Download bacteria and viruses

Bacteria and Viruses
Discovery of Viruses
– Dmitri Ivanovski (1892)
• demonstrated that the cause of tobacco mosaic disease was
found in the liquid extracted from infected plants.
– Martinus Beijerinck (1897)
• suggested that tiny particles in the juice caused the disease,
and he named these particles viruses, after the Latin word
for “poison”
– Wendell Stanley (1935)
• isolated crystals of tobacco mosaic virus
• living organisms do not crystallize, so Stanley inferred that
viruses were not truly alive
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.
Structure and Composition
– Viruses differ widely in terms of size and
structure
• Note no scientific names
– Most viruses are so small they can be seen
only with the aid of a powerful electron
microscope
Structure and Composition
– The protein coat surrounding a virus is called
a capsid.
– Some viruses, such as the influenza virus,
have an additional membrane that surrounds
the capsid.
– The simplest viruses contain only a few
genes, whereas the most complex may have
more than a hundred genes.
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.
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.
Two Modes of Viral Infection
Lytic Infection
Lysogenic Infection
• Virus enters cell, inserts
• Virus enters cell, makes
its DNA into host cell
copies of itself and causes
DNA
the host cell to rupture
Kill host
cell to
• Viral DNA lies dormant in
• Cell dies quickly
make
host DNA, replicates with
more
host DNA
• Ex – cold, flu
virus
particles
• Eventually virus
enters lytic cycle
•Ex – herpes, HIV
Lytic Infections
– In a lytic infection, a virus enters a bacterial
cell, makes copies of itself, and causes the
cell to burst, or lyse
• Ex: Bacteriophage T4 causes such an infection
Lytic Infections
– Bacteriophage T4
has a DNA core
inside a protein
capsid that binds to
the surface of a host
cell.
Lytic Infections
– The virus injects its DNA into the cell.
Lytic Infections
– The cell then begins to make messenger
RNA (mRNA) from the viral genes.
Lytic Infections
– The viral mRNA is translated into viral
proteins that chop up the cell’s DNA.
Lytic Infections
– Controlled by viral genes, the host cell’s
metabolic system makes copies of viral
nucleic acid.
Lytic Infections
– The host cell’s metabolic system also makes
copies of capsid proteins.
Lytic Infections
– The viral nucleic acid and capsid proteins are
then assembled into new virus particles.
Lytic Infections
– The host cell lyses, releasing hundreds of
virus particles that go on to infect other cells.
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.
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.
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.
Lysogenic Infection
– Some bacterial viruses cause a lysogenic
infection.
Lysogenic Infection
– In a lysogenic infection a host cell is not
immediately taken over.
Lysogenic Infection
– The viral nucleic acid is inserted into the host
cell’s DNA.
Lysogenic Infection
– The viral DNA is then copied along with the
host DNA without damaging the host.
Lysogenic Infection
– Viral DNA multiplies as the host cells
multiply.
Lysogenic Infection
– In this way, each generation of daughter cells
derived from the original host cell is infected.
Lysogenic Infection
– Bacteriophage DNA that becomes
embedded in the bacterial host’s DNA is
called a prophage.
Lysogenic Infection
– The prophage may remain part of the DNA of
the host cell for many generations.
Lysogenic Infection
– Influences from the environment—radiation,
heat, etc—trigger the prophage to become
active.
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.
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.
Common Diseases Caused by Viruses
Type of Virus
Nucleic Acid
Disease
Oncogenic
viruses
DNA
cancer
Retroviruses
RNA
cancer, AIDS
Adenoviruses
DNA
respiratory
infections
Herpes viruses
DNA
chickenpox
Poxviruses
DNA
smallpox
The Common Cold
– Cold viruses attack with a very simple, fastacting 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.
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.
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.
HIV
– When a retrovirus infects a cell, it makes a
DNA copy of its RNA using an enzyme called
reverse transcriptase
HIV
– The copy inserts itself into the DNA of the
host cell.
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.
Viruses and Cells
– All viruses are nonliving 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
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
Viruses and Cells
– Some of the main differences between cells
and viruses are summarized in this chart.
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.
Section Summary
• How do viruses reproduce?
– By infecting a living cell
• 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 (lytic
infection), while others initially persist in an
inactive state within the host (lysogenic infection).