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BIOL 3340
Chapter 5
Chapter 5
Viruses
&
Other Acellular Agents
Viruses
Viruses:
• are noncellular or Acellular
infectious agents
Virology:
• study of viruses
Virologists:
• scientists who study viruses
General features of Viruses
Viruses are infectious agents with both living
and nonliving characteristics.
1. Living characteristics of viruses
 a. They reproduce at a fantastic rate, but only in
living host cells.
 b. They can mutate.

…General features of Viruses
2. Nonliving characteristics of viruses
a. They are acellular, that is, they contain
no cytoplasm or cellular organelles.
 b. They carry out no metabolism on their
own and must replicate using the host
cell's metabolic machinery.

…General features of Viruses
virus particles contains either DNA or RNA
(not both)
 Nucleic Acid is surrounded or coated by a
protein shell (capsid)
 Some viruses possess a membrane-like
envelope surrounding the particle

…General Properties of viruses
of 1 molecule of DNA or RNA
enclosed in coat of protein
 may have additional layers
 cannot reproduce independent of living cells
nor carry out cell division as procaryotes and
eucaryotes do
 consists
The Size and Morphology of
Selected Viruses
Figure 5.2
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8
Generalized Structure of Viruses
The Structure of Viruses


Virion size range is ~10-400 nm
All virions contain a nucleocapsid which is
composed of nucleic acid (DNA or RNA) and a
protein coat (capsid)


some viruses consist only of a nucleocapsid, others
have additional components
Envelopes
virions having envelopes = enveloped viruses
 virions lacking envelopes = naked viruses

Capsids
large macromolecular structures which
serve as protein coat of virus
 protect viral genetic material and aid in its
transfer between host cells
 made of protein subunits called
protomers

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11
Helical Capsids-Tobacco Mosaci
Virus

shaped like hollow tubes with protein walls
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12
Influenza Virus – an Enveloped Virus
with a Helical Nucleocapsid
Figure 5.4
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13
Attachment
Receptor sites:
 specific
surface structures on host to which
viruses attach
 specific for each virus
 can be proteins, lipopolysaccharides, techoic
acids, etc.
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14
Entry into the Host
most bacterial viruses (bacteriophages)
inject their nucleic acid into host
 eucaryotic viruses usually enter the
cytoplasm with the genome still enclosed.

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15
Figure 5.8
16
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Fusion with Host Membrane
Figure 5.12 (a)
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Permission required for reproduction or display.
17
Generalized Illustration of Virus
Reproduction
Figure 5.11
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18
Figure 5.22
19
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Top ten Infectious diseases

http://search.yahoo.com/search;_ylt=A
0geu510a4tJJPoAS8VXNyoA?p=list+of+
deadly+virus&y=Search&fr=yfp-t-501-s

Assignment: Ebola, Cholera, Shanti,
AIDS, West Nile, Small pox, Birds Flu.
Bibliography
http://en.wikipedia.org/wiki/Scientific_metho
d
 https://files.kennesaw.edu/faculty/jhendrix/bi
o3340/home.html
 Lecture PowerPoints Prescott’s Principles of
Microbiology-Mc Graw Hill Co.
 http://student.ccbcmd.edu/courses/bio141/lec
guide/unit3/viruses/ssvir.html
