IS A VIRUS?
A virus is a small amount of infectious genetic material that causes
disease. There are many different types of viruses. They differ
in size, shape, type of nucleic acid, type of host and disease they
cause. Virus particles are too small to be seen without the aid
of an electron microscope capable of magnifying objects more than
100,000 times actual size (Fig.).
of organisms are subject to infection by viruses. Viruses enter
and take control of the cells of their hosts and direct their own
reproduction using mostly host enzymes, energy and materials such
as amino acids and bases. A virus consists of a nucleic acid (RNA
or DNA) surrounded by one or a few specific proteins. The protein
component of the virus is often referred to as the ‘coat protein’
or ‘capsid protein’. Viral proteins including proteins
key to viral reproduction, movement of virus within the host, spread
to noninfected hosts and symptoms are coded for the virus nucleic
acid. Viruses contain only enough nucleic acid to code for typically
4-10 proteins. They fullfill their needs for survival by genetically
directing the host to use its resources for the reproduction of
the virus. In just a few days of infection, millions of virus particles
are produced per infected cell.
obligate parasites in that they are completely dependent on hosts
for their reproduction and have adapted efficient and specific ways
to move within a plant and spread between plants by interacting
with vectors, pollen, seed and tubers to insure their survival.
They have the ability to adapt to new hosts and environments because
they readily mutate during reproduction.