Pathogenetics

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Pathogenetics
©2006 Lee Bardwell
Bio 97
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Pathogenetics lecture outline - 1
What are bacteria?
What are viruses?
Antibiotic resistance is a big problem
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
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Genetics of pathogens - 2
How do bacteria become resistant to antibiotic (genetic mechanism)?
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?
Transformation
Viral Transduction
©2005 Lee Bardwell
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Bacterial life cycle
s
Circular
chrm
Bacteria
Prokaryotic
Unicellular
Haploid
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
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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 micro-organisms (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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Inhibition of bacterial cell-wall synthesis by certain* antibiotics
*penicillin, vancomycin, others
How do antibiotics kill bacteria ?
NOTE: movies won’t work on downloads/pdfs
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No need
to memorize
BONUS
MATERIAL !
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No need
to memorize
BONUS
MATERIAL !
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Inhibition of bacterial protein synthesis by macrolide* antibiotics
*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 low-copy-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 F-factor 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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Fig. 7.16
In the process of transduction,
bacterial DNA is transferred from
one bacterial cell to another by a phage
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The transferred DNA may be
integrated into the host chrm
by recombination
Fig. 7.16
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Brief review of diploid genetics
©2005 Lee Bardwell
Bio 97
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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 non-functional
(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
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L.O.F. alleles
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
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L.O.F. alleles II
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

PHENOTYPE
PROGRESSIVE INVOLUTARY MOVEMENTS INCLUDING CHOREA (GREEK=DANCE),
COGNITIVE DEFICITS, AND PSYCHIATRIC DISORDERS

GENETICS
FREQUENCY = 1/10,000 (European origin)

AUTOSOMAL DOMINANT - COMPLETE PENETRANCE

LATE ONSET: 35-50 years of age
10-20 YEAR COURSE

ANTICIPATION -TRINUCLEOTIDE REPEAT EXPANSION (pp. 427-8)

NO EFFECTIVE TREATMENT OR CURE


Disease of
the Day
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HD is characterized by neuronal degeneration
MATLEKLMKAFESLKSFQQQQQQQQQQQQQQQQQQQQQQQPPP

PPPPPPPPQLPQPPPQAQPLLPQPQPPPPPPPPPPGPAVAEEPLHRP

KKELSATKKDRVNHCLTICENIVAQSVRNSPEFQK… (3142 total aa’s)
HUNTINGTIN protein
350 kD protein

10-11 kb transcript

ubiquitously expressed

function unknown

correlation between repeat
size and age of onset
Gene cloned by Huntington’s Disease
Collaborative Research
Group 1993
The repeated trinucleotide is AGC
which encodes Q (glutamine)
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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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Sex Selection
©2005 Lee Bardwell
Using genetic techniques
To pick the sex of your kid
Ethical
Issue
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Sex Selection
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
©2005 Lee Bardwell
Ethical
Issue
Using genetic techniques
To pick the sex of your kid
How ?
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Sex Selection
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
©2005 Lee Bardwell
Ethical
Issue
Picking the sex of your kid
Why ?
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Sex Selection
©2005 Lee Bardwell
Ethical
Issue
Picking the sex of your kid
How do you feel about it ?