The Amazing World of Viruses

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Viral Viral HistoryHistory

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Discovery of Discovery of VirusesViruses

•Beijerinck (1897) coined the Latin name “virus” meaning poison•He studied filtered plant juices & found they caused healthy plants to become sick

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Tobacco Mosaic Tobacco Mosaic VirusVirus

•Wendell Stanley (1935) crystallized sap from sick tobacco plants•He discovered viruses were made of nucleic acid and protein

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SmallpoxSmallpox•Edward Jenner (1796) developed a smallpox vaccine using milder cowpox viruses•Deadly viruses are said to be virulent•Smallpox has been eradicated in the world today

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Viewing VirusesViewing Viruses•Viruses are smaller Viruses are smaller than the smallest cellthan the smallest cell•Measured in Measured in nanometersnanometers•Viruses couldn’t be Viruses couldn’t be seen until the seen until the electron microscope electron microscope was invented in the was invented in the 2020thth century century

Interdependence

Viruses and organisms relyon their environmentand other species for survival.

VirusesBut, Viruses are NOT cells.

A virus is an infectious agent made up of:

•a core of nucleic acid (RNA or DNA)

•a protein coat

Alive or not?Even scientists disagree as to whether or not viruses are alive. What do you think? Look at the chart on the next page to help you decide.

CELLCELL VIRUSVIRUS

Made of cells YES NO

Obtain & use energy

YES NO

Grow & develop

YESOnly inside a living cell

Reproduce YESOnly inside a living cell

Respond & adapt

YES YES

Contain RNA or DNA

YES YES

Characteristic

of life

Alive or not?Most scientists do not

consider viruses to be living because don’t have the

necessary parts to reproduce on their own.

They MUST infect a cell and use its parts to reproduce.

Structureof a virus

Because viruses don’t carry around the necessary parts to reproduce themselves they are very simple structures.

ENVELOPE

NUCLEIC

ACIDCAPSID

Structureof a virus

Consists of 2 parts:

1. A Nucleic acid -either DNA or RNA

2. Capsid – A protein Coat that surrounds and protects the DNA/RNA

ENVELOPE

NUCLEIC

ACIDCAPSID

Types of Viruses1. Standard DNA based Virus2. Retro Virus3. Bacteriophage

Basic VirusA strand of nucleic acid

surrounded by a protein coat.

DNA RNA Protein

RetrovirusA virus that

contains RNA instead of DNA

Human Immunodeficiency Virus (HIV) is a retrovirus

HIV causes AIDS

ENVELOPE

RNA

CAPSID

BacteriophageA virus that

infects only bacteria.

CAPSID

DNA

TAIL FIBER

HEAD

CAPSID

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Helical VirusesHelical Viruses

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Polyhedral Polyhedral VirusesViruses

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Complex Complex VirusesViruses

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Ticking time bombs . . .

Viruses CANNOT reproduce, EXCEPT inside a living cell. They

invade a living cell and let the cell work for them.

Virus CycleA Viral Infection has 2 possible stages:

1.Lytic Cycle

2.Lysogenic Cycle

*Some viruses go straight to the lytic phase and never enter the lysogenic phase

Virus Cycle

1. Lytic Cycle - stage where virus is actively reproducing and killing host cells

2. Lysogenic Cycle - stage where virus is inactive and host cells remain unharmed

Lytic Cycle1. The virus injects its DNA into the

host cell

2. The DNA uses the host cell’s resources and organelles to make new viruses.

3. The viruses cause the cell to burst

4. The cell dies and the newly made copies of the virus are released to go infect new cells.

5. The cycle continues.

1 2 3

4 5

Cycle Illustration

Lysogenic Cycle1. The Virus injects its DNA into the host

cell.

2. The DNA inserts itself into the host cell’s DNA and stays there inactive.

3. There are NO symptoms of the infection at this stage.

4. Eventually the viral DNA will remove itself from the host cell’s DNA and the lytic cycle will begin.

LyticCycl

e

Lyso-Genic

Cycle

Role of virusesAll viruses are parasites: they benefit while

harming the host cell/organism.

Viruses cause disease in every kind of organism—animals, plants, fungi, protists, and bacteria.

Are hard to cure, but many can be prevented by

•Good hygiene•Immunization

Viral diseases

• HIV• Measles• Smallpox• Influenza• Chicken pox

• Common cold• Herpes• Warts• Mononucleosi

s• Mumps

Some viral diseases

MEASLES

Mumps

SMALLPOX

HERPES

1918 Flu

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Herpes VirusHerpes Virus

SIMPLEX I and II

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AdenovirusAdenovirus

COMMON COLD

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Influenza VirusInfluenza Virus

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Chickenpox Chickenpox VirusVirus

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Papillomavirus – Papillomavirus – Warts!Warts!

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Helical VirusesHelical Viruses

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Polyhedral Polyhedral VirusesViruses

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Complex Complex VirusesViruses

Linkshttp://www.virology.net/Big_Virology/BVFamilyIndex.html

http://www.virology.net/Big_Virology/BVFamilyIndex.html

Biology, Kenneth R. Miller & Joseph Levine, 1991, Prentice-Hall, Inc., Englewood Cliffs NJ.

Biology: The Web of Life, Eric Strauss & Marylin Lisowski, 1998, Scott Foresman Addison Wesley, Menlo Park CA

TAKS Power Tools, Glencoe Science, Glencoe McGraw-Hill, 2001, New York, NY.

Microsoft Office Clipart

Sources