Microbial Models I: Genetics of Viruses and Bacteria 5 November, 2001 Text Chapter 18

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Microbial Models IGenetics of Viruses and
Bacteria5 November, 2001Text Chapter 18
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Virus Basics - part I
Viruses are genetic parasites that are smaller
than living cells. They are much more complex
than molecules, but clearly not alive, since they
lack their own metabolism and reproductive
capacity.
Viruses replicate by invading cells and using the
metabolic and reproductive capacity of the cell
to make hundreds or thousands of new virus
particles. Viruses cause disease because the
genetic takeover impairs the normal function of
the cell.
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Viruses consist of a protein capsid and DNA or
RNA genome.
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Virus Basics - part II
Viruses are specialized. Each recognizes and
invades a narrow range of cell types in one or a
few closely related species.
The protein coat functions in recognition of the
host cell, invasion, and protection of the viral
genome outside the host cell.
The viral genetic material can be DNA or RNA.
Virus usually have only a few genes (usually
10-20). These commonly specify coat and
structural proteins, regulatory proteins used to
take over host gene expression systems, and
proteins that process or assemble completed
virus.
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Viral Reproduction
A viral infection begins when a virus recognizes
a host cell.
The viral genome is then replicated and
transcribed by host enzymes. Viral coat and
structural proteins are translated and processed.
Viral components self-assemble into new virus
particles. These particles exit the cell and can
infect new cells. Often, the cell is destroyed
in the process.
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Phage T4 is a structurally complex phage with a
simple life cycle.
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Phage lambda can use the lytic or lysogenic cycle.
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Enveloped Animal Virus
Animal viruses sometimes incorporate parts of the
host cell membrane, including viral proteins that
are processed and inserted in the membrane during
viral replication. These viruses usually do not
lyse the host cell but may severely impair its
function, as the metabolic resources of the cell
are diverted to viral replication. The membrane
helps the virus evade detection by the host
immune system.
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HIV
The HIV virus is an enveloped virus. Its genome
is single stranded RNA that encodes an efficient
and complex life cycle with only five major
genes.
Two of these genes code for the structure of the
virus.
One gene codes for reverse transcriptase and
integrase activities.
Two genes code for transcription factors.
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• Because viral infections occur inside cells, they
  are often not accessible to the immune system.
  The virus is only vulnerable when it is between
  cells.
• At this time, the three dimensional shape of
  proteins on the outside of the virus can be
  recognized as foreign and destroyed. Vaccines
  against viral diseases train the immune system to
  recognize and destroy viral coat proteins.

• Viruses that target vulnerable cell populations
  like polio and HIV are especially damaging.
• Some viruses can cause cancer by introducing or
  activating oncogenes.

• Analysis of viral sequence indicates that viruses
  are escaped genes that become mobile with the
  help of transposable elements.

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Bacterial Model Systems
Escherichia coli is the best-studied organism.
It is still far from completely understood. It
makes an excellent model organism because it is
Small Readily Cultured (Fast-Growing) Haploid Sm
all Genome (4300 genes) Mobile Genetic Elements
(Plasmids and Phages) Asexual Reproduction Rapid
Evolution (10-7 mutations per gene per
replication.)
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Bacteria can exchange genetic material.
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Transformation, Transduction, and Conjugation
Transformation occurs when bacteria take up DNA
from their surroundings. (Think of the R to S
transformation that introduced us to the idea of
DNA as the genetic material.) You will take
advantage of this bacterial property in lab this
week. When you mix plasmid DNA with calcium
chloride-treated E. coli cells, some of the cells
will be transformed when they take up the plasmid.
Transduction is the movement of DNA from one
bacterium to another by bacteriophages.
Conjugation is the direct transfer of genes
between joined bacteria.
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Transduction
Generalized transduction can occur for any
gene. Specialized transduction can only occur
for genes near the prophage insertion site.
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Conjugation
The order of gene transfer during Hfr conjugation
is the basis for the construction of bacterial
gene maps.
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