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CH. 18: THE GENETICS OF
VIRUSES & BACTERIA
Well-researched: Pneumococcus,
Escherichia coli, bacteriophages, TMV
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Bacteria
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Bacterial cell shapes
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Gram staining
Gram + (top), Gram – (bottom)
Classification of bacteria according to
what color stain they retain.
A sample of bacteria (on a slide) is first
stained with a violet dye.
The slide is then rinsed with ethanol.
If the violet stain washes off, a red dye is
added (“counterstaining”).
Depending on the structure of the cell
walls, some types of bacteria (such as
staphylococcus and streptococcus) retain
the violet stain and are called Grampositive.
Other types (such as pseudomonas and
salmonella) retain the red, but not the
violet, stain and are called Gram-negative.
This technique is named after the Danish
bacteriologist Hans C. J. Gram (1853-1938)
who invented it in 1884.
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The E.coli Chromosome:
• Bacterial chromosome is
GENERALLY a single,
continuous (circular) thread
of double- stranded DNA.
• Approx. 1 mm long when
fully extended (only 2mm in
diameter); contains about
4.7 million base pairs.
• More recently, it has been
found that bacteria may
actually have different kinds
of chromosomal material
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• Prokaryotic Cells Replicate their DNA in a
bidirectional fashion (replication)
• Replication begins at a specific base sequence.
diagram:
“theta” replication
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Transcription and Its Regulation
• Transcription begins when RNA polymerase
(enzyme) begins formation of an mRNA strand
along DNA strand, beginning at promoter site.
• A segment of DNA that codes for one specific
protein is known as a structural gene
• There may be several "start" and "stop"
codons along the mRNA strand, marking the
beginning and end of each structural gene.
– "leader" sequence (of nucleotides) at 5’ end
– "trailer" sequence (of nucleotides) at 3’ end
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The Need for Regulation
This is regulated by several means:
• 1) can be induced by presence of a
material (ex. Lactose presence induced E. coli
to synthesize beta-galactosidase enzyme)
• 2) can be inhibited: presence of substance
prevents formation of an enzyme
"repressible" (ex: E. coli; tryptophan inhibits
tryptophan forming enzymes)
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The Lac Operon
The Operon Model (1965 Nobel Prize) François Jacob, André Lwoff, and
Jacques Monod "for their discoveries concerning genetic control of enzyme and
virus synthesis"
- arose from study of mutant cells
studies done on E. coli cells making the enzyme: beta-galactosidase ; found it
was "blocked" by a repressor binding to operator.
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Operator – gene that activates
transcription of a structural gene
Regulator - This gene codes for a
repressor protein
Repressor: protein that can bind to
the operator gene, thus obstructing*
the promotor (blocks the RNA
polymerase from moving along
("reading") the molecule  no
mRNA transcription can occur.
*when the repressor is removed,
mRNA transcription begins
animations of lac and tryp operons
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CAP-cAMP Complex
• The catabolite gene activator protein (CAP) is
a sequence-specific DNA binding protein that
starts transcription of some gene systems and
represses others, when bound to cyclic
adenosine monophosphate (cAMP)
• this CAP-cAMP complex binds to the promotor
and maximizes transcription.
• It increases the binding of RNA polymerase,
among other things
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Recombinant DNA: involves modifying/combining DNA from a
variety of different sources and inserting these altered
molecules into other cells, in which the "new "genes are
expressed.
Bacterial plasmids are often used in genetic
engineering, as a “gene of interest” can be inserted
easily into them, then cloned into hundreds of copies
when the bacteria multiply.
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Bacterial Reproduction
Binary fission
• Bacteria cell goal: to grow and
multiply
• many can double number every
20 minutes!
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Plasmids and Conjugation
• Although bacterial chromosomes carry all the
genes necessary for growth and reproduction
of the cell, they also carry additional DNA
molecules called Plasmids
• Plasmids usually carry only between 2 to 30
genes; generally small)
• 2 important types:
– "sex factor" plasmids = F (fertility)
– "drug resistance" plasmids = R (resistance)
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The F Plasmid
• We now know that the fertility
factor or F factor is a very large
(94,500 bp) circular dsDNA
plasmid; it is generally
independent of the host
chromosome.
http://www.mun.ca/biochem/cou
rses/3107/Lectures/Topics/conjug
ation.html.
• They make a cell capable of
conjugation.
• F+ (male) "donor" cells: make pili
(protein "bridges" that form to
connect 2 cells for transfer of
genetic material)
• F- cells lack the F plasmid and
can’t form pili (female); these are
"recipient" cells
“rolling-circle replication”
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Hfr Cell: Sometimes the F factor gene can be incorporated
within the main bacterial chromosome. Because such strains
transfer chromosomal genes very efficiently, they are called Hfr
(high frequency of recombination).
This can then transfer a portion of bacterial chromosome to a Fcell:
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R plasmids = resistance plasmids
• Plasmids carrying genes responsible for antibiotic (or antibacterial
drug) resistance among bacteria may pass them to other species
of bacteria as well!
ex: E. coli -------> Shigella (dysentery)
• "Horizontal gene transfer“ also called Lateral gene transfer (LGT),
is any process in which an organism incorporates genetic material
from another organism without being the offspring of that
organism.
• By contrast, vertical transfer occurs when an organism receives
genetic material from its ancestor, e.g. its parent or a species
from which it evolved.
• It is most often thought of as a sexual process that requires a
mechanism for the mobilization of chromosomal DNA among
bacterial cells.
•
http://www.sci.sdsu.edu/~smaloy/MicrobialGenetics/topics/geneticexchange/exchange/exchange.html
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Because they are unable to reproduce sexually, bacterial
species have acquired several mechanisms by which to
exchange genetic materials
• Transformation - the uptake of naked DNA is a common mode of
horizontal gene transfer that can mediate the exchange of any part
of a chromosome; this process is most common in bacteria that are
naturally transformable; typically only short DNA fragments are
exchanged.
• Conjugation - the transfer of DNA mediated by conjugal plasmids or
conjugal transposons; requires cell to cell contact but can occur
between distantly related bacteria or even bacteria and eukaryotic
cells; can transfer long fragments of DNA.
• Transduction - the transfer of DNA by phage requires that the
donor and recipient share cell surface receptors for phage binding
and thus is usually limited to closely related bacteria; the length of
DNA transferred is limited by the size of the phage head.
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VIRUSES
diagram: bacteriophages
•
•
TMV
consists of a molecule of nucleic acid encased in a protein coat (capsid) contain no other "cell machinery",
but can move from cell to cell and utilize the host cell’s "machinery" to replicate the viral genes "obligate
intracellular parasites"
Viral nucleic acids vary: may be either DNA or RNA; double or single-stranded, circular or linear.
Viral classifications
– Viruses are generally classified by size (nm), shape/capsid, and type of nucleic acid they contain.
CHARACTERISTICS:
1. Contain a varying, but small amount of DNA (or RNA) surrounded by protein
T7 bacteriophage has DNA and ~100 genes;
The poliovirus has 7,600 RNA nucleotides;
the vaccinia (cowpox) virus has 240,000 DNA nucleotide pairs
2. Small viruses that don’t have room for a lot of DNA use overlapping genes
3. Retroviruses (ex: HIV) are RNA viruses that use an enzyme called Reverse Transcriptase to make DNA to
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replicate itself during infection stage)
Viroids and Prions
• Viroids:
(mostly cause plant diseases)
• are tiny strands of RNA, usually only a few
hundred nucleotides long.
• Viroids can interfere with a plant's
metabolism.
• Generally speaking, where viroids come
from and how they can disrupt the host
cell are not known.
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Prions
•
•
•
•
•
•
Prions are infectious agent, a misshapen protein, which have been defined as "small
proteinaceous infectious particles which resist inactivation by procedures that modify nucleic
acids".
The discovery that proteins alone can transmit an infectious disease has come as a considerable
surprise to the scientific community.
Prion diseases are often called spongiform encephalopathies because of the post mortem
appearance of the brain with large vacuoles in the cortex and cerebellum.
Probably most mammalian species develop these diseases. Specific examples include:
– Scrapie: sheep
– TME (transmissible mink encephalopathy): mink
– CWD (chronic wasting disease): mule deer, elk
– BSE (bovine spongiform encephalopathy): cows
Humans are also susceptible to several prion diseases:
– CJD: Creutzfeld-Jacob Disease
– GSS: Gerstmann-Straussler-Scheinker syndrome
– FFI: Fatal familial Insomnia
– Kuru
– Alpers Syndrome
http://www-micro.msb.le.ac.uk/3035/prions.html
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Viral Cycles
• Lytic cycle - causes destruction of the host cell quickly
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–
–
–
–
Attachment- phage attaches to cell surface
Entry- phage injects DNA into host cell
Replication- phage DNA "tells" host to make more phage DNA and protein coats
Assembly- new viruses are assembled; host cell becomes a "virus factory"
Release- cell lyses (breaks open), releasing viruses
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Lysogenic Cycle
• some viruses incorporate their DNA into a cell’s
chromosome.
• The cell may then cause a sudden eruption of viral activity
(can remain latent for many generations)
• Temperate bacteriophages- viruses that can integrate their
DNA into bacterial chromosome at specific sites
• Prophage- an integrated bacteriophage
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Viral Transduction
• the transfer of cellular DNA from one host cell
to another by means of viruses = recombinant
DNA
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Transduction
1. General Transduction
Transduction is bacterial
chromosome DNA transfer
mediated by a virus. This is a
one way nonreciprocal virus
mediated transfer from one
bacterial cell to another.
2. Restricted (specialized)
Transduction
The situation in which a
particular phage will
transduce only specific
regions of the bacterial
chromosome.
* can you think of the implications to
genetic engineering? gene therapy?
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Introducing Lambda
• Lambda is the best studied of the temperate bacteriophages
– Viral form: linear, double-stranded (2 free ends)
– When inserted into a bacterial cell, it becomes circular
• integrase: enzyme produced by a retrovirus that enables its
genetic material to be integrated into the DNA of the infected
cell.
Lambda plaques on
an E.coli culture
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Transposons
• discovered by Barbara McClintock
• Are segments of DNA that can move around to different positions in the
genome of a single cell.
• These mobile segments of DNA are sometimes called "jumping genes".
• In the process, they may cause mutations increase (or decrease) the
amount of DNA in the genome.
• Also contain a gene for enzyme "transposase"
At each end, they contain a sequence of repeats
• direct repeats -ATTCAG-ATTCAG• often used to I.D. insertion points
• indirect repeats -ATTCAG-GACTTA• Can carry genes for mutations, protein synthesis, drug resistance, etc...
from one place in the genome to another
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Many transposons move by a "cut and paste" process: the transposon is cut out of its
location (like command/control-X on your computer) and inserted into a new
location (command/control-V).
This process requires an enzyme — a transposase — that is encoded within some of
these transposons.
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