Biomedical Engineering Reference
In-Depth Information
TABLE 21.1
Characteristics and Comparison between Virus and Living Microorganisms
Characteristic
Virus
Living Microorganisms
Size
Generally less than 200 nm
Generally more than 200 nm
Nucleic acid
DNA or RNA
DNA and RNA
Outer covering
Protein coat; some with envelope
Complex membrane, wall,
or both
Reproduction
Requires host
Generally self-replicating
Metabolism
Utilizes host metabolic machinery
Utilizes own metabolic
machinery
Cultivation
Must be grown on media
Generally grown on
containing cells
cell-free media
Source
: From Indoor Environmental Quality Corporation. IEQ Fact Sheets. 2004. Virus. http://www.germology.
com/fact_sheets.htm. Accessed December 20, 2004.
lipids. Therefore, the type of lipid and carbohydrate within a given viral envelope is depend-
ent on the particular host [16]. Distinguishing virus from other living microorganisms is the
fact that they lack their own means of replication. The general comparisons between virus
and living microorganisms along with their characteristics are given in Table 21.1.
21.1.2
Viral Taxonomy (Classification)
Animal virus (virus that require animal hosts) are classified based on
size;
●
the nature of the nucleic acid: DNA or RNA, single-stranded or double-stranded,
segmented or single molecule;
●
the type of viral replication: type of nucleic acid, type of participating enzymes,
location of replication within the host cell;
●
capsid structure: either polyhedral, helical, or binal; and
●
presence or absence of viral envelope.
●
21.1.3
How Virus Cause Disease
A broad spectrum of disease in all living organisms (including humans) is caused by virus.
To understand how virus cause these diseases, one must understand the basics of viral
replication (i.e., reproduction involving the host's cellular mechanisms). There are essen-
tially six steps to this process, details of which are varied and complex for a given type of
virus. Each stage of the replication cycle influences the type of disease and its
timing. The six basic steps are:
1.
Attachment (adsorption): the virion attaches to the host cell.
2.
Penetration: the virion breaks through the cell wall or membrane.
3.
Uncoating: the virion releases its nucleic acid from the protein coat.
4.
Replication: to produce more viral nucleic acid and viral proteins, the virion
exploits the host's cellular mechanisms for reproduction.
5.
Assembly (maturation): viral nucleic acids and viral proteins are reassembled.
6.
Release: virions are released from the host cell; this generally results in cell and
tissue damage and further activation of the immune system; the manifestations
of this process (as well as its location within the host) determine the type and
extent of disease.
