Living or Not? Viruses
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 not—they can only
reproduce by means of using another cell’s machinery—has been a
source of debate for many years. Because of their in-between status,
viruses do not fit into the taxonomic system; neither do they commonly
appear on the SAT II. All you need to know about viruses appears
below.
Structure
All viruses have a protein capsid or head region that
contains genetic material. The genetic material can be either DNA,
RNA, or even in some cases a limited number of enzymes. Some viruses
also have an elaborate protein tail region. The tail aids in binding
to the surface of the host cell and penetrating the surface of the
host so that the virus’s genetic material can be introduced.
Virus “Life Cycle”
Though the details of virus infection and replication
vary greatly with the type of host a particular virus attacks, all
viruses share four basic steps in their replication cycles:
-
Attachment: Using specialized protein
structures located on the exterior of the capsid or tail, the virus
latches onto the cell it will attack and hijack. The protein structures
are specific to specific cells. A virus that can attach to a bacterium
is unlikely to be able to attack animal cells.
- Penetration: The
virus breaks through the cell wall and cell membrane, releasing
its genetic material into the host cell.
- Replication
and assembly: The viral genetic material hijacks the cell machinery.
Host ribosomes begin to produce viral proteins and nucleic acids.
The virus uses the host cell to assemble many new viruses.
- Release: Viruses
are bad guests. In addition to the production of new viruses, the
viral genetic material usually forces the host cell to produce an
enzyme that kills, or lyses, the host and breaks it open, freeing
the many new viruses to go and hunt new host cells to attack. Almost
always, the host cell is killed when it is invaded by a virus.
