Transcript Chapter 18 - Canyon ISD

Chapter 18
Genetics of Viruses and Bacteria
Virus Discovery
• 1883: Adolf Mayer (German) was looking for
the cause of the tobacco mosaic disease
– Stunts growth of tobacco and gives leaves a
mottled look
– Disease was contagious and could be transmitted
by spraying sap extracted from diseased leaves
onto healthy ones
– Thought it was caused by an unusually small
bacteria that could not be seen by the microscope
Magnification Demonstration
• Dimitri Ivanowsky (Russia) passed sap from
infected tobacco leaves through a filter designed
to remove bacteria
– Sap still caused disease
– Thought that they were too small to be filtered, or
made a toxin that caused the disease
• 1897: Marntinus Beijerinck (Dutch) discovered
the infectious agent could reproduce (therefore is
not a toxin)
– Pathogen could only reproduce within the host it
infected
• Sprayed and sprayed and sprayed
– Could not be cultivated on petri dishes
• 1935: Wendell Stanley (USA) crystallized the
virus with the electron microscope
Viral Anatomy
• Infectious particles consisting of nucleic acid
enclosed in a protein coat
• Genome: may be double stranded DNA, single
stranded DNA, double stranded RNA, or single
stranded RNA
– DNA virus or RNA virus
– Usually organized in a single linear or circular
molecule of nucleic acid
• Capsid is the protein shell that encloses the
genome
– may be rod shaped, polyhedral, or more complex
• Viral envelopes are membranes that cloak the
capsid
– Derived from the membrane of the host cell
– Adds to offense
Obligate Intracellular Parasites
• Can reproduce only within a host cell
• Viruses lack enzymes for metabolism and have no
equipment for making their own proteins
• Each virus can infect only a limited range of hosts,
called host range
– Can be wide (rabies) or narrow (only infect E. coli)
• Viruses identify their host by the same way
enzymes find their substrates- lock and key
• Eukaryotic viruses are generally tissue specific
– Cold virus attaches to respiratory tissue
Reproduction
• Lytic
• Lysogenic
Lytic Cycle
• Phage reproduction that cumulates in death
of the host cell
• Virulent phage can only reproduce by the lytic
cycle
• Turns cell into a phage producing factory, and
the cell lyses and releases the products
Lysogenic Cycle
• Replicates the phage genome without
destroying the host
• Could undergo both lytic and lysogenic =
temperate phage
• Phage DNA is incorporated into host DNA at
the prophage
• Every time the cell divides, it replicates the
phage DNA also and passes it to daughter cells
– Switch from lysogenic to lytic is environmental
Clay Video
Animal Viruses- Viral Envelope
RNA as Genetic Material
• Retroviruses undergo reverse direction of
genetic information
– Reverse transcriptase transcribes DNA from and
RNA template (RNADNA)
• New DNA integrates into a chromosome in the host cell
– Called a provirus, and it never leaves the host cell
– Examples: HIV and AIDS
Cause of Viral Diseases
• Some viruses cause the infected cells to produce
toxins that lead to disease symptoms, and some
have molecular components that are toxic
• How much damage a virus causes depends on the
ability of the infected tissue to regenerate
– Cold virus repairs respiratory tissue relatively fast
– Polio virus damage is permanent
– Symptoms of viral infections actually result in the
body’s efforts at defending against the infection
Prevention
• Vaccines are harmless variants or derivatives
of pathogenic microbes that stimulate the
immune system to mount defenses against
the actual pathogen
– “Vacca” is Latin for cow
• First vaccine, against smallpox, consisted of cowpox
virus
– Edward Jenner learned that milkmaids who had contracted
cowpox were resistant to smallpox
– Scratched a boy with a needle bearing fluid from an infected
milkmaid’s sore and the boy resisted smallpox
Treatment
• Antibiotics don’t work
• Most viruses need few nutrients to live
• New medication is working on blocking the
action of reverse transcriptase
Emerging Viruses
• AIDS
– Sudden appearance in 1980’s
• Nipah Virus
– Malaysia; killed 105 in 1999 and destroyed pig
markets
How Does it Happen?
• Mutation of existing diseases
– Flu
• Spread of existing viruses from one species to
another
– ¾ of new viruses come from animals
– Hantavirus
• Dissemination of virus from a small isolated
population spreads to widespread epidemics
– AIDS
Viruses and Cancer
• 1911: Peyton Rous discovered a virus that causes
cancer in chickens
• Virus that causes hepatitis B also causes liver
cancer
• Virus that causes mono has been linked to several
types of cancer in parts of Africa
• HPV  cervical cancer
• Viruses have oncogenes that mess with the cell
cycle
– Want more and more copies of its genome in the host
Tiny Infectious Agents
• Viroids are tiny molecules of naked circular
RNA that infect plants
– Disrupt the metabolism of a plant cell and stunt its
growth
– Caused 10 million coconut trees to die in
Philippines
– Cause errors in the regulatory systems that control
growth
• Prions are infections proteins
– Cause mad cow disease and Creutzfeldt-Jakob
disease
– It is a misfolded form of a protein normally
present in brain cells
– When the prion gets into a cell containing the
normal form, it converts it to the prion version
Creutzfeldt-Jakob Disease
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Blurred vision (sometimes)
Changes in gait (walking)
Confusion, disorientation
Dementia that occurs over a few weeks or months
Hallucinations
Lack of coordination (for example, stumbling and falling)
Muscle stiffness
Muscle twitching
Myoclonic jerks or seizures
Nervous, jumpy feelings
Personality changes
Sleepiness
Speech impairment
CJD is rarely confused with other types of dementia (such as Alzheimer's disease) because in CJD,
the symptoms get worse much more quickly. Both forms of CJD are different than dementia
because the symptoms progress quickly to disability and death
Bacterial Genome
• Major component of the bacterial genome is
one double-stranded, circular DNA molecule
• Consists of about 4.6 million base pairs coding
for 4,300 genes
– 100 times more than viruses, but one thousandth
as much as eukaryotic cells
• DNA is found in the nucleiod, a region not
bound to the membrane like a nucleus
• When DNA needs to be copied, it starts at a
single origin of replication and proceeds both
ways around the circular chromosome
Proliferation
• E. coli growing under optimal conditions can
divide every 20 minutes
• A single cell can produce a colony of 108 new
cells in 12 hours
• In the human colon, E. coli reproduces enough
to replace the 2 x 1010 bacteria lost in feces
daily
Reproduction
• Binary fission is a type of asexual reproduction
• Therefore, they are essentially clones of the
parent cell
– Mutations occur every million divisions
• 2,000 mutations in your colon replacements
– New mutations can have a significant impact on
the genetic diversity when reproductive rates are
very high because of short generation times
• Leads to the evolution of bacteria
Genetic Recombination
• Transformation
• Transduction
• Conjugation
Transformation
• Alteration of a bacterial cell’s genotype by the
uptake of naked, foreign DNA from the
surrounding environment
– Oswald Avery Experiment
• The foreign allele is incorporated into the
bacterial chromosome, replacing the native allele
• Many species of bacteria have surface proteins
specialized for the uptake of naked DNA
Transduction
• Phages carry bacterial genes from one host
cell to another
– Generalized transduction: some of the host’s DNA
gets packaged with the new phage during lytic
cycle and transferred to a new host
– Specialized transduction: requires infection from a
temperate phage, and a piece of the prophage
gets picked up and sent out when it turns to the
lytic cycle
Conjugation
• Direct transfer of genetic material between
two bacterial cells that are temporarily joined
• “Bacterial Sex”
• DNA transfer is one way
• “Male” uses appendages called sex pilli to
attach to the DNA recipient, the “female”
– Pulls the two cells together, and a bridge is formed
for DNA transfer
Manly Bacteria
• The “maleness” ability to form sex pilli and
donate DNA during conjugation comes from a
piece of DNA called the F factor
– F stands for fertility
– Could exist as a segment of DNA or as a plasmid
Plasmids
• Small, circular, self-replicating DNA molecule
separate from the bacterial chromosome
• Can undergo reversible incorporation into the
cell’s chromosome
• An episome is a genetic element that can exist
wither as a plasmid or as a part of the
chromosome
• Generally beneficial to the cell
– Contains F factor that may be advantageous in a
changing environment that no longer favors existing
strains in a bacterial population
F Factor
• “Males” are considered F+
• It is heritable and contagious
– F- “females” that undergo conjugation become F+
R Plasmids
• 1950- Japan; began to notice that patients
suffering from bacterial dysentery did not
respond to antibiotics that had once been
effective
• R plasmids transfer resistance to its cell
– Some code for enzymes that attack antibiotics
themselves
• How do so many resistance genes become a part
of one plasmid?
– Transposon
+
Transposons
• “Transposable genetic element”
• Piece of DNA that can move from one location
to another in a cell’s genome.
• Never exist independently
• Bring multiple genes for antibiotic resistance
into a single R plasmid by moving the genes to
that location from different plasmids
• Do a copy and paste or cut and paste of the
genes to put them in new locations
Insertion Sequences
• Simplest bacterial transposons
• Consist only of the DNA necessary for the act
of transposition
• Uses inverted repeats to know where the
Transposase gene is located
Composite Transposons
• Include extra genes other than ones only
needed for transposition
– Resistance genes
• May help bacteria adapt to new environments
• Transposons are found in other organisms as
well
– Barbara McClintock and maize
Gene Expression Control
• Bacteria live in crazy and unpredictable
environments
• The E. coli in our colons are dependent on the
food we eat for their nutrients
• However, if they do not get enough of a
certain amino acids they need, they will make
their own by changing more readably
available amino acids into the one they are
lacking
Operons
• When the bacteria must make their own
amino acids, all the enzymes for that
particular metabolic pathway are synthesized
at one time
– The switch to turn it on/off is called an operator
• Can allow or block RNA polymerase to attach and
synthesize the amino acids
– The operator, the promoter, and the genes they
control are operons
Trp Operon
• Pathway to make Tryptophan
• Repressible Operon
• Pathway is generally on
– RNA polymerase can bind to the promoter and transcribe
the genes of the operon
• Can be switched off by a protein called the repressor
– Binds to the operator and blocks RNA polymerase
• Made by a regulatory gene
• Tryptophan itself acts as a corepressor because when
present, it will sit on the operator and turn the switch
off
Inducible Operons
• lac operon is usually OFF, but in the presence
of lactose, lactose will bind to the inducer to
inactivate the repressor
• Lac operon codes for genes that allow bacteria
to take in lactose, which is not normally in the
nutrient medium