Transcript Ch. 18 – Microbial Models of DNA

Microbial Models of DNA
Microbes : microscopic organisms like
bacteria and viruses, can include fungi and
protista
Bacteria : kingdoms Eubacteria and
Archeabacteria (formerly Monera),
Prokaryotic cell, unicellular, one circular
piece of DNA and multiple plasmids
Viruses : nonliving – DNA or RNA and a
protein coat
Usefulness
• While we are familiar with disease causing
bacteria and viruses (pathogens), most bacteria
are harmless and actually helpful
• Because the have simple systems – the study of
their genetics has helped us understand more
complex systems, like our own.
• They have some specialized systems that have
helped us study disease
• They transfer genetic material between cells –
an important area of genetic research.
Discovery of Viruses
• Late 1880’s – Mayer, Ivanowsky and
Beijerinck; plants can spread disease to
each other, disease is smaller than a
bacteria and can be within the sap
• Early 1900’s - Stanley ; crystallized TMV
(tobacco mosaic virus)
– Crop was still big part of economy
• Actually saw virus with electron
microscope
Viruses
• 20 nm (nanometers) – smaller than ribosome
• Cells can not be crystallized
• Nucleic acid plus protein
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–
–
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Protein called a capsid
Variety of shapes…. Including the icosahedron
dsDNA, ssDNA, ssRNA and dsRNA
Linear or circular
4 – 100’s genes
• Bacteriophages are viruses that infect bacteria
– Phages named T1 – T7
Viral Reproduction
• Viruses are host specific (range)
– Phages only bacteria, some only E.coli
– Rhinovirus (cold) only humans
– Rabies – all mammals
• Proteins fit with receptor cells
– Some viruses of eukaryotes are tissue
specific (URT)
• Lytic vs lysogenic life cycles
– See handouts
Viral Life Cycles
LYTIC cycle
• Virus attaches to host
• Injects nucleic acid
• Host reads nucleic acid
and then
• manufactures viral DNA
and proteins
• Virus is assembled
• Escapes host cell
• Infects other cells
LYSOGENIC cycle
• Same as lytic UNTIL
• Cell replicates – BEFORE
copying and making viral
instructions
• This takes along viral
nucleic acids to new
generations of host, as
part of the host genome
• At some point, lots of host
offspring are triggered to
start lytic cycle
• “latent” phase
Control of Viral Genes
• Bacteria posses restriction endonucleases,
which are enzymes that degrade viral DNA as it
enters
• Go lysogenic instead – if phage is added to a
specific site within bacterial DNA then it’s a
prophage and it codes for a protein that
represses the other phage genes.
• Some bacteria are actually more harmful
because they posses phages that make toxins
• Viruses can also be ‘enveloped’ by host to help
them hide
Retroviruses
• These viruses double stranded RNA that is
a template from which an enzyme called
reverse transcriptionase can make DNA
• “backwards”
• An example is HIV
Vaccines
• Colds – epithelial cells in throat can repair themselves efficiently
• Polio – attaches to nerve cells that cannot be repaired
• 1796 – Edward Jenner – noticed that cow pox (mild) makes one
immune to small pox (more harmful).
• Used fluid from cow (vacca) pox blister to give people cow pox,
making them immune to small pox.
• Small pox has since been eradicated (WHO)
• Fewer cases of polio, rubella, measles and mumps since vaccines
• Rabies vaccines for pets
• Newer vaccines for chicken pox (varicella) and HPV (gardasil)
• Flu shots are vaccines against the flu – varies from year to year and
location to location
– Globalization
– Evolution of viral genome
– CDC and WHO
Emergent Diseases
• More cases, cases in new location, more deadly
cases and spreading to a new host make a
disease an emergent disease
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HIV
Hantavirus
Ebola ! ! ! !
The plague (emergent diseases aren’t all viral) – not
currently emerging…. But still exists
– Lyme disease
– Asian Bird Flu
– ???? Certain cancers
Link between cancer and virus
• Some viruses do cause cancers in animals
– Example is feline leukemia
• Versions of oncogenes are found in normal cells
– something triggers them
• Viruses may activate proto-oncogenes
• Viroids = circle of RNA that only infects plants,
replicate in plant cell to interfere with enzymes
• Prions = infectious proteins that convert normal
proteins to infectious proteins
– Ex. “Mad Cow” aka Crueutzfeldt –Jacob disease
History
• Viral genomes often have much in
common with the hosts DNA
– Could have evolved as either plasmids or
transposons
– Plasmids are small circular pieces of DNA that
replicate rapidly with in bacteria
– Transposons are pieces of DNA that move
from location to location with in a genome
Bacterial Genetics
• Bacteria are cells – prokaryotic cells with a cell
membrane, cytoplasm with ribosomes, and a circular
piece of DNA in the nucleoid region. May also have
plasmids
• Bacteria are mm,
• genome is about 4300 genes which is 100 x virus and
1/1000 of a euk. Cell
• DNA is 500 longer than the cell
• Divide by binary fission
• Adaptable
– Lots of habitats and host and modes of nutrition
– Natural selection happens quickly because of generation time
(hours to days)
Genetic Recombination
• Transformation
– Uptake of DNA from the environment
– S. pneumonia coats
• Transduction
– Phages carry genes from one bacterial host to another
– May have research possibilities
• Conjugation
– Bridge called pili between two bacteria and plasmids are transferred….
No new cells, not reproduction
– Transfer resistance to antibiotics – called R factor for resistance
– Conjugation requires bacteria to have F (fertility) factor
• Transposons
– Regions of DNA that move within a bacteria, either to a new place on
the chromosome or between the chromosome and the plasmids
– Originally called ‘jumping genes’ by McClintock, they moved within the
genome of indian corn, turning the color genes on and off
Control of Bacteria genes
• Restriction endonucleases
• Transposons
• Feed back inhibition from OPERONS
– Transcription unit
– Promoter : RNA polymerase attaches
– Operator : acts as switch
– Regulatory gene
– Repressor : keeps operator off
– Inducer : lets operator be on
Operons
Off to on / and on to off
• trp operon
• See pg. 338
• If no tryptophan –
then operon on and
making tryp.
• If lots of trp – then
repressor in operator,
no need to make if
you already have
• lac operon
• See pg. 339
• No lactose, no need
for lactase enzyme.
• Lactose actually
inactivates the
repressor to lactase
can be made