Viruses STRUCTURE, CLASSIFICATION AND PHYSIOLOGY OF VIRUSES

Viruses
Viruses

At the boundary of life, between the
At the boundary of life, between the
macromolecules (which are not alive) and the
macromolecules (which are not alive) and the
prokaryotic cells (which are), lie the viruses and
prokaryotic cells (which are), lie the viruses and
bacteriophages (phages).
bacteriophages (phages).
These twilight creatures are
These twilight creatures are
parasites
parasites


responsible for causing many diseases in living
responsible for causing many diseases in living
things (herpes and HIV in humans, for example).
things (herpes and HIV in humans, for example).
Viruses are found everywhere.
Viruses are found everywhere.
Viruses consist of a
Viruses consist of a
core of nucleic acid,
core of nucleic acid,
either DNA or RNA, and a protective
either DNA or RNA, and a protective
coat of protein molecules and

coat of protein molecules and
sometimes lipids.
sometimes lipids.
Viruses -
The Boundary of Life

In isolation, viruses and
In isolation, viruses and
bacteriophages show none of the
bacteriophages show none of the
expected signs of life. They do not
expected signs of life. They do not
respond to stimuli, they do not grow,
respond to stimuli, they do not grow,
they do not do any of the things we
they do not do any of the things we
normally associate with life.
normally associate with life.
Strictly speaking, they should not be
Strictly speaking, they should not be
considered "living" organisms at all.
considered "living" organisms at all.
However, they are more complex than
However, they are more complex than
a lifeless collection of macromolecules
a lifeless collection of macromolecules
and they do show one of the most
and they do show one of the most
important signs of life: the ability to
important signs of life: the ability to

reproduce at a fantastic rate
reproduce at a fantastic rate
but only in a host cell
but only in a host cell
.
.



Bacteriophages attack bacteria
Bacteriophages attack bacteria
(prokaryotes)
(prokaryotes)
viruses attack eukaryotic cells.
viruses attack eukaryotic cells.
Viruses and bacteriophages invade cells
Viruses and bacteriophages invade cells
and use the host cell's machinery to
and use the host cell's machinery to
synthesize more of their own
synthesize more of their own
macromolecules.
macromolecules.


Once inside the host the bacteriophage or
Once inside the host the bacteriophage or
virus will either go into a
virus will either go into a
Lytic

Lytic
Cycle
Cycle


-
-
destroying the host cell during
destroying the host cell during
reproduction.
reproduction.
or
or
It will go into a
It will go into a
Lysogenic Cycle
Lysogenic Cycle
- a
- a
parasitic type of partnership with
parasitic type of partnership with
the cell
the cell


The Lytic Cycle
The Lytic Cycle

The Lysogenic Cycle
The Lysogenic Cycle

A
A
provirus
provirus


is a
is a
DNA virus that has been inserted into a
DNA virus that has been inserted into a
host cell chromosome.
host cell chromosome.

A
A
retrovirus
retrovirus
injects the enzyme, reverse transcriptase into the cell
injects the enzyme, reverse transcriptase into the cell
to copy viral RNA into DNA
to copy viral RNA into DNA
.
.

HIV is a
HIV is a
retrovirus injecting the enzyme, reverse

retrovirus injecting the enzyme, reverse
transcriptase into the cell to copy viral RNA into DNA
transcriptase into the cell to copy viral RNA into DNA
.
.



Viruses are host specific – a protein on the surface of the virus
has a shape that matches a molecule in the plasma membrane
of its host, allowing the virus to lock onto the host cell.

HIV
HIV
doesn’t target just any cell, it goes right for the cells that
doesn’t target just any cell, it goes right for the cells that
want to kill it. “Helper" T cells are HIV's primary target. These
want to kill it. “Helper" T cells are HIV's primary target. These
cells help direct the immune system's response to various
cells help direct the immune system's response to various
pathogens.
pathogens.

HIV undermines the body's ability to
HIV undermines the body's ability to
protect against disease by depleting T
protect against disease by depleting T
cells thus
cells thus
destroying the immune system

destroying the immune system
.
.
The virus can infect 10 billion cells a day,
The virus can infect 10 billion cells a day,
yet only 1.8 billion can be replaced daily.
yet only 1.8 billion can be replaced daily.

After many years of a constant battle, the body
After many years of a constant battle, the body
has insufficient numbers of T-Cells to mount an
has insufficient numbers of T-Cells to mount an
immune response against infections. At the point
immune response against infections. At the point
when the body is unable to fight off infections, a
when the body is unable to fight off infections, a
person is said to have the disease
person is said to have the disease
AIDS
AIDS
.
.


It is not the virus or the disease that ultimately
It is not the virus or the disease that ultimately
kills a person; it is the inability to fight off
kills a person; it is the inability to fight off
something as minor as the common cold.
something as minor as the common cold.