Biomedical Engineering Reference
In-Depth Information
defined as noncellular infectious agents consisting of a single type of nucleic acid (either
RNA or DNA) surrounded by a coat of protein. Because they are incapable of independ-
ent reproduction, they are labeled as nonliving or noncellular. All organisms, including
other microbes, are susceptible to viral attack, although virus are generally specific for a
given host [13].
A virion is therefore defined as nothing more than nucleic acid surrounded by a pro-
tein shell, the components of which taken together represent one infectious particle
[14]. The outer protein shell is referred to as the capsid, which consists of repeating pro-
tein subunits called capsomeres. The architectural arrangement of the capsid, or its
structure, determines a virus's general form, classified as polyhedral, helical, or binal
(see Figure 21.2). Slightly more complex arrangements are present in virus that have
animal hosts. A notable addition is the presence of a membranous envelope surround-
ing the capsid; the virus having capsid are termed “enveloped,” whereas those that lack
the feature are called “naked.” Connecting the envelope to the capsid is a layer of
matrix proteins. These proteins also serve to strengthen the envelope that itself com-
prises glycoproteins (a protein-carbohydrate complex) embedded within lipid from the
host's cell membrane. Glycoproteins may project from the envelope's surface in the
form of spikes of varying lengths and shapes, which serve an important role in attach-
ment and infection [15].
The origin of each component determines the virus's resistance and survival, which
means it is important to distinguish between components that are coded by the virus's genes
(genes are segment of RNA or DNA) vs. those that are coded by the host's genes. Only the
virus codes the proteins of the viral envelope. The host cell codes the carbohydrate and
Nucleic acid
Capsomere
Nucleic acid
Capsid
Capsid
Capsid
Polyhedral
Helical
Binal
FIGURE 21.1
The types of virus structures. (From Indoor Environmental Quality Corporation. IEQ Fact Sheets. 2004. Virus.
http://www.germology.com/fact_sheets.htm. Accessed December 20, 2004.)
FIGURE 21.2
Directigen™ Flu A
B Rapid Test Kit. (From www.bd.com/clinical/products/direct/fluaw.asp. Accessed
December 18, 2004.)
