Download H1N1 Modeling Infection

Viruses at Work
Please read the following encyclopedia article about viruses and then get together with your
group to try to develop a better understanding of how they work. There aren't any
illustrations of viruses to accompany this article, but try to imagine them as you read.
Virus
virus, parasite with a noncellular structure composed mainly of nucleic acid within a
protein coat. Viruses usually are too small (100–2,000 Angstrom units) to be seen
with the light microscope and thus must be studied by electron microscopes. In one
stage of their life cycle, in which they are free and infectious, virus particles do not
carry out the functions of living cells, such as respiration and growth; in the other
stage, however, viruses enter living plant, animal, or bacterial cells and make use of
the host cell's chemical energy and its protein- and nucleic acid–synthesizing ability
to replicate themselves.
The existence of submicroscopic infectious agents was suspected by the end of the
19th cent.; in 1892 the Russian botanist Dimitri Iwanowski showed that the sap from
tobacco plants infected with mosaic disease, even after being passed through a
porcelain filter known to retain all bacteria, contained an agent that could infect other
tobacco plants. In 1900 a similarly filterable agent was reported for foot-and-mouth
disease of cattle. In 1935 the American virologist W. M. Stanley crystallized tobacco
mosaic virus; for that work Stanley shared the 1946 Nobel Prize in Chemistry with J.
H. Northrup and J. B. Summer. Later studies of virus crystals established that the
crystals were composed of individual virus particles, or virions. By the early 21st
cent. the understanding of viruses had grown to the point where scientists
synthesized (2002) a strain of poliovirus using their knowledge of that virus's genetic
code and chemical components required.
Viral Structure
Typically the protein coat, or capsid, of an individual virus particle, or virion, is
composed of multiple copies of one or several types of protein subunits, or
capsomeres. Some viruses contain enzymes, and some have an outer membranous
envelope. Many viruses have striking geometrically regular shapes, with helical
structure as in tobacco mosaic virus, polyhedral (often icosahedral) symmetry as in
herpes virus, or more complex mixtures of arrangements as in large viruses, such as
the pox viruses and the larger bacterial viruses, or bacteriophages. Certain viruses,
such as bacteriophages, have complex protein tails. The inner viral genetic
material—the nucleic acid—may be double stranded, with two complementary
strands, or single stranded; it may be deoxyribonucleic acid (DNA) or ribonucleic
acid (RNA). The nucleic acid specifies information for the synthesis of from a few to
50 different proteins, depending on the type of virus.
Viral Infection of a Host Cell
A free virus particle may be thought of as a packaging device by which viral genetic
material can be introduced into appropriate host cells, which the virus can recognize
by means of proteins on its outermost surface. A bacterial virus infects the cell by
attaching fibers of its protein tail to a specific receptor site on the bacterial cell wall
and then injecting the nucleic acid into the host, leaving the empty capsid outside. In
H1N1: The Coming Plague
From
1
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2007, Columbia University Press. All rights reserved.
viruses with a membrane envelope the nucleocapsid (capsid plus nucleic acid)
enters the cell cytoplasm by a process in which the viral envelope merges with a
host cell membrane, often the membrane delimiting an endocytic structure (see
endocytosis) in which the virus has been engulfed.
Within the cell the virus nucleic acid uses the host machinery to make copies of the
viral nucleic acid as well as enzymes needed by the virus and coats and enveloping
proteins, the coat proteins of the virus. The details of the process by which the
information in viral nucleic acid is expressed and the sites in the cell where the virus
locates vary according to the type of nucleic acid the virus contains and other viral
features. As viral components are formed within a host cell, virions are created by a
self-assembly process; that is, capsomere subunits spontaneously assemble into a
protein coat around the nucleic core. Release of virus particles from the host may
occur by lysis of the host cell, as in bacteria, or by budding from the host cell's
surface that provides the envelope of membrane-enveloped forms.
Some viruses do not kill host cells but rather persist within them in one form or
another. For example, certain of the viruses that can transform cells into a cancerous
state (see cancer) are retroviruses; their genetic material is RNA but they carry an
enzyme that can copy the RNA's information into DNA molecules, which then can
integrate into the genetic apparatus of the host cell and reside there, generating
corresponding products via host cell machinery. Similarly, in bacterial DNA viruses
known as temperate phages, the viral nucleic acid becomes integrated into the host
cell chromosomal material, a condition known as lysogeny; lysogenic phages are
similar in many ways to genetic particles in bacterial cells called episomes (see
recombination)
Process and Procedures
1. With the help of your group mates, each of you please create a diagram of a virus based
upon the description given in this article. Be sure to include the following labels: capsid,
envelope, protein tail, nucleic acid.
2. Be sure to include an explanation of the role that each of the parts of a virus carries out.
In some cases, it may not be entirely clear, but make your best attempt.
3. In your group, create a diagram of the life cycle of a virus as it reproduces using a cell.
You may want to use labels to help explain what is happening in each step of the process.
Ask if you are not sure about some part of the cycle, and please make this process as clear
as possible. Be sure to get a copy of this into your notebook.
Analysis
1. If we are infected with a virus, what happens to the viruses that are released from a
cell? What would this tell you about how rapidly a virus infection spreads?
2. Why are programs that attack our computers called viruses? Please explain.
H1N1: The Coming Plague
From
2
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2007, Columbia University Press. All rights reserved.