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Because most viruses are extremely well adapted to their host organism, virus
structure varies greatly. However, there are some general structural
characteristics that all viruses share.
Figure %: General virus structure
All viruses have a capsid or head region that contains its genetic material.
The capsid is made of proteins and glycoproteins. Capsid contruction varies
greatly among viruses, with most being specialized for a particular virus's host
organism. Some viruses, mostly of the type infecting animals, have a membranous
envelope surrounding their capsid. This allows viruses to penetrate host
cells through membrane fusion. The virus's genetical material rests inside the
capsid; that material can be either DNA, RNA, or even in some cases a limited
number of enzymes. The type of genetic material a virus contains is used in
classification, and is discussed in Virus
Classification.
In addition to the head region, some viruses, mostly those that infect bacteria,
have a tail region. The tail is an often elaborate protein structure. It aids in
binding to the surface of the host cell and in the introduction of virus genetic
material to the host cell.
Virus "Life" Cycles
Figure%: Generalized Replication of Viruses
Though the details of virus infection and replication vary greatly with host
type, all viruses share 6 basic steps in their replication cycles. These are:
1) attachment; 2) penetration; 3) uncoating; 4) replication; 5) assembly;
6)release. As shown in , the virus must first attach
itself to the host cell. This is usually accomplished through special
glycoprotiens on the exterior of the capsid, envelope or tail. Next,
penetration occurs, either of the whole virus or just the contents of the
capsid. If the entire capsid enters, the genetic material must be uncoated to
make it available to the cell's replication machinery. Replication of genetic
material takes place, as well as the production of capsid and tail proteins.
Once all of the necessary parts have been replicated, individual virus particles
are assembled and released. Release often takes place in a destructive manner,
bursting and killing the host cell.
Some viruses have a slightly more complicated replication cycle involving
lytic and lysogenic phases. The lytic phase is similar to that described
above, with virus particles infecting and being replicated. In the lysogenic
phase, however, viral genetic material that has entered the host cell becomes
incorportated in the cell and lies dormant. It is passed on to the progeny of
the infected cells. Eventually, the lytic phase will start again, and cells that
were never infected themselves, but carry the viral genetic material will begin
to produce new virus particles.
