A virus is a biological
particle composed of genetic material and
protein. A typical virus consists of
either RNA or DNA encased in a protein
coat called a capsid. When a virus causes
a disease, the virus is said to be
virulent (VIR-yuh-lunt). If the virus
does not cause disease immediately, it is
said to be temperate. Viruses are constructed
of compounds usually associated with
cells, but they are not considered living
organisms. They have some, but not all,
of the characteristics of life listed in
Chapter 1. They have no nucleus,
cytoplasm, organelles, or cell membrane.
They do not reproduce by either mitosis
or meiosis, nor are they capable of
carrying out cellular functions. Study
Table 19-1, in which viruses are compared
and contrasted with cells.
Because they are
not cells, viruses can only reproduce by
invading a host cell and using the
enzymes and organelles of the host cell
to make more viruses. They are therefore
obligate intracellular parasites, which
means they require a host cell to
reproduce. Outside a host cell a virus is
a lifeless particle with no control of
its movements. It is spread on the wind,
in water, in food, or via blood or other
bodily secretions.
Studying
Viruses
Scientists knew
as early as 1800 that some factor existed
that was smaller than bacteria and that
could transmit disease. However, they
were unaware of the nature of the factor
until 1935, when Wendell Stanley
(1904-1971) first isolated the virus that
causes tobacco mosaic disease. The
electron microscope was the first tool
that permitted scientists actually to
look at viruses. Recent advances such as
tissue culture, serology,
electrophoresis, and nucleic acid
sequencing help virologists learn more
about viruses.
Tissue culture
is the growing of living cells in a
controlled medium. This technique is the
foundation for viral research. By
