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VIRUSES – Note
o Not considered living things
o Described somewhere between living and nonliving things
o Contain genetic material, but lack structures necessary for metabolism,
reproduction & growth
o Need a host cell to reproduce
o Cause infectious diseases – from common cold to rabies and AIDS
Size and Structure
 1892 – first virus (tobacco mosaic disease) was discovered
 1935 – tobacco mosaic virus was chemically isolated
Plants – leaf cells
 Viruses infect
Animals – respiratory or digestive tracts
Bacteria
 Consists of
o nucleic acid  DNA or RNA
o Protein coat called capsid
o Some contain an envelope
o Some have protein spikes
 Shapes  rod-shaped – tobacco mosaic
 polyhedral – herpes
 helical – mumps
DNA Viruses produce RNA
Genetic Makeup
RNA Viruses make DNA
Directs production of viral proteins OR
Direct the production of proteins by host
May combine w/host DNA  producing new viruses
RNA may make DNA w/aid of reverse transcriptase
 DNA produces new RNA  produces proteins that
produce new viruses (retrovirus like in AIDS)
Standard: 10d. Viruses
Reproduction
Bacteriophages (phages) -- are viruses that attack bacterial cells
-- their host cells grow rapidly and easily in the lab cultures
-- many have a tadpole-shaped outer protein that transports its nucleic acid
Lytic Cycle
Phage injects its DNA into host cell
Cell forced to produce new phages
Lysogenic Cycle
Phage injects its DNA into host cell
Phage’s nucleic acid merges with the host’s
DNA
Cell becomes so full that it burst thus releasing Cell divides & phage DNA divides with it
a new phage generation that infect more
thereby producing more cells with viral DNA
bacteria
Viruses and Disease (see figure 30-30)
HIV  AIDS
o HIV virus that causes AIDS was identified in the US and France in 1984
o HIV attacks helper T cells  immune response weakens
o “people do not die from AIDS, but rather from complications of diseases and infections
that their bodies cannot fight.
o There is no cure for AIDS, but research continues
TUMOR VIRUSES  Oncogenes
DNA tumor virus directs constant production of viral DNA & proteins – causing rapid cell
division & growth (no new viruses, but cells grow and divide without control)
1798 first vaccine against smallpox developed
1884 first rabies vaccine developed
1954 polio vaccine developed
Control mechanisms:
Natural immunity – body’s own immune system
Vaccines – weak or dead virus used to stimulate production of antiviral substances
Interferon – produced by a cell infected with virus & inhibits virus reproduction
*Notes:
1. There is no cure for viral infections however, vaccines can prevent them
2. Antibiotics are not effective against viruses
Standard: 10d. Viruses