Viruses are responsible for many human diseases and come in a variety of shapes and sizes, but all share certain properties. Viruses are composed of nucleic acid and proteins and can only reproduce inside a host cell. When outside a cell, a virus exists as a virion which contains a small amount of genetic material. Viruses infect specific host cells, like plants, animals, or bacteria, by binding to membrane receptors on the surface of those cells.

1. VIRUSES

2. Viruses are responsible for several human
diseases, including AIDS, polio, flu, cold
sores, measles and some type of cancer.
Viruses occur in a wide variety of very
different shapes, sizes and constructions,
but all of them share certain properties.

3. What are viruses?
macromolecular aggregates composed by
nucleic acid and proteins.
Obligatory intra-cellular parasites
that is,
they cannot reproduce unless present
within a host cell.

4. In one virus one form of
nucleic acid can be
present
(DNA or RNA)

5. Outside of a living cell, the virus exists
as a particle, or virion, which is little
more than a macromolecular package.
The virion contains a small amount of
genetic material that, depending on the
virus, can be single-stranded or double-
stranded, RNA or DNA.

6. Depending on the specific virus,
the host may be a plant, animal,
or bacterial cell.

7. Virus diversity

9. adenovirus
adenovirus, which causes respiratory infections in mammals,
has an icosahedral capsid
DNA a doppia elica, 30-42,000 bp

10. Hiv - human immunodeficiency virus
the protein capsid is surrounded
by a lipid-containing outer envelope
that is derived from the modified
plasma membrane of the host cell as
the virus buds from the host-cell
surface.

11. Each virus has on its surface a protein that is able to bind to
a particular surface component of its host cell. For example,
the protein that projects from the surface of the HIV
particle (labeled gp120, the molecular mass 120,000 daltons)
interacts with a specific protein (called CD4) on the surface
of certain white blood cells, facilitating the entry of the virus
into the cell. The interaction between viral and host proteins
determines the specificity of the virus, that is, the types of
host cells that the virus can enter and infect.
The virus that causes rabies, for example, is able to infect
many different types of mammalian hosts, including dogs,
bats, and humans on the contrary, human flu viruses are
generally able to infect only the respiratory epithelial cells of
human hosts.

12. There are two basic types of viral infection. (1) In most cases, the
virus arrests the normal synthetic activities of the host and redirects
the cell to use its available materials to manufacture viral nucleic acids
and proteins, which assemble into new viruses. Viruses, in other words,
do not grow like cells; they are assembled from components directly
into the mature-sized virions. Ultimately, the infected cell is killed by
plasma membrane rupture (lyses) and the release of a new generation
of viral particles, capable of infecting neighboring cells, occurs.

13. Binding to membrane specific
receptors

14. (2) In other cases, the infecting virus does not induce the
death of the host cell, but instead inserts (integrates)
its DNA into the DNA of the host cell. The integrated viral
DNA is called “provirus”.

15. Lytic cycle Lysogenic cycle
Bacteriophage

17. RNA virus infection