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characteristics of living things
dichotomous keys
Dec 2­3:01 PM
viruses are not considered to be living organisms
do not have cellular structure and are not included in the six kingdoms
viruses invade the cells of organisms and use the metabolism of a host cell to reproduce
a virus consists of strands of DNA or RNA surrounded by a protective protein coat called a capsid
Dec 2­3:03 PM
viruses can come in various shapes
Dec 2­3:07 PM
Where do viruses come from?
a virus cannot reproduce without a host
viruses probably started as fragments of nucleic acids that escaped from their original cells
nucleic acids are complex compounds carrying hereditary information
this would suggest that viruses and their hosts (cells) are closely related and why viruses are able to infect certain cells but not others
Dec 2­3:09 PM
Viral Replication of a T4 Virus
(a) Attachment:
T4 virus attaches to a specific receptor site on a hosts cell wall (e­coli bacteria)
weak bonds form joining the virus to the host
(b) Entry:
T4 injects its nucleic acid (DNA) into the bacteria host
the viral DNA passes into the cell while the capisid remains outside
Dec 2­3:12 PM
(c) Replication
hosts protein synthesis is stopped by viral degradation of the host DNA
the hosts metabolism instead starts to replicate the viral DNA
(d) Assembly
spontaneous assembly of new viruses occur in the host cell
Dec 2­3:16 PM
(e) Lysis and release
the host cell membrane and cell wall break open (lyses)
this releases the new virus particles and the cell dies
*see diagrams on pages 122 and 123
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Pages 122 ­ 126
Page 126: #'s 1,2,5,6,10
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