Viruses10B Part I

Intro Viruses are strange particles that are

not classified as living but made of biological or organic compounds

They have to have a living host to multiply

Their main purpose – cause disease and often death

Discovery of Viruses 1890s – Dimitri Iwanowski filtered particles

that were causing tobacco mosaic disease and thought a toxin produced from a bacterium

Later in the 1930s, Wendell Stanley was able

to isolate and purify those particles that were multiplying inside the cells of tobacco leaves

Virus – comes from a Latin word meaning poison…

A Virus…Many scientists were stumped.

Virus particles do not multiply when isolated and alone

But somehow the virus grows and reproduces when placed on living tissue…

Is the virus alive? Or a non-living chemical? Do the cells make more virus? Does the virus alter the genes of the leaf cells? Does the virus supply genes to the cell to make

more virus?

Structure of VirusesViruses are made of 2 main

parts:Core – DNA or RNA (never both)Capsid – outer covering made of

tough protein; could also be surrounded by an envelope

Structure cont…

Virus ShapesVirus shapes are determined by their

DNA/RNAShapes include:

Helix – spiral shape (Tobacco mosaic virus) Icosahedron – 20 triangular sides Spherical – (HIV) Cylindrical and spiral - Ebola

Classification Viruses are classified based on:

1. What type of nucleic acid they contain; either DNA or RNA

2. Shape3. Presence or absence of an envelope 4. Method of infecting and replicating inside a

host cell

Viroids and Prions Even smaller than viruses are other agents

that cause disease! viroids and prions

A viroid is simply a short circular strand of RNA without a capsid or envelope that can still replicate inside a host cell Can cause Hepatitus D and plant diseases

A prion is smaller than even a viroid and is an abnormally shaped protein that can cause disease Can cause BSE (bovine spongiform encephalitis)

Or aka – mad cow disease

Smallpox “The most dangerous epidemic is the

smallpox...which sweeps at times like a storm of death over the land.”

Richard Burton, 1860 Dr. Edward Jenner, an English doctor and

scientist in the late 1700s noticed that milkmaids and dairy workers seemed to be immune to the deadly smallpox disease

He developed a way to expose an individual to small amounts of cowpox (similar to smallpox) that would make that person immune to the actual disease

Vaccines This process called vaccination (“of cows”) is not a cure for a disease but rather a way of

developing an immunity by exposing someone to a weaker or similar form of a

disease

• 200 years after Dr. Jenner developed the vaccination process, the World Health Organization has almost eliminated the global threat of smallpox.