Download Prescott`s Microbiology, 9th Edition Chapter 6 –Viruses and Other

Prescott’s Microbiology, 9th Edition
Chapter 6 –Viruses and Other Acellular Infectious Agents
GUIDELINES FOR ANSWERING THE MICRO INQUIRY QUESTIONS
Figure 6.1 Which capsids are icosahedral? Which are helical? Which have complex symmetry?
Viruses pictured with icosahedral symmetry include human papilloma virus, herpes virus, and
X174 phage. Those with helical capsids include Rhabdovirus and tobacco mosaic virus. Those
with complex capsids include vaccine, T phages, tailed phages, and Mimivirus.
Figure 6.5 Are the capsomers at the vertices of an adenovirus pentamers or hexamers? What is
the difference between a pentamer and hexamer?
Looking at figure 6.5b, the light blue balls at the vertices represent the capsomers. It can be
clearly seen that they contact five other coat proteins, seen as yellow balls, thus they are
pentamers. Since viral protein coats are called capsids, the individual units of capsids are called
capsomers. Capsomers are made of protomers, which consist of one or more viral coat proteins.
A capsomer made of five protomers is called a pentamer or penton, while a capsomer made of
six protomers is called a hexamer or hexon.
Figure 6.7 Why is T4 said to have binal symmetry?
Binal means two types of symmetry in the coat. For T4, the head has icosahedral symmetry and
the tail has helical.
Figure 6.10 Which of these mechanisms involves the production of a vesicle in the host cell?
Which involves the interaction between viral proteins and host cell membrane proteins?
Entry into the host cell via endocytosis by enveloped and nonenveloped viruses involve the
formation of a vesicle or endosome within the host cell. All three types of viral entry illustrated
involve the interaction of viral proteins with host cell membrane proteins (receptors).
Figure 6.13 Why do the empty capsids remain attached to the cell after the viral genome enters
the host cell?
Empty viral capsids remain attached to the host bacterial cell because attachment was mediated
by a specific receptor-ligand interaction.
Figure 6.15 Why is a lysogen considered a new or different strain of a given bacteria species?
A lysogen is considered a new or different strain of a given bacterial species because it contains
phage DNA (containing viral genes) that confers new properties on the lysogenic bacteria (by
expressing those viral genes), with one new common characteristic being that the cell cannot be
reinfected by the same virus.
Figure 6.19 What would happen to a plate with individual plaques if it were returned to an
incubator so that the viral infection could continue?
Plates that have individual viral plaques if reincubated will have continuation of the lytic cycle
and the virus will continue to infect host cells and lyse them upon maturation. The plaques will
continue to get larger with the spread of virus until they coalesce and all cells have been lysedyou will eventually get a clear plate because all bacteria will be lysed. Thus, the plaque assay is
time dependent.
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