes

b. shown to cause tumors on potatoes

5. Little is known about how they spread.

C. PRION1. A self replicating protein2. Causes “Scrapie” (degenerates nervous system in sheep and goats)3. Causes Mad Cow Disease4. Causes Creutzfeldt-Jakob disease and Gerstmann- Strassler Syndrome in humans a. neurological disease

D. RETROVIRUS1. Virus with RNA and an enzyme called “Reverse Transcriptase”2. Enters cells and enzyme copies viral RNA into DNA.3. DNA then becomes a “provirus” (virus DNA integrated with host cells DNA)

E. Specificity

1. Viruses are specific in the cells they attack.

2. Plant viruses only attack plants and animal viruses only attack animals.

3. May only attack one species (polio and measles…humans only)

4. Why? host cells have receptor proteins on their membranes that give off signals. Viruses only identify certain ones.

III. Virus Replication – there are two ways in which viruses can reproduce.

A. The Lytic Cycle

1. discovered in the 1940’s

2. This type of replication causes an “Active Infection” – the infection occurs immediately.

3. The viruses that replicate this way are “Virulent viruses” – they kill the host cell they invade.

4. Steps: a. Virus attaches to the cell membrane (specific) b. Virus injects DNA into the cell c. Virus DNA takes over the cell, telling it to replicate the

viral DNA and make new capsids. d. Capsids and DNA assemble into new viruses e. An enzyme from the virus “Lyses” (breaks) the cell membrane, releasing new viruses.

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B

C

D

E

B. Lysogenic Cycle1. discovered in 19532. This type of replication causes a “Latent Infection” – the infection DOES NOT occur immediately.3. Examples of this type of virus: Herpes virus, HIV, 4. Steps: a. Virus attaches to the cell membrane b. Virus injects its DNA into the cell c. Viral DNA forms a circle inside the host cell d. The viral DNA attaches to the host cell’s DNA e. The viral DNA becomes a part of the host cell’s DNA f. The host cell replicates by mitosis (any new cell has viral DNA in it) g. Viral DNA pinches off of the host cell’s DNA and forms a circle. h. This initiates the lytic cycle, therefore killing the cell.

A, B

C

D, E

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H

H