Transcript The Genetics of Viruses and Bacteria

The Genetics of Viruses
and Bacteria
Microbial Models
Viruses come in many shapes and
sizes
Compare the size of a Eukaoryotic
cell, Bacterial Cell and a Virus
Herpes Virus
Measles
Polio
Ebola Virus
Discovery of the Virus
 Adolph
Meyer a German Scientist studied
the Tobacco Mosaic Virus.
 Thought
it was caused by a very small
bacteria because it could not be viewed
through the microscope.
Tobacco Mosaic Virus
Tobacco Mosaic Virus
Dimitri Ivanosky a Russian
Scientist
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Filtered the sap to get rid of the bacteria.
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The plants still received the infection when
sprayed with the filtered sap.
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Still thought the pathogen were very small
bacteria.
Martinus Beijerink a Dutch Botanist
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Discovered that this infectious particle could
reproduce.
Sprayed plants with filtered sap and their sap
infected other plants.
Infection was not diluted on subsequent
infections.
Could not grow outside the host in culture
medium.
Could not be inactivated with alcohol like
bacteria
Wendell Stanley an American
Scientist
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Finally crystallized
this infectious particle
and viewed it under
the electron
microscope.
Viral Composition
 Capsid
– protein coat
 Sometimes
an envelope – glycoproteins
acid – DNA or RNA. Never both.
Can be single or double stranded.
 Nucleic
 Some
have tail fibers – Bacteriophage T4
Viruses Are Obligate Intracellular
Parasites
 They
lack their own enzymes to perform
metabolism and reproduction.
 They utilize the host’s enzymatic
machinery to accomplish these tasks.
 Viruses have a host range or are host
specific.
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Rabies infects more than one host
Eukaryotic viruses are usually tissue specific.
• Rhinoviruses, Adenoviruses, Herpes, HIV
Reproductive Cycles of Virus
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Lytic Cycle – destroys the host cell
Viral proteins are translated by host enzymes
and new viral particles are produced.
Viral particles are assembled and the host cell is
lysed. Host cell death occurs.
Bacterial cells possess restriction
endonucleases that destroy foreign DNA.
The bacterial DNA is methylated to protect from
destruction.
Lytic Cycle
Lysogenic Cycle Can Be Used For
Cloning
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Viruses can infect without destroying the host
cell.
They integrate their DNA into the host cell and
turn off their own genes.
These types of viruses are called temperate
viruses.
Bacterial cells that possess these viral genes are
celled prophages.
Viral DNA can be replicated along with the host
cell’s DNA.
Lysogenic Cycle
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Lysogenic Viruses can be Triggered to
Become Lytic Viruses
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Radiation, chemicals or any host cell stress can
cause the virus to enter the lytic cycle and
destroy the host cell.
 Some prophages express prophage genes that
alter the phenotype of the host cell.
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Bacteria produce endotoxins that originate from viral
genes.(Diptheria, Scarlet Fever and Botulism)
Genes can be inserted into bacterial cells using
viruses in a process called Transduction.
HIV Infection
HIV Life Cycle
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HIV Infection
RNA as Viral Genetic Material
 Messenger
RNA serves as the template
for new genetic material.
 Reverse transcriptase produces DNA from
mRNA.
 Newly made DNA integrates into the host
chromosome.
 Unlike prophages, proviruses never leave.
 The virus now is referred to as a provirus.
 Viruses that do this are called retroviruses.
 The
host’s RNA polymerase transcribes
viral RNA from the DNA.
RNA serves as both a template and
mRNA.
RNA viruses mutate more rapidly because
replication of RNA does not have to
undergo the same proofreading steps as
replicating DNA.
Transduction
Viroids
 Viroids
are tiny molecules of naked
circular RNA that infect plants.
- only several hundred nucleotides long.
- a molecule can be an infectious agent.
- disrupt metabolism by interferring with
the host genome.
Prions
Prions are infectious proteins.
- cause degenerative brain diseases like
scrapes and Creutzfeldt-Jakob disease.
- abnormal shaped brain proteins induce
normal proteins to assume an abnormal
shape propagating itself.
Prions
The Genetics of Bacteria
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Bacterial genome
Circular double stranded DNA in the nucleoid region.
(remember prokaryotes do not have a nucleus)
Plasmids are in addition to the genome
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• Circular double stranded DNA
• May carry extra chromosomal DNA called plasmids.
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Bacteria have about 4,300 genes, 500 x more than a
virus but about 1/10th of that of a typical eukaryotic
cell.
Divide by binary fission
DNA synthesis is bidirectional and can occur very
quickly.
• E.coli can divide in 20 minutes
Genetic recombination of Bacteria
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Transformation
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Bacteria take up naked DNA from the environment.
• If expressed this can change the phenotype of the bacterial
cell.
• Remember how the smooth strain of streptococcus
pneumonia was transformed into the rough strain of
streptococcus pneumonia.
• Biotechnologist stimulate bacteria to take up foreign DNA
with CaCl2.
• Human genes like insulin can be transferred into bacteria and
large quantities of insulin can be produced.
Genetic Recombination of Bacteria
 Transduction
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Viruses (phages) are used to carry bacterial
genes from one host cell to another.
Two kinds of transduction
• Generalized Transduction
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During the lytic cycle of the virus some of the host DNA
can be accidentally packaged in the viral particle.
When the virus infects a new host cell it injects the DNA
from the previous host.
It is called generalized transduction because the
bacterial DNA is taken up randomly by the virus.
Transduction
Genetic Recombination in Bacteria
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Specialized Transduction
• Involved the lysogenic cycle
• The viral DNA integrates into the host bacterial cell
and is called a prophage.
• The virus then enters the lytic cycle and
accidentally takes DNA that is flanking the insertion
site of the viral DNA.
• When the virus infects another host cell only
certain genes are transferred to the new host cell.
Conjugation
Genetic Recombination in Bacteria
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Conjugation
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Direct transfer of genetic material between bacteria.
Involves a sex pilus
• A bridge that forms between two cells.
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Involves the transfer of a small segment of DNA or a small
circular segment of DNA called a plasmid containing genes that
code for the formation for the sex pili.
• The plasmid is called the F (fertility) factor
• Bacteria that have the F factor are ( F + ) and are considered male.
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The f factor replicates and is donated to the recipient cell.
• The recipient cells are ( F - ) and considered to be female.
• The recipient cell becomes ( F + ) when it receives the F factor.
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When the F factor integrates into the main chromosome then the
cell is called an Hfr cell.
Hfr cells have the tendency to recombine with the host
chromosome of another ( F - ) cell.
Conjugation
Other types of Plasmids
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Episomes
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A genetic element that can either exist as a plasmid
or part of a bacterial chromosome.
Usually beneficial to bacteria because:
• it allows then to survive adverse conditions
• F factors promote recombination which introduces new
genes that can perhaps facilitate survival.
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R Plasmids
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These plasmids carry genes for antibiotic resistance.
Bacteria can transfer antibiotic resistance from one
cell to other by transferring the R plasmid.
Very dangerous antibiotic resistant bacteria have
been created in hospitals
Transposons
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Transposons
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Often called “jumping genes”
Code for its own enzymes to excise itself from the genome and
insert itself somewhere else.
Transposons also copy themselves and insert the copy
somewhere else in the genome.
Transposons can jump from a plasmid to the main chromosome
and visa versa.
One main difference between genetic recombination through
transformation, transduction, conjugation and transposons are:
• In transformation, transduction, conjugation depends on base
pairing between homologous regions of DNA. Transposons do not
require similar or identical sequences of DNA to pair with.
Operons
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A mechanism that bacteria use to control their
metabolic needs.
 A group of genes are either turned on or off
depending the metabolic needs of the organism.
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Involve catabolic and anabolic pathways.
• Catabolic pathways break down substances and typically are
turned off until the substance to be metabolized is present.
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These are known as inducible operons
• Anabolic pathways are usually turned on until there is
enough product is made.
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These are known as repressible operons.
Operons
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Anatomy of a Repressible Operon
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These operons are normally turned on until the repressor is
activated and binds to the operator to block RNA polymerase
from binding to the promoter.
Operator
• regulatory switch
• If a molecule is bound to the operator, then RNA polymerase cannot
bind to the promoter and mRNA cannot be transcribe and thus the
protein product cannot be made.
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Repressor
• The molecule that binds to the operator is the repressor.
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Corepressor
• Binds to the repressor and activates it so that it can bind to the
operator.
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Regulatory Gene
• Synthesizes the repressor molecule
Trp operon
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Repessible operon present in E.coli that
synthesizes tryptophan.
 When enough tryptophan is made it acts as a
corepressor and binds to the repressor to and
converts the repressor to its active form.
 The activated repressor binds to the operator
and blocks RNA polymerase from binding to the
promoter thus preventing transcription.
Trp operon
Operons
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Anatomy of a Inducible Operon
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These operons are normally turned off until the repressor is
inactivated. At this point the repressor falls off the operator and
RNA polymerase and binds to the promoter.
Operator
• regulatory switch
• If a molecule is bound to the operator, then RNA polymerase cannot
bind to the promoter and mRNA cannot be transcribed and the
protein product cannot be synthesized.
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Repressor
• The molecule that binds to the operator is the repressor.
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Inducer
• Binds to the repressor and converts it to the inactive from and it falls
off the operator. Regulatory Gene
• Synthesizes the repressor molecule
lac operon
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The lac operon is only turned on when lactose
is present because it produces enzymes
necessary to break down lactose.
 It is an inducible operon
 The inducer in allolactose which binds to the
repressor which is already bound to the operator
and inactivates it.
 The inactivated repressor falls off the operator
and the genes are transcribed by RNA
polymerase.
lac operon
An Example of Positive Gene
Regulation
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E.coli would prefer to use glucose as an energy source.
It will produce the enzymes to break down lactose into its
components galactose and glucose if glucose is not available.
How does E.coli sense the glucose concentration and send a
message to the operon to produce or not produce the enzyme
necessary to break down lactose (Beta galactosidase)?
A molecule called cyclic AMP(cAMP) builds up when glucose is in
low concentrations.
cAMP binds to cAMP receptor protein (CRP) and the cAMP/CRP
complex binds to a site upstream from the promoter and bends the
DNA in such a way that it is easier for RNA polymerase to bind to
the operon.
This stimulates transcription and is positive regulation.
When there is an abundance of glucose the cAMP levels drop and
the cAMP/CRP complex cannot form.
Up regulation of the lac operon