Biology 2014-2015

Viruses
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Section 19-2

 

§         Virus:  the Latin word meaning “poison”

 

§         Was given this name by scientist named Beijerinck

 

HISTORY

 

§         In 1892, a Russian biologist named Ivanovski studied the disease of tobacco plants.  The disease is known as tobacco mosaic.

 

§         Ivanovski noticed that when you rubbed the juice from infected leaves on a healthy plant, the healthy plant contracted the disease.

 

§         Next, Ivanovski filtered the juice from infected leaves through a filter with pores small enough to filter out bacteria.

 

§         He checked the fluid for bacteria and found none.

 

§         When this fluid was rubbed on the healthy plant, they contracted the disease.

 

§         During Ivanovski’s day, scientists thought that bacteria were the smallest possible agents of disease.

 

§         Six years later, Beijerinck repeated Ivanovski’s work and concluded that an invisible agent must be causing the disease, the virus.

 

§         In 1935, the virus was found!  Dr. Wendell Stanley ground and removed fluid from over a ton of diseased tobacco leaves.

 

§         From the fluid, he isolated a teaspoon of needlelike crystals.

 

§         When stored in a bottle, they did nothing.  BUT, when placed in water and rubbed on leaves, they caused the disease.

 

§         From Stanley’s work, it was determined that viruses were very small and that they were non-living in crystal form.  They needed to be placed in living cells to be able to reproduce.

 

HOW BIG ARE VIRUSES?

 

§         Viruses are larger than a molecule but smaller than the smallest cells.

 

§         All viruses have one thing in common: They enter living cells, and once inside, use the machinery of the infected cells to produce more viruses.

 

§         To see the tobacco mosaic virus, you need an electron microscope.  The virus then needs to be magnified 200,000 times just to be seen.

 

WHAT MAKES UP A VIRUS?

 

§         Viruses are composed of:

 

Ø       The outer protein coat that is called the capsid.

 

Ø       The protein coat carries the genetic material that is either a strand of DNA or RNA.

 

Ø       The capsid binds to the surface of a cell and “tricks” the cell into allowing it inside.

 

§         There are a wide variety of shapes for viruses: oval shaped with tiny necks, some are shaped like spheres, cubes or needles.

 

VIRUSES ARE SUBCELLULAR!

 

 

§         This means that they lack organization of cells.

 

§         They do not contain any of the structures of a cell that is used for making protein or high energy molecules.

 

HOW DO VIRUSES WORK?  HOW DO VIRUSES REPRODUCE?

 

§         All viruses have the same basic life cycles.

 

§         When it is outside a cell, a virus does not show any signs of life. (grow or carry on respiration).

 

TO BECOME ACTIVE, A VIRUS REQUIRES A HOST CELL!

 

§         A host cell is any cell that is attacked (has the viral DNA or RNA inserted into its cytoplasm) by a virus.

 

§         VIRULENCE is a virus’s ability to cause disease.

 

§         Viruses only invade specific kinds of cells.  This is why they are classified according to their host cell.

 

§         There are bacteria, plant and animal viruses.

 

§         Tobacco mosaic attacks only certain types of plant tissue.

§         Polio only attacks one kind of nerve cell in the brain and spinal cord producing paralysis.

 

§         Human papillomavirus infects cells in the outermost layer of the skin causing warts.

 

VIRAL DISEASES CANNOT BE TREATED WITH ANTIBIOTICS!

 

§         Prevention of viruses is by the use of vaccines.

 

WHAT ARE OTHER VIRUSLIKE PARTICLES?

 

§         Viroids cause disease in plants.

 

§         These are single-stranded RNA molecules that have no surrounding capsids.

 

§         They disrupt the metabolism of the plant cell and stunt the growth of the entire plant.

 

§         Prions cause disease in animals.

 

§         These are “protein infectious particles” which contain no DNA or RNA.

 

§         They cause disease by forming protein clumps and eventually damage nerve tissue.

 

Lytic Cycle

 

1.  Phage attaches tail down to special site called the receptor site on a bacterias cell surface.

 

2.  Tail is like a hypodermic.  Sheath on the tail penetrates the cell wall and membrane.  The viral DNA is injected into the bacterium.

 

3.  In minutes, the viral DNA serves as the genetic code and has complete control of cell activity.  New DNA sequence codes for more viral DNA to be made.

 

4.  The bacteria becomes a factory for new viruses.  During assembly, proteins coded for by phage DNA act as enzymes that put new viruses together.

 

5.  An enzyme digests the bacterial wall from within. The cell breaks open releasing new viruses. The new phages attack other bacteria.

 

Lysogenic Cycle

 

1.  The virus attacks a bacterium. The DNA or RNA is injected.

 

2.  Genetic material attaches to the bacterial chromosome or host DNA.

 

3.  The virus does not cause the bacteria to produce more like the lytic virus.  Instead, it acts like an extra gene.  In this stage, the virus is called a prophage.  The prophage replicates whenever the host bacterium reproduces.

 

4.  As the bacteria divides and multiplies, the viral genetic material is being replicated too.  There is a copy in every bacteria made. Each bacterial offspring is infected with a prophage.  It causes no harm to the host cell.

 

5.  When the environmental conditions are right, the prophage becomes virulent and takes over the host cell.  It enters the lytic cycle, proceeding with replication and destroying the host cell.