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Microbial Models I:
Genetics of Viruses
and Bacteria
8 November, 2004
Text Chapter 18
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.
Viruses consist of a protein capsid and DNA or RNA genome.
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.
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.
Phage T4 is a structurally complex phage with a simple life cycle.
Phage lambda can use the lytic or lysogenic cycle.
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.
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.
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.
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
Small Genome (4300 genes)
Mobile Genetic Elements (Plasmids and Phages)
Asexual Reproduction
Rapid Evolution (10-7 mutations per gene per replication.)
Bacteria can exchange genetic material.
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 chloridetreated 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.
Transduction
Generalized transduction can
occur for any gene.
Specialized transduction can
only occur for genes near the
prophage insertion site.
Conjugation
The order of gene
transfer during Hfr
conjugation is the basis
for the construction of
bacterial gene maps.