Download Ch. 8: Presentation Slides

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Bio 97
Pathogenetics
©2006 Lee Bardwell
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Pathogenetics lecture outline - 1
• What are bacteria?
• What are viruses?
• Antibiotic resistance is a big problem
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What is an antibiotic?
Why don't antibiotics kill us?
Why don't antibiotics kill viruses?
• How do bacteria become resistant to
antibiotic (biochemical mechanism)?
©2005 Lee Bardwell
Genetics of pathogens - 2
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• How do bacteria become resistant to
antibiotic (genetic mechanism)?
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What's a plasmid?
What's a transposable element?
How do these things move from cell to cell?
• What are other ways for DNA to move
from cell-to-cell?
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Transformation
Viral Transduction
©2005 Lee Bardwell
Bacterial life cycle
Circular
chrm
s
Bacteria
• Prokaryotic
• Unicellular
• Haploid
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Question
Does the statement
“loss-of-function alleles
are often recessive”
make sense when applied
to bacteria?
©2005 Lee Bardwell
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Viruses and Phages
• Non-cellular micro-organism
• Consist minimally of DNA or RNA genome
and some protein
• Infect host cells
• Can replicate only within host cells
• No intrinsic metabolism- relies on host
for energy, precursor molecules
• No ribosomes - relies on host for protein
synthesis
©2005 Lee Bardwell
Antibiotic resistance is a big
problem in bacteria
• Most bacteria isolated from clininal infection
are resistant to multiple antibiotics
• Some are resistant to all antibiotics in
routine use
• Some of what used to be the best
antibiotics (very effective, few side effects)
are now virtually useless
©2005 Lee Bardwell
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What is an antibiotic ?
• A substance that kills or halts the
growth of a micro-organism
(typically a bacterium)
• Usually made by other microorganisms (fungi, other bacteria)
• Examples
• Pencillin
• Streptomycin
• Chloramphenicol
©2005 Lee Bardwell
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Why don’t antibiotics kill us ?
• Bacteria and humans share many core processes
 Bacteria polymerases resemble human polymerases
 Bacterial ribosomes resemble human ribosomes, etc.
• But there are some things that are completely
different
 bacteria have a peptidoglycan cell wall and we don’t
• The best (least toxic to us) antibiotics target
these “completely different” structures
©2005 Lee Bardwell
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How do antibiotics kill bacteria ?
Inhibition of bacterial cell-wall
synthesis by certain* antibiotics
QuickTime™ and a
GIF decompressor
are needed to see this picture.
*penicillin, vancomycin, others
NOTE: movies won’t work on downloads/pdfs
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BONUS
MATERIAL !
No need
to memorize
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BONUS
MATERIAL !
No need
to memorize
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Inhibition of bacterial protein
synthesis by macrolide* antibiotics
QuickTime™ and a
GIF decompressor
are needed to see this picture.
*erythromycin, azithromycin, others
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Why don’t antibiotics
kill viruses ?
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How do bacteria become resistant ?
They can acquire
certain _____ that
encode ______ that
function to neutralize
the antibiotic
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DNA can move between bacteria
• Many DNA sequences in bacteria are
mobile – they can be transferred between
individuals and even between species
• Plasmids
• Transposable Elements
• Antibiotic resistance genes are often
contained in these mobile elements
• Other means of moving DNA between cells
• Transformation
• Viral Transduction
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Plasmids
Plasmids
are circular DNA
molecules that replicate
independently of the bacterial
chromosome
• They often carry antibiotic
resistance genes
• They are used in genetic
engineering as gene
transfer vectors
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F factor Plasmid
• The F (fertility) factor plasmid is a lowcopy-number plasmid ~100 kb in length,
and is present in 1–2 copies per cell
• It replicates once per cell cycle and
segregates to both daughter cells in cell
division
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F factor and Conjugation
• Conjugation is a process in which
DNA is transferred from bacterial
donor cell to a recipient cell by
cell-to-cell contact
• The transfer is mediated by a
tube-like structure called a pilus,
formed between the cells,
through which the plasmid DNA
passes
• The ~20 proteins that make up
the pilus are encoded by the Ffactor plasmid
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Mobility of smaller plasmids
• Many small plasmids don’t
have the genes necessary
for pillus formation
• They can recombine with F
and tag along for the ride.
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Transposable Elements
• Transposable elements are DNA sequences that
can jump from one position to another within a
chrm, or from one DNA molecule to another
• Bacterial TE’s often contain antibiotic resistance
genes
• They can jump into plasmids, and move with ‘em
• The smallest and simplest are 1–3 kb in length
and encode the transposase protein required for
transposition and one or more additional proteins
that regulate the rate of transposition
• TE’s are also found in eukaryotes, including
humans
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Bacterial Transformation
• The process of genetic alteration by pure
DNA is transformation
• Recipient cells acquire genes from DNA
outside the cell
• DNA is taken up by cell and often recombines
with genes on bacterial chromosome
• Bacterial transformation showed that DNA is
the genetic material (Avery, MacLeod,
McCarty 1944)
• Transformation may alter phenotype of
recipient cells
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Transduction
• In the process of transduction, bacterial DNA is
transferred from one bacterial cell to another by
a phage
• The transferred DNA may be integrated into the
host chrm by recombination
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Phage Life Cycle
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In the process of transduction,
bacterial DNA is transferred from
one bacterial cell to another by a phage
Fig. 7.16
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The transferred DNA may be
integrated into the host chrm
by recombination
Fig. 7.16
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Bio 97
Brief review of
diploid genetics
©2005 Lee Bardwell
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Dominance, Recessiveness
– Heterozygous genotype --> normal
phenotype
– The mutant allele is ___________
©2001 Lee Bardwell
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Dominance, Recessiveness
– Heterozygous genotype --> disease
phenotype
– The mutant allele is ___________
©2001 Lee Bardwell
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Mutant (disease causing) alleles
• Loss-of-function mutations --> allele encodes a
protein that...
– Is not made, or has a reduced function, or is nonfunctional
• (e.g Hemophilia) Usually recessive
• Gain-of-function mutations --> allele encodes a
protein that...
– has a new, disease-causing function
• (e.g Huntington’s disease --> mutant protein forms toxic
aggregates) Usually dominant
©2005 Lee Bardwell
L.O.F. alleles
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• In many cases, having only half the
normal amount of a given protein is
okay
– Heterozygous genotype --> normal
phenotype
– The mutant (loss-of-function) allele is
recessive
©2001 Lee Bardwell
L.O.F. alleles II
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• In some cases, having only half the
normal amount of a given protein is
NOT okay
– Heterozygous genotype --> disease
phenotype
– Loss-of-function allele is dominant
(= haploinsufficiency)
©2001 Lee Bardwell
HUNTINGTON’S DISEASE
Disease of
the Day
PHENOTYPE
• PROGRESSIVE INVOLUTARY MOVEMENTS INCLUDING CHOREA
(GREEK=DANCE),
COGNITIVE DEFICITS, AND PSYCHIATRIC DISORDERS
GENETICS
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FREQUENCY = 1/10,000 (European origin)
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AUTOSOMAL DOMINANT - COMPLETE PENETRANCE
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LATE ONSET: 35-50 years of age
10-20 YEAR COURSE
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ANTICIPATION -TRINUCLEOTIDE REPEAT EXPANSION (pp. 427-8)
NO EFFECTIVE TREATMENT OR CURE
HD is characterized by neuronal degeneration
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HUNTINGTIN protein
MATLEKLMKAFESLKSFQQQQQQQQQQQQQQQQQQQQQQQPPP
PPPPPPPPQLPQPPPQAQPLLPQPQPPPPPPPPPPGPAVAEEPLHRP
KKELSATKKDRVNHCLTICENIVAQSVRNSPEFQK… (3142 total aa’s)
350 kD protein
10-11 kb transcript
The repeated trinucleotide is AGC
which encodes Q (glutamine)
ubiquitously expressed
function unknown
correlation between repeat
size and age of onset
Gene cloned by
Huntington’s Disease
Collaborative Research
Group 1993
Most accepted current theory:
The extended glutamine tracts are thought to
promote the formation of toxic aggregates, leading
to cell death
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Ethical
Issue
Sex Selection
Using genetic techniques
To pick the sex of your kid
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©2005 Lee Bardwell
Ethical
Issue
Sex Selection
Using genetic techniques
To pick the sex of your kid
How ?
• Sperm sorting
– (X is heavier) 75% success if boy is desired, 90% if girl is
desired
• Preimplantation Genetic Diagnosis
– In Vitro Fertilization
– Determine sex of 4-8 cell embryos
– Only implant the ones you want
• Chorionic Villus Sampling - sample the placenta
– Done at 8-9 weeks of gestation
• Amniocentesis - sample the amniotic fluid
– Done at 4-5 months
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©2005 Lee Bardwell
Ethical
Issue
Sex Selection
Picking the sex of your kid
Why ?
• To prevent the birth of sons to
women carriers of X-linked
recessive diseases
• For family “balancing”
• Because of economic and cultural
pressures that lead to sons being
more desirable
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©2005 Lee Bardwell
Ethical
Issue
Sex Selection
Picking the sex of your kid
How do you feel about it ?
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©2005 Lee Bardwell