Viruses whose capsids are not covered by an envelope are known as non enveloped viruses.The capsid of
a nonenveloped virus protects the nucleic acid from nuclease
enzymes in biological fluids and promotes the virus's attachment
to susceptible host cells.
When the host has been infected by a virus, the host immune
system is stimulated to produce antibodies (proteins that react
with the surface proteins of the virus). This interaction between
host antibodies and virus proteins should inactivate the virus
and stop the infection. However, some viruses can escape antibodies
because regions of the genes that code for these viruses'
surface proteins are susceptible to mutations. The progeny of
mutant viruses have altered surface proteins, such that the antibodies
are not able to react with them. Influenza virus frequently
undergoes such changes in its spikes. This is why you can get  influenza more than once. Although you may have produced antibodies to one influenza virus, the virus can mutate & infect again.


Viruses may be classified into several different morphological
types on the basis of their capsid architecture. The structure of
these capsids has been revealed by electron microscopy and a
technique called X-ray crystallography.

Helical viruses:

Helical viruses resemble long rods that may be rigid or flexible.
The viral nucleic acid is found within a hollow, cylindrical capsid
that has a helical structure . The viruses that
cause rabies and Ebola hemorrhagic fever are helical viruses.

Polyhedral Viruses:

Many animal, plant, and bacterial viruses are polyhedral, or many sided,
viruses. The capsid of most polyhedral viruses is in the shape  of an icosahedron, a regular polyhedron with 20 triangular faces and
12 corners. The capsomeres of each face form an
equilateral triangle. An example of a polyhedral virus in the shape
of an icosahedron is the adenovirus.
Another icosahedral virus is the poliovirus.

Enveloped Viruses:

As noted earlier, the capsid of some viruses is covered by an envelope.
Enveloped viruses are roughly sphericaL When helical or
polyhedral viruses are enclosed by envelopes, they are called
enveloped helical or enveloped polyhedral viruses. An example of an
enveloped helical virus is the influenza virus.
An example of an enveloped polyhedral (icosahedral) virus is the
herpes simplex virus.

Complex Viruses
Some viruses, particularly bacterial viruses, have complicated
structures and are called complex viruses. One example of a
complex virus is a bacteriophage. Some bacteriophages have capsids
to which additional structures are attached .Another example of complex viruses are poxviruses, which do not contain clearly identifiable capsids but have several coats around the nucleic acid.

Cited By Kamal Singh Khadka
Msc Microbiology TU


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