Viruses

Chapter 19.2

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

Viral AnatomyA. Scientist could not observe viruses until the

invention of the electron microscope.B. Structure of Viruses

1. A layer of protein (called the __capsid__) surrounds a core of hereditary material (called the _nucleic acid core-DNA or RNA_).

2. DNA and RNA have the same functions in viruses – Both can be double or single stranded.

C. Shape and Size1. Viruses have many different shapes___-

spherical, oval, long, narrow, cylindrical.2. Size- compares to that of a yeast cell 25 to 250

nanometers ( a nanometer is one millionth of a millimeter)

Viral Replication and the Lytic Cycle

A. Viruses _cannot_ carry out the functions of a living cell (no nucleus or cytoplasm).

B. _Replication or reproduction_ occurs only within a living cell.

C. Steps in the _Lytic cycle_ include: *(bacteriophage_ = infects bacteria)

Steps of the Lytic Cycle1. __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.

Lytic Cycle

Adsorption and Host Cell Penetration

Eclipse

(replication)Formation of new phages

(assembly)

Lysis

(host cell rupture)

Lysogenic Cycle (Viruses that reproduce without killing their hosts are

called temperate viruses):1. These viruses have 2 alternate forms of reproduction.

The lytic cycle or the lysogenic cycle.2. The process begins very similarly to the lytic infection -

_inserting__ the genome and becoming dormant.3. During the lysogenic cycle one of the temperate phage’s

genes remains active. It codes for a repressor protein keeping the repressor protein keeping the prophage genes inactive.

4. From here on the cells reproduce regularly. Each _carrying the hidden___ genes of the virus.

5. 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.

Lysogenic Cycle

Lytic Cycle Lysogenic cycle

Viruses and DiseaseA. Only specific viruses can infect certain types of cells.B. _Plant_ Viruses

1. Tobacco Mosaic, Tomato Bushy Stunt Disease, Southern Bean Mosaic Disease are examples.

2. Plants can become discolored or deformed. (Tumors)3. May not kill plants but can make plants unusable as

food or weaken the plant.C. Bacterial Viruses—ex: bacteriophageD. Animal Viruses

1. Birds, many times, are viral reservoirs for other animals.

2. _Rabies_, cold sores, herpes, yellow fever, _A.I.D.S._, mumps, measles, etc. are all examples of animal viruses.

Viral DiseasesA. The _virulence__ of a disease virus

depends on three factors: 1. The ability of a virus to _absorb and

penetrate__ a cell.2. The ability of the virus to _multiply__.3. The _number of viral particles_ infecting

the organism.

B. Influenza_ and the _common cold_-contaminated droplets in the air

Common Viral DiseasesVirus Disease Virulence

Influenza FluAir-bourne

High Virulence

Rhinovirus Common coldAirbourne

High Virulence

HIV AIDSBlood bourneBody Fluids

Low Virulence

HPV Genital WartsBody Fluids

Sexual ContactLow Virulence

Herpes Simplex II Genital HerpesBody Fluids

Sexual ContactLow Virulence