Viruses are extremely small infectious agents that invade cells of all types.
Once inside another cell, viruses become hijackers, using the cells' machinery
to produce more viruses. Whether viruses constitute living organisms or merely
conglomerations of molecules has been a source of debate for many years.
Are they alive?
One of the fundamental hallmarks of life is the ability to reproduce. Whether
viruses have this ability is key to a debate over their status as living things.
Some argue that since viruses cannot reproduce independently, they are not
alive. However, similar to viruses, there are a few prokaryotes that are
obligate parasites and cannot reproduce without a host. But these
prokaryotes show another hallmark of life that viruses lack: growth. Once
assembled, a virus does not change in size or chemical composition. They lack
the machinery for producing energy to drive such biological processes. This
makes them radically different from any known organism.
Viruses do, however, show some characteristics of living things. They are made
of proteins and glycoproteins like cells are. They contain genetic information
needed to produce more viruses in the form of DNA or RNA. They evolve to adapt
to their hosts. So while it is doubtful viruses are truly alive, they are
clearly very similar to living organisms.
Virus Diversity
There is more diversity among viruses than among all groups of living organisms
combined. New varieties are constantly being described. It would be nearly
impossible to describe all of the groups of viruses and their characteristics
briefly. Instead, we will look at the characteristics shared by all viruses,
such as their basic structure and general replicative cycle. Then we will
discuss the features used to classify new viruses, such as shape and form of
genetic material.
