Lesson Overview
Studying the Human Genome
Lesson Overview
20.1 Viruses
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
Most viruses have proteins on their surface
membrane or capsid that bind to receptor
proteins on the host cell.
The proteins “trick” the cell to take the virus,
or in some cases just its genetic material, into
the cell.
Once inside, the viral genes are eventually
expressed and may destroy the cell.
Lesson Overview
Studying the Human Genome
Structure and Composition
Most viruses infect only a very specific kind of cell.
Plant viruses infect plant cells.
Most animal viruses infect only certain related
species of animals.
viruses that infect bacteria are called
bacteriophages.
Lesson Overview
Studying the Human Genome
Viral Infections
What happens after a virus infects a cell?
Inside living cells, viruses use their genetic information to make
multiple copies of themselves.
Some viruses replicate immediately, while others initially
persist in an inactive state within the host.
Lesson Overview
Studying the Human Genome
Types of Viral Infections
• Lytic infection
• Lysogenic Infection
Lesson Overview
Studying the Human Genome
Lytic Infections
In a lytic infection, a virus enters a bacterial cell, makes copies of
itself, and causes the cell to burst, or lyse.
Bacteriophage T4 is an example of a bacteriophage that causes such
an infection.
Lesson Overview
Studying the Human Genome
Lytic Infections
Bacteriophage T4 has a DNA
core inside a protein capsid that
binds to the surface of a host
cell.
Lesson Overview
Studying the Human Genome
Lytic Infections
The virus injects its DNA into the cell.
Lesson Overview
Studying the Human Genome
Lytic Infections
Controlled by viral genes, the host cell’s metabolic system makes
copies of viral nucleic acid.
Lesson Overview
Studying the Human Genome
Lytic Infections
The host cell lyses, releasing hundreds of virus particles that go on to
infect other cells.
Lesson Overview
Studying the Human Genome
Lytic Infections
A lytic virus is similar to an outlaw in the
Wild West of the American frontier in the
demands the virus makes on its host.
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
Some bacterial viruses cause a lysogenic infection.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
In a lysogenic infection a host cell is not immediately taken over.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
The viral nucleic acid is inserted into the host cell’s DNA.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
The viral DNA is then copied along with the host DNA without damaging
the host.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
Viral DNA multiplies as the host cells multiply.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
In this way, each generation of daughter cells derived from the original
host cell is infected.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
Bacteriophage DNA that becomes embedded in the bacterial host’s
DNA is called a prophage.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
The prophage may remain part of the DNA of the host cell for many
generations.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
Influences from the environment—radiation, heat, etc—trigger the
prophage to become active.
Lesson Overview
Studying the Human Genome
Lysogenic Infection
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.
Certain kinds of cancer also begin with an infection by viral RNA.
Lesson Overview
Studying the Human Genome
The Common Cold
Cold viruses attack with a very simple, fast-acting infection.
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.
Lesson Overview
Studying the Human Genome
The Common Cold
The host cell’s ribosomes mistake the viral RNA for the host’s own
mRNA and translate it into capsids and other viral proteins.
The new capsids assemble around the viral RNA copies, and within 8
hours, the host cell releases hundreds of new virus particles to infect
other cells.
Lesson Overview
Studying the Human Genome
Common Cold Video- How the Flu Virus infects
• A virus infects a host cell and then uses it to make more
viruses
• The viruses then infect more cells, until the host is weakened
or killed
• Here is a video that explains how a common cold virus infects
cells
http://www.youtube.com/watch?v=Rpj0emEGShQ
Lesson Overview
Studying the Human Genome
HIV
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
How HIV Infects the Cell Video
HIV virus attacks a cell and then uses it to make more
viruses
http://www.youtube.com/watch?v=l-2ilZA-_aw
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
Viruses have many of the characteristics of living things. After infecting
living cells, viruses can reproduce, regulate gene expression, and even
evolve.
Lesson Overview
Studying the Human Genome
Viruses and Cells
Some of the main differences between cells and viruses are
summarized in this chart.
Lesson Overview
Studying the Human Genome
Viruses and Cells
Although viruses are smaller and simpler than the smallest cells, it is
unlikely that they were the first living organisms.
Because viruses are dependent upon living organisms, it seems more
likely that viruses developed after living cells.
The first viruses may have evolved from the genetic material of living
cells. Viruses have continued to evolve, along with the cells they infect,
for billions of years.