Viruses

1
Viruses

• Chapter 19.2

2
Viruses

• Discovery 1892, Dimitri Iwanowski, A Russian
  Scientist- Disease agent is filterable.
• 1935, Wendell Stanley, an American Scientist,
  isolated the first virus, the tobacco mosaic virus

3
Viral Anatomy

• Scientist could not observe viruses until the
  invention of the electron microscope.
• Structure of Viruses
• A layer of protein (called the __capsid__)
  surrounds a core of hereditary material (called
  the _nucleic acid core-DNA or RNA_).
• DNA and RNA have the same functions in viruses
  Both can be double or single stranded.
• Shape and Size
• Viruses have many different shapes___-spherical,
  oval, long, narrow, cylindrical.
• Size- compares to that of a yeast cell 25 to 250
  nanometers ( a nanometer is one millionth of a
  millimeter)

4
Viral Replication and the Lytic Cycle

• Viruses _cannot_ carry out the functions of a
  living cell (no nucleus or cytoplasm).
• _Replication or reproduction_ occurs only within
  a living cell.
• Steps in the _Lytic cycle_ include
• (bacteriophage_ infects bacteria)

5
Steps of the Lytic Cycle

1. __Adsorption_ tail of bacteriophage (adsorption
   sites) attach to the receptor sites on specific
   bacteria.
2. __Host Cell Penetration__- enzyme in tail makes
   opening in cell wall of bacterium and nucleic
   acid is injected from virus into bacterium.
3. __Eclipse (Replication)_- the nucleic acid of the
   bacteriophage causes the cell to form new viral
   parts.
4. __Formation of New Phages (assembly)__- viral
   parts assemble to form complete bacteriophages-
   usually 100 to 300 new viruses formed. (Can
   produce _1000_ viruses in one day)
5. Host Cell Rupture- cell rupture (_Lysis_) occurs
   as the host cell bursts, releasing new viruses.

6
Lytic Cycle
Adsorption and Host Cell Penetration
Lysis (host cell rupture)
Eclipse (replication)
Formation of new phages (assembly)
7
Lysogenic Cycle

• (Viruses that reproduce without killing their
  hosts are called temperate viruses)
• These viruses have 2 alternate forms of
  reproduction. The lytic cycle or the lysogenic
  cycle.
• The process begins very similarly to the lytic
  infection -_inserting__ the genome and becoming
  dormant.
• During the lysogenic cycle one of the temperate
  phages genes remains active. It codes for a
  repressor protein keeping the repressor protein
  keeping the prophage genes inactive.
• From here on the cells reproduce regularly. Each
  _carrying the hidden___ genes of the virus.
• Eventually the virus genes in each new cell
  complete the cycle- new phage DNA and proteins
  are synthesized and assemble into virus
  particles, then the cell bursts releasing virus
  particles.

8
Lysogenic Cycle
Lysogenic cycle
Lytic Cycle
9
Viruses and Disease

• Only specific viruses can infect certain types of
  cells.
• _Plant_ Viruses
• Tobacco Mosaic, Tomato Bushy Stunt Disease,
  Southern Bean Mosaic Disease are examples.
• Plants can become discolored or deformed.
  (Tumors)
• May not kill plants but can make plants unusable
  as food or weaken the plant.
• Bacterial Virusesex bacteriophage
• Animal Viruses
• Birds, many times, are viral reservoirs for other
  animals.
• _Rabies_, cold sores, herpes, yellow fever,
  _A.I.D.S._, mumps, measles, etc. are all examples
  of animal viruses.

10
Viral Diseases

• The _virulence__ of a disease virus depends on
  three factors
• The ability of a virus to _absorb and penetrate__
  a cell.
• The ability of the virus to _multiply__.
• The _number of viral particles_ infecting the
  organism.
• Influenza_ and the _common cold_ -contaminated
  droplets in the air

11
Common Viral Diseases
Virus Disease Virulence
Influenza Flu Air-bourne High Virulence
Rhinovirus Common cold Airbourne High Virulence
HIV AIDS Blood bourne Body Fluids Low Virulence
HPV Genital Warts Body Fluids Sexual Contact Low Virulence
Herpes Simplex II Genital Herpes Body Fluids Sexual Contact Low Virulence