1. VIRUSES Organisms that bridge between living and non-living things.
Composed of:
Viral disease is immediate – virulent
Disease is not immediate – temperate

2. Why are viruses considered ‘not alive’?
No nucleus, cytoplasm, organelles or cell membrane
Cannot reproduce thru meiosis or mitosis
They are not cells; they are intracellular parasites
No control of movement outside cells
How are they spread?

3. Viral Structure Icosahedron – 20 triangular faces
Polio virus; ‘common’ cold viruses
Helical (rod-shaped) – helical nucleic acid & protein coat
Rabies, mumps viruses

4. Structures (con’t) Enveloped virus – i.e. influenza
Spherical lipid bilayer surrounds nucleic acid; surface protein spikes
Complex - nucleic acid inside two protein coats, in turn covered by lipid layer and glycoproteins (i.e. – HIV)
Capsid (coat) = lipids and glycoproteins

5. Structures (con’t)
Bacteriophage – infects bacteria

6. Two major classifications of all viruses
DNA virus – makes RNA which makes more particles; or join host DNA before beginning production
RNA virus – enter and make proteins directly using host ribosomes, or retroviruses – use reverse transcriptase to make DNA from RNA, DNA makes new RNA which makes proteins. HIV is a retrovirus

7. Viral variations
Viroid – strand of RNA w/no protein capsid; disrupt cell functions & use cell enzymes to make new viroids
Prions – glycoprotein with a 250 amino acid polypeptide; Kuru – degenerative nerve disease.

8. Reproduction and Evolution
Lytic cycle – virulent strains; cycle occurs in 5 phases (1-2 days)
1. Adsorption – recognizes receptor site
2. Entry – nucleic acid injected into host cell
3. Replication – host reproduces viral components
4. Assembly - of new viruses
5. Release – lysis caused by viral enzyme

9. Reproduction (con’t.)
Lysogenic cycle – may be dormant for months or years
Temperate virus (phage) {i.e. Herpes Simplex I} injects DNA into host cell; viral DNA integrated into host DNA – prophage. Host cell reproduces and copies viral DNA routinely
External stimuli (sunshine, UV radiation) triggers viral lytic cycle – cold sores.
Transduction -