Viruses
Learning Objectives
 Explain how viruses reproduce.
 Explain what happens after a virus infects a cell.
The Discovery of Viruses
1892
Dmitri Ivanovski
1897
Martinus Beijerinck
1935
Wendell Stanley
Virus Reproduction
A virus is nonliving.
Viruses can reproduce only by infecting living cells.
Virus Structure and Composition
Capsid: protein coat surrounding a virus
Viral Infections
Viruses use their genetic information to reproduce inside living cells.
Lysogenic
infection
Lytic
infection
Lytic Infections
The virus injects DNA into
a bacterium.
Viral genes are transcribed
by the host cell.
Viral enzymes lyse the
bacterium’s cell wall.
The new viruses
escape.
The proteins and
nucleic acids assemble
into new viruses.
The bacterium
makes new viral
proteins and nucleic
acid.
Lytic Infections Analogy
A lytic virus is similar to the Wild West of the American frontier.
The host cell’s
DNA is chopped up.
Virus uses host cell
to make viral DNA
and viral proteins.
The host cell bursts,
releasing hundreds
of virus particles.
Lysogenic Infections
The prophage may replicate
with the bacterium for many
generations.
The viral DNA inserts
itself into the bacterial
chromosome.
Prophage
The virus injects DNA
into the bacterium.
The prophage can exit the
bacterial chromosome and
enter a lytic cycle.
An RNA Virus: The Common Cold
Once the cold virus has penetrated the host’s cells, it uses the
host’s cellular machinery to replicate itself.
The virus makes many
copies of its RNA.
Cytoplasm
The copies are
translated by the host
into new viral parts.
The parts assemble
into new viruses and
burst from the host cell.
An RNA Virus: HIV
HIV makes a DNA copy of itself that inserts into the host’s DNA.
There, it may remain inactive for many cell cycles.
A DNA copy of the
viral RNA is made.
The parts assemble into
new viruses and burst
from the host cell.
DNA
The copy is
inserted into the
host’s genome.
Cytoplasm
It is later transcribed
and translated into
new viral parts.
Viruses and Cells
Summary of Viruses
• Viruses reproduce by infecting living cells.
• Some viruses replicate immediately;
others initially persist in an inactive state
within the host.
Lysogenic infection
Lytic infection