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Characteristics of Viruses – Part 1
Similarities and Differences between Viruses and Cells
If you ever had a cold or the flu, you probably hosted viruses. A virus is an
infectious agent made up of a core of nucleic acid and a protein coat. Viruses are
not cells. Unlike plant and animal cells, a virus package does not include many
parts , such as a nucleus, a membrane or organelles that a cell has. Examples of the
cellular organelles that are absent in viruses include: ribosomes, mitochondria and
chloroplasts. Although viruses are not cells, they do have organized structural parts.
Compared to even the smallest cell, a virus is tiny. The virus that causes
polio, for example, measures only 20 nanometers in diameter. One nanometer is
one billionth of a meter. At that size, 3000 polioviruses could line up across the
period at the end of this sentence.
All viruses have at least two parts: a protective protein coat and a core of
nucleic acid. The protein coat around the core of the nucleic acid is called a capsid.
Depending on the virus, the capsid may consist of one or several kinds of protein.
The capsid protects the viral nucleic acid core from its environment.
In cells, DNA is the hereditary material. Some viruses also contain DNA,
while other viruses contain only RNA. In viruses containing RNA, the RNA
functions as the hereditary material.
Compared to a cell, a virus has a relatively simple existence. Viruses do not
eat, respire, or respond to environmental changes as cells do. It should not surprise
you, therefore, to learn that viruses have fewer genes than cells have. While a
human cell may contain about 100,000 genes and the bacterial cell about 1000, a
virus may contain only 5 genes.
An Influenza Virus
Envelope
An envelope is made of
an outer lipid layer and
an inner protein layer
surrounding the capsid.
RNA
The influenza virus
has a total of eight
strands of RNA. Many
viruses contain DNA
rather than RNA.
Projections
Capsid
The proteins in a capsid
are determined by the
genes in the virus.
The spike-like projections
on the viral envelope
help the virus recognize
and attach to a host cell.
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