VIRUSES Chapter 13, pp 323-340
A) characteristics (Table 13-1, p326)
1) properties
a) obligate intracellular parasites
b) nucleic acid = RNA or DNA
c) genome surrounded by protein coat = capsid, may have envelop
around coat
Fig 13-2, p 325
2) host range = viruses tend to be species and cell specific
a) animal viruses
b) plant viruses
c) bacteria viruses = bacteriophages or phages
3) size = 20 - 300 nm (Fig 13-3, p325)
4) shape = variable (Fig 13-1, p324)
a) helical
b) polyhedral
c) enveloped
d) complex
B) cultivation and assaying of viruses
1) cultivation
a) tissue (cell) culture
b) embryonated eggs
c) bacteria in culture (for phages)
2) assaying (quantitation) viruses
a) biological effects (on cells)
- plaques in bacteria cultures
- cytopathic effects = rounding, giant cells, inclusion bodies
b) count viral particles under electron microscopy
c) hemagglutination = agglutination or clumping of red blood
cells (RBC)
C) multiplication of bacteriophages (T even phages - Fig 13-5, p
328) = lytic cycle
1) attachment (to a specific cell surface structure e.g. pili of
flagella)
2) penetration = only DNA enters
3) biosynthesis of viral components
4) maturation = spontaneous assembly of viral particles
5) release following lysis of cell (lytic cycle)
D) Lysogeny = phage DNA integrates into bacterial chromosome
(lysogenic or temperate phages)
integrated viral DNA = prophage Fig 13-6, p329 = phage lambda
1) prophage may impart new properties to the bacteria = lysogenic
conversion
e.g. Corynebacterium diphtheriae = toxin is encoded for
by prophage;
scarlet fever results from toxin encoded for by prophage in Streptococcus
Table 13-3, p331
2) prophage may excise back out and enter lytic cycle
Here specific bacterial genes may be taken along with the virus
(specialized
transduction Fig 13-11, p 334)