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

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?

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

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

Structures (con’t) Bacteriophage – infects bacteria

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

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.

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

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 -