1.
2.
3.
Review What do viruses depend on for their
reproduction
Review Describe each of the two paths viruses
may follow once they have entered a cell
Compare and Contrast Compare the structure of
a virus to the structure of both a prokaryotic cell
and a eukaryotic cell. Use a graphic organizer of
your choice to organize the information
CH 20 VIRUSES AND PROKARYOTES
20.1 Viruses
Viruses




Nonliving particle made of proteins, nucleic acids,
and sometimes lipids
Can reproduce only by infecting living cells
Wide variety of size and structure
Very small
 Need
electron microscope to see.

Capsid
 Protein
coat surrounding a virus
 Proteins on their surface membrane that trick receptor
cells on host cell into accepting its genetic material

Contain a few to a few hundred genes.


Most viruses infect only a very specific kind of cell
Bacteriophages
 Viruses


that infect bacteria
Plant viruses infect plant cells
Most animal viruses infect only certain related
species of animals.
Lytic Infections



Virus enters a bacterial cell
Makes copies of itself
Causes the cell to burst, or
lyse
 Bacteriophage
T4.

The virus injects its DNA into the cell.

Cell then begins to make messenger RNA (mRNA)
from the viral genes.

Viral mRNA is translated into viral proteins that
chop up the cell’s DNA.

Host cell’s metabolic system makes copies of viral
nucleic acid and capsid proteins.

Viral nucleic acid and capsid proteins are then
assembled into new virus particles.

Host cell lyses, releasing hundreds of virus particles
that go on to infect other cells.


Lytic virus is similar to
an outlaw in the Wild
West
Outlaw eliminates the
town’s existing
authority.

Outlaw demands to be
outfitted with new
equipment from the
local townspeople.

Outlaw forms a gang
that leaves the town to
attack new
communities.
Lysogenic Infection

Host cell is not immediately taken over.


Viral nucleic acid is inserted into the host cell’s DNA
Prohage
 Bacteriophage
DNA that becomes embedded in the
bacterial host’s DNA.

Viral DNA is then copied along with the host DNA
without damaging the host.


Viral DNA multiplies as the host cells multiply
Each generation of daughter cells derived from the
original host cell is infected.

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.

Removes itself from the host cell DNA, directs the
synthesis of new virus particles, and now becomes
an active lytic infection.


Cold viruses attack with a very simple, fast-acting
infection
Capsid settles on a cell, typically in nose, and is
brought inside, where a viral protein makes many
new copies of the viral RNA.

Host cell’s ribosomes mistake the viral RNA for it’s
own and translates it into capsids and other viral
proteins.


New capsids assemble around the viral RNA copies
Host cell releases hundreds of new virus particles
to infect other cells within 8 hours.
HIV



Human immunodeficiency virus (HIV)
Acquired immune deficiency syndrome (AIDS)
Retrovirus
 Genetic
information is copied from RNA to DNA.

Enter cell and makes a DNA copy of its RNA.

Copy inserts itself into the DNA of the host cell.


HIV is similar to a lysogenic infection
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 must infect living cells in order to grow and
reproduce, taking advantage of the nutrients and
cellular machinery of their hosts
After infecting living cells, viruses can reproduce,
regulate gene expression, and even evolve.