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

IcosahedronIcosahedron – 20 triangular faces Polio virus; ‘common’ cold viruses

HelicalHelical (rod-shaped) – helical nucleic acid & protein coat

Rabies, mumps viruses

Structures (con’t)

EnvelopedEnveloped virus – i.e. influenza Spherical lipid bilayer surrounds nucleic

acid; surface protein spikes ComplexComplex - 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)

BacteriophageBacteriophage – infects bacteria

Two major classifications of all viruses

DNADNA virus – makes RNA which makes more particles; or join host DNA before beginning production

RNARNA 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

ViroidViroid – strand of RNA w/no protein capsid; disrupt cell functions & use cell enzymes to make new viroids

PrionsPrions – glycoprotein with a 250 amino acid polypeptide; Kuru – degenerative nerve disease.

Reproduction and Evolution

Lytic cycleLytic 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 cycleLysogenic 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.

TransductionTransduction -