Download 20.1 Viruses - Hermantown Community Schools

Lesson Overview
Studying the Human Genome
Lesson Overview
20.1 Viruses
Lesson Overview
Studying the Human Genome
Discovery of Viruses
In 1892, Dmitri Ivanovski demonstrated that the cause of tobacco
mosaic disease was found in the liquid extracted from infected plants.
In 1897, Martinus Beijerinck suggested that tiny particles in the juice
caused the disease, and he named these particles viruses, after the
Latin word for “poison.”
In 1935, Wendell Stanley isolated crystals of tobacco mosaic virus.
Since living organisms do not crystallize, Stanley inferred that viruses
were not truly alive.
Lesson Overview
Studying the Human Genome
Discovery of Viruses
A virus is a nonliving particle made of proteins, nucleic acids, and
sometimes lipids.
Viruses can reproduce only by infecting living cells.
Lesson Overview
Studying the Human Genome
Structure and Composition
Viruses differ widely in terms of size and structure.
Most viruses are so small they can be seen only with the aid of a
powerful electron microscope.
Lesson Overview
Studying the Human Genome
Structure and Composition
The protein coat surrounding a virus is called a capsid.
The simplest viruses contain only a few genes, whereas the most
complex may have more than a hundred genes.
Lesson Overview
Studying the Human Genome
Structure and Composition
Most viruses have proteins on their capsid that bind to receptor proteins
on the host cell. Most viruses infect only a very specific kind of cell.
The proteins “trick” the cell to take the viral DNA/RNA into the cell.
Once inside, the viral genes are eventually expressed and may destroy
the cell.
Lesson Overview
Studying the Human Genome
Viral Infections
Inside living cells, viruses use their DNA/RNA to make multiple
copies of themselves. Some viruses replicate immediately (lytic infection),
others persist in an inactive state within the host (lysogenic infection).
Lesson Overview
Studying the Human Genome
Lytic Infections
In a lytic infection, a virus enters a host cell, makes copies of itself,
and causes the cell to burst, or lyse.
- The virus injects its DNA into the cell.
- The cell then makes mRNA, from the viral genes, which is translated
into viral proteins that chop up the cell’s DNA.
- Controlled by viral genes, the host cell’s metabolic system makes
copies of viral nucleic acid and capsid proteins, which are then
assembled into new virus particles.
- The host cell lyses, releasing hundreds of virus particles that go on to
infect other cells.
Lesson Overview
Studying the Human Genome
Lytic Infections
An outlaw in the Wild West is a good
analogy for a lytic virus
First, the outlaw eliminates the town’s
existing authority.
In a lytic infection, the host cell’s DNA is
chopped up.
Lesson Overview
Studying the Human Genome
Lytic Infections
Next, the outlaw demands to be outfitted
with new equipment from the local
townspeople.
In a lytic infection, the viruses use the
host cell to make viral DNA and viral
proteins.
Lesson Overview
Studying the Human Genome
Lytic Infections
Finally, the outlaw forms a gang that
leaves the town to attack new
communities.
In a lytic infection, the host cell bursts,
releasing hundreds of virus particles.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
-
-
In a lysogenic infection a host cell is not immediately taken over.
The viral nucleic acid is inserted into the host cell’s DNA, which is then
copied along with the host DNA without damaging the host.
In this way, each generation of daughter cells is infected.
Bacteriophage DNA that becomes embedded in the bacterial host’s
DNA is called a prophage.
The prophage may remain part of the DNA of the host cell for many
generations.
Influences from the environment—radiation, heat, etc—trigger the
prophage to become active.
It then removes itself from the host cell DNA, directs the synthesis of
new virus particles, and now becomes an active lytic infection.
Lesson Overview
Studying the Human Genome
A Closer Look at Two RNA Viruses
About 70 percent of viruses contain RNA rather than DNA.
In humans, RNA viruses cause a wide range of infections, from
relatively mild colds to severe cases of HIV.
Lesson Overview
Studying the Human Genome
The Common Cold (Lytic)
A capsid settles on a cell, typically in the host’s nose, and is brought
inside, where a viral protein makes many new copies of the viral RNA.
The host cell’s ribosomes translate the viral RNA into capsids and other
viral proteins.
Within 8 hours, the host cell releases hundreds of new virus particles to
infect other cells.
Lesson Overview
Studying the Human Genome
HIV (lysogenic)
The deadly disease called acquired immune deficiency syndrome
(AIDS) is caused by an RNA virus called human immunodeficiency virus
(HIV).
HIV belongs to a group of RNA viruses that are called retroviruses.
The genetic information of a retrovirus is copied from RNA to DNA
instead of from DNA to RNA.
Lesson Overview
Studying the Human Genome
HIV
When a retrovirus infects a cell, it makes a DNA copy of its RNA.
Lesson Overview
Studying the Human Genome
HIV
The copy inserts itself into the DNA of the host cell.
Lesson Overview
Studying the Human Genome
HIV
Retroviral infections are similar to lysogenic infections of bacteria. Much
like a prophage in a bacterial host, the viral DNA may remain inactive
for many cell cycles before making new virus particles and damaging
the cells of the host’s immune system.
Lesson Overview
Studying the Human Genome
Viruses and Cells
All viruses are parasites. Parasites depend entirely upon other living
organisms for their existence, harming these organisms in the process.
Viruses must infect living cells in order to grow and reproduce, taking
advantage of the nutrients and cellular machinery of their hosts.
Lesson Overview
Studying the Human Genome
Viruses and Cells
Some of the main differences between cells and viruses are
summarized in this chart.