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BACTERIAL GENETICS
Prof. Manal Baddour
INTRODUCTION
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Bacteria have four important advantages for
"traditional types of genetic experiments":
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1. They are haploid (no masking).
2. New generation is produced every 20 minutes.
3. Easy to grow in ENORMOUS NUMBERS.
4. Individual members of these large populations
are GENETICALLY IDENTICAL (or very nearly
so).
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DEFINITIONS
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The science of genetics defines and analyzes
heredity, or constancy and change in the vast
array of physiologic functions that form the
properties of organisms.
 The unit of heredity is the gene, a segment of
DNA that carries in its nucleotide sequence
information for a specific biochemical or
physiologic property.

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The chemical basis for variation in phenotype is
change in genotype, or alteration in the
sequence of DNA within a gene or in the
organization of genes.
 Restriction enzymes cleave DNA at specific
sites, giving rise to DNA restriction fragments
 Plasmids are small genetic elements capable of
independent replication

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DNA inserted into plasmids can be placed under
control of high-expression bacterial promoters
that allow encoded proteins to be expressed at
high levels.
 Bacterial
genetics fostered development of
genetic engineering, a technology that has
been responsible for tremendous advances in the
field of medicine.

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THE WATSON-CRICK MODEL
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Top: Adenine-thymine pair.
Bottom: Guanine-cytosine pair.
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THE PROKARYOTIC GENOME
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Prokaryotic genomes consist of a single circular
DNA molecule containing from 580 kbp to more
than 5220 kbp of DNA.
 Many bacteria contain additional genes on
plasmids that range in size from several to 100
kbp.
 DNA circles (chromosome and plasmid), which
contain genetic information necessary for their
own replication, are called replicons.

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BACTERIAL GENES
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Membranes do not separate bacterial genes from
cytoplasm as in eukaryotes.
 With few exceptions, bacterial genes are haploid.
 Restriction
enzymes
(restriction
endonucleases) provide bacteria with a mechanism to
distinguish between their own DNA and DNA
from other biologic sources.

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THREE MAIN TYPES OF MUTATIONS
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a. Antibiotic resistance
• StrR=streptomycin resistant; able to grow when
the antibiotic streptomycin is added to the
medium.
• StrS=streptomycin
sensitive; killed in the
presence of streptomycin.
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FOR BACTERIA
Can use upper or lower case letter for resistant
vs. sensitive or a dash (Str-r).
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b. Nutritional mutants
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Examples of nutritional mutants:
Pro+
able to synthesize the amino acid proline
Prounable to synthesize the amino acid proline
Nutritional mutants include defects in the synthesis of:
Amino acids (examples Leu, leucine; Pro, proline, Met, methionine,
His, histidine)
Vitamins
(examples Thi, thiamine; Bio, biotin)
Nucleotides
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
prototrophs: able to synthesize all their components from simple
molecules (from a minimal medium)
auxotrophs: mutants unable to synthesize one or more of their
components from simple molecules.
Usually the wild type trait is the ability to synthesize the
compound (they are prototrophs).
Mutants usually are defective in the gene for an enzyme needed to 11
synthesize the compound and thus require that compound to be
added to the medium.
c. Carbon source mutants
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Generally these cannot use particular sugars as a
source of energy.
 Example: Mutants in genes in the lactose metabolic
enzymes (lac Z or lac Y) cannot use lactose; must be
supplied with glucose or some other source to grow.

Lac Z+ can grow in medium containing lactose.
Lac Z- can’t grow in medium containing lactose.
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Some bacterial species can invade higher
organisms because they possess specific genes for
pathogenic determinants.
 These genes are often clustered together in the
DNA and are referred to as pathogenicity
islands.
 Pathogenicity islands contain diverse genes
important for pathogenesis—including adhesins,
invasins, and exotoxins—as well as those that
are probably involved in mobilization.

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BACTERIAL REPLICATION
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The replication of bacterial DNA begins at one
point and moves in both directions (ie,
bidirectional replication) from there.
 The two old strands of DNA are separated and
used as templates to synthesize new strands
(semiconservative replication).

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PLASMIDS
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Non-essential DNA molecules in bacteria
that replicate independent of the bacterial
chromosome.
Intracytoplasmic extrachromosomal double
stranded circular DNA
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Usually small circular DNAs
Some are large (example, fertility plasmids)
Copy number per cell can vary
Encode a few genes
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PLASMID GENES
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Plasmids carry genes associated with specialized
functions.
 Many plasmids carry genes that mediate their
transfer from one organism to another as well as
other genes associated with acquisition or
rearrangement of DNA.
 Swift spread among bacterial populations of
plasmid-borne resistance to antibiotics.
 Plasmids also carry IS elements, which are
important in the formation of high-frequency
recombinant (Hfr) strains.
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Major types:
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R plasmids: Resistance plasmids. Contain genes
for drug or antibiotic resistance. These are the types
of plasmids from which most of the recombinant DNA
vectors are derived.
 F plasmids: Fertility plasmids. Involved in transfer
of genes between bacteria by conjugation (bacterial
mating).

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INSERTION SEQUENCES AND
TRANSPOSONS
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Many DNA sequences in bacteria are mobile and
can be transferred between individuals and
among species.
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MAIN TYPES OF MOBILE ELEMENTS:
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1-3 kb long
can be found in plasmids or in the chromosome
can mediate integration of plasmids into the
chromosome; example F plasmid integration to
form an Hfr
contain a gene for a tranposase enzyme;
regulates mobility of the element
Can cause insertion mutations.
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IS elements=insertion sequences
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Transposons
are genetic elements that contain several kbp of
DNA, including the information necessary for
their migration from one genetic locus to another.
 Unlike plasmids, transposons do not contain
genetic information necessary for their own
replication.
 larger than IS elements
 usually contain at least one additional gene, often
for antibiotic resistance
 often are arranged with the resistance gene in
the middle, flanked by IS elements
 contain a gene for a transposase enzyme
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Significance of mobile elements:
 Rapidly spread beneficial traits between bacteria,
such as drug resistance
 Responsible for “horizontal transfer” of genes
across species boundaries
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BACTERIOPHAGE
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Viruses associated with prokaryotes
 The nucleic acid molecule of bacteriophages is
surrounded by a protein coat
 Bacteriophages
exhibit a wide variety of
morphologies
 Many phages contain specialized syringe-like
structures (ie, tails) that bind to receptors on the
cell surface and inject the phage nucleic acid into
a host cell

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Lytic phages produce many copies of
themselves as they kill their host cell.
 Temperate phages are able to enter a nonlytic
prophage state in which replication of their
nucleic acid is linked to replication of host cell
DNA.
 Bacteria
carrying prophages are termed
lysogenic.

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REPLICATION OF BACTERIOPHAGE
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BACTERIOPHAGE LAMBDA
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LYSOGENY
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TRANSFER OF DNA
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Interstrain transfer of DNA among prokaryotes
is widespread.
 Bacterial genetic exchange is typified by transfer
of a relatively small fragment of a donor genome
to a recipient cell.
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MECHANISMS OF GENE
TRANSFER
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HOW DO BACTERIA EXCHANGE GENES?
 Three broad mechanisms mediate efficient movement
of DNA between cells: conjugation, transduction, and
transformation.
 In conjugation, only one strand of DNA is
transferred. The recipient completes the structure of
double-stranded DNA by synthesizing the strand that
complements the strand acquired from the donor.
 In transduction, donor DNA is carried in a phage
coat and is transferred into the recipient by the
mechanism used for phage infection.
 Transformation, the direct uptake of donor DNA by
the recipient cell, may be natural or forced. Relatively
few bacterial species are naturally competent for
transformation.
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BACTERIAL CONJUGATION
EXCHANGE OF GENES BETWEEN BACTERIAL CELLS
BY CELL TO CELL CONTACT
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"Bacterial conjugation is the process in which DNA is
transferred from a bacterial donor cell to a recipient
cell by cell-to-cell contact.
 It has been observed in many bacterial species and is
best understood in E.coli, in which it was discovered
by Joshua Lederberg in 1951.“

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CONJUGATION
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Plasmids are the genetic elements most
frequently transferred by conjugation.
 Genetic functions required for transfer are
encoded by the tra genes, which are carried by
self-transmissible plasmids.
 Some self-transmissible plasmids can mobilize
other plasmids or portions of the chromosome for
transfer.
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The ability to transfer DNA by conjugation is
dependent on the presence of a cytoplasmic entity
termed the fertility factor, or F.
Cells carrying F are termed F+
Cells without F are F-.
F is a small, circular DNA element that acts like a
minichromosome. It is an example of a plasmid.
F contains approximately 100 genes; these give F
several important properties:
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1. F can replicate its DNA, which allows F to be
maintained in a cellular population that is dividing.
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3. F+ cells can transfer the newly synthesized copy of
the circular F genome to a recipeint (F-) cell that
lacks such a genome; note that a copy of F always
remains behind in the donating cell.
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2. Cells carrying F produce pili: minute proteinaceous
tubules that allow the F+ cells to attach to other cells
and maintain contact with them.
4. When a donor cell transfers a copy of its cytoplasmic
F to an F- cell, the recipient cell also becomes an F+
cell, because it now contains a circular F genome.
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6.
Occasionally, F leaves the cytoplasm and
integrates itself into the host bacterial
chromosome. When this occurs, F can also
transfer the host chromosomal markers to the
recipient cell along with its own DNA.
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F+ cells are usually inhibited from making
contact with other F+ cells and do not usually
transfer the F genome to F+ cells.
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5.
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BACTERIAL TRANSFORMATION
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"Transformation" is simply the process where
bacteria manage to "uptake" or bring in a piece of
external DNA .
 Usually, this process is used in the laboratory to
introduce a small piece of PLASMID DNA into a
bacterial cell.
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TRANSFER OF NAKED DNA BETWEEN BACTERIA
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some bacteria can be transformed by simply
mixing DNA with bacteria
for E. coli and some other bacteria, must pretreat
the cells with salt solution (such as calcium
chloride) to make the membrane more permeable
to take up DNA
typically procedure for E. coli involves
pretreating
cells
with
calcium
chloride,
incubation of cells with DNA on ice, brief heat
shock, followed by an incubation at 37˚C to allow
the cells to express the new DNA
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DNA IS THE GENETIC MATERIAL
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The
First
demonstration
of
bacterial
transformation was through experiments done
by Frederick Griffith (in London) in 1928. He
found there were two different types of the
bacterium Streptococcus pneumoniae:
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An "S" or SMOOTH coat strain, which
is lethal to mice.
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An "R" or rough strain, which will not
hurt the mouse.
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Griffith found that he could heat inactivate the smooth strain.
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•However, if he were to take a mixture of the heat-inactivated S strain,
mixed with the R strain, the bacteria would die.
•Thus there was some Material in the heat-killed S strain that was
responsible for "transforming“ the R strain into a lethal form.
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Their work continued on in the U.S., and in 1944,
Oswald Avery, C.M. MacLeod, and M. McCarty
carefully demonstrated that the ONLY material
that was responsible for the transformation was
DNA - thus, DNA was the "Genetic material“
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The genetic transfer of
streptomycin resistance
(strr) to the streptomycin
sensitive (strs) cells of
E.coli. The recovery of
strs cells depends on the
concentration of the strr
DNA.
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TRANSDUCTION
CELL TO ANOTHER MEDIATED BY VIRUSES
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Generalized transduction:
Any gene
transferred. Carried out by lytic viruses.
can
be
Specialized transduction: Only one or two genes can
be transferred. Carried out by some lysogenic
viruses such as bacteriophage lambda.
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THE EXCHANGE OF GENES FROM ONE BACTERIAL
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