Biol. 106 Lecture Topic # 16 Diversity: Viruses

I. Viruses not in any KINGDOM!

A. Discovered late in 19th century

1. Smallpox, tobacco mosaic- infectious agent could not be seen w/ light microscope

2. agent could pass through filters that stopped bacteria

B. Structure

1. extremely small (15 - few 100 nanometers)

2. largest virus just approaches size of smallest bacteria

3. basic virus = nucleic acid molecule containing viral genome

4. surrounded by a protein coat = capsid

a. capsid protects genetic material in passage from one host cell to another

b. capsid responsible for host specificity

c. capsid has a # of protein subunits that determines virus particle's shape

5. some viruses have a membranous outer envelope of glycoprotein and lipid

6. some viruses may also have a few enzymes

7. viral genome may consist of a molecule of DNA or RNA but NOT both!

C. How Differ from living cells?

D. Reproduction

1. Lytic Cycle - invades cell, reproduce, dispersed upon lysis, destroys cell and take over metabolic machinery, cell is quickly killed. ex.,

2. Lysogenic Cycle - invades cell, eventually enter lysis, does not kill host cell quickly. ex. ,

Transduction = virus may bring part of the host genome with them when released and incorporated into new host cell. ex.,

3. Budding

E. Classification of Viruses

1. shape of capsid 3. single or double stranded

2. presence of RNA or DNA 4. presence of envelope around capsid

F. Notable Viruses etc.

1. Retroviruses

2. Oncogenic viruses (tumor)

3. Viroids (Theodor Diener)

G. Viruses and Evolution:

1. Extreme specialization for parasitism

2. Free-living noncellular predecessors

3. Fragments of genetic material

H. Evolutionary Implications of Viruses:

1. Host distribution is peculiar, ex.'s

2. Important Selective Force in Evolution of Cellular Organisms:

evolve rapidly,

may mutate often,

rapid generation time,

move genes from one organism to the next.