Download 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

Filtered the sap to get rid of the bacteria.

The plants still received the infection when
sprayed with the filtered sap.

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

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
Transduction
Lysogenic Viruses can be Triggered to
Become Lytic Viruses

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.
Transduction
HIV Infection
HIV Life Cycle
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.
HIV Infection
 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.
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