viruses organisms that bridge between living and non-living things. composed of: viral disease is...
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VIRUSES
Organisms that bridge between living and non-living things.
Composed of:
Viral disease is immediate – virulent
Disease is not immediate – temperate
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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?
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Viral Structure
IcosahedronIcosahedron – 20 triangular faces Polio virus; ‘common’ cold viruses
HelicalHelical (rod-shaped) – helical nucleic acid & protein coat
Rabies, mumps viruses
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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
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Structures (con’t)
BacteriophageBacteriophage – infects bacteria
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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
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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.
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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
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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 -