Chapter 7 Microbial Genetics
Plasmids
? 1-1000 KB in size
? Typical plasmid 1/20 of chromosome
? Most are circular double-stranded
DNA,some linear ds DNA
? Transmitted from cell to cell via
conjugation process
? Some can integrated into
chromosome
? Can carry a variety of genes for
production of toxin,resistance to
antibiotics and heavy metals et al.
Circular genetic elements that
reproduce autonomously and have
an extra-chromosomal existence:
Plasmid
? Conjugative,plasmids which govern their own transfer by
cell-to-cell contact are called conjugative
? Tra region,a set of genes within the plasmid that control the
transmissability by conjugation
? Hfr (high frequency of recombination),strains of bacteria
that transfer large amounts of chromosomal DNA during
conjugation
? Supercoil,plasmids isolated from the cells are in supercoiled
configuration
? Plasmid separation,by ultracentrifugation or electrophoresis
? Curing of plasmids,elimination of plasmids from host cells
by various treatments.
Plasmids
? Replication,Most plasmids of gram-positive
bacteria replicate by a rolling circle
mechanism.
? Copy number,The number of plasmids in a
cell,can range from only 1-3 copies to 100
copies.
? Incompatibility,Two different types of
plasmids can not coexist in a cell.
? Episomes,Plasmids having the ability to
integrate into host chromosome
F-Plasmid-Fertility Plasmids
? 100 KB
? Can be cured with
acridine orange
? Incompatibility (inc)
? Origin of replication
(oriS)
? Transposable elements
(Tn)
? tra region
? phi,phage inhibition
? IS (insertion sequence)
? rep,replication
functions
Cell to Cell Transfer of Plasmids
? Conjugative,Plasmids that govern their own transfer
by cell-to-cell contact are called conjugative (not all
plasmids are conjugative)
? Some conjugative plasmids can transfer genetic
information between distintly related organisms
(between gram-positive and gram-negative bacteria,
between bacteria and plant cells,and between bacteria
and fungi),it is important for evolution.
Conjugation (接合作用）
Plasmid Biology
Episomes
Plasmid Biology
Types of Plasmids and Their
Biological Significance
? The presence of plasmids in a cell can have a
profound influence on the cell’s phenotype:
– the ability of conjugation
– the ability of Rhizobium to interact with plants
– the resistance to antibiotics and heavy metals
– the degradation of octane,camphor et al
– the production of enterotoxin
– the applications in genetic engineering
Types of Plasmids and Their Functions
Resistance Plasmids (R-Plasmids),
the most well studied plasmids
? The emergence of bacteria
resistant to several antibiotics is
medically significant
? Resistance can be transferred via
cell-to-cell contact
? This could be one of the reasons
for the rapid rise of multiply
resistant strains
? Plasmid recombination is one
mean by which multiply
resistant organisms might have
first arisen
? Infectious nature of the R
plasmids permits rapid spread of
the characteristic through
populations
? Typical example,plasmid R100
The presence of multiple antibiotic resistance is due to the fact that a
single R plasmid contains a variety of genes coding for different
antibiotic inactiviation enzymes
Biochemical
mechanism
of resistance
mediated by
R plasmids
Engineered
plasmids-Artificial
plasmids
? Incorporation
into artificial
plasmids of
genes from a
wide variety of
sources has
made possible
the transfer of
genetic
material across
virtually any
species barrier
Conjugation and Chromosome Mobilization
? Conjugative plasmids possess genetic
information to code for sex pili and for some
proteins needed for DNA transfer
? Rolling circle replication occurs for DNA
transfer during conjugation
F plasmid of E,coli has the
special property of being able
to mobilize the chromosome
so that it can be transferred
during cell-to-cell contact.
Transfer of
plasmid DNA
by conjugation
? The F plasmid of
an F+ cell is
being transferred
to a F- recipient
cell
Details of the replication
and transfer process
Conjugation and Chromosome Mobilization:
F+ and F- strains
? F+ strains,cells possessing an unintegrated F plasmid.
? F- strains,cells which can act as recipients for F+ or
Hfr,F- strains lack F plasmid.
? F plasmid provides its host cell with:
– ability to synthesize the F pilus
– mobilization of DNA for transfer to another cell
– alteration of surface receptors so that the cell is no
longer able to behave as a recipient in conjugation
Detection of
Genetic
Conjugation
Integration of an F plasmid
into the chromosome with the
formation of an Hfr,IS elements
are the sites of insertion.
homology
Hfr strain
? Hfr strains
arise as a
result of the
integration
of the F
plasmid into
the
chromosome
Manner of formation
of different Hfr strains
? The direction in
which the F factor is
inserted determines
which of the
chromosomal genes
will be inserted first
into the recipient
Important Concept,F’ plasmids
? Integrated F plasmids may be occasionally
excised from the chromosome and bring
some chromosomal genes with itself into
the liberated F plasmid.
? F’-mediated transfer resembles specialized
transduction in that only a restricted group
of chromosomal genes can be transferred.
Interrupted
Mating
? Mixing Hfr
and F- cells.
? Shake the
mixture
violently at
various time.
? Plate the
samples on
selective
medium for
recombinant
to grow.Mapping the
order of genes
Transposons and Insertion Sequences
? Transposition,the process by which gene moves
from one place to another in the genome.
? Transposable elements,transposition of genes is
linked to the presence of special genetic elements
called transposable elements.
? Three types of transposable elements in bacteria:
– Insertion sequences (IS)
– Transposons (Tn)
– Some special viruses (such as Mu)
Three types of transposable elements in bacteria
? Insertion sequences (IS),about 1000 nucleotides,
carry only information to move them to new
location (IS1,IS2 and IS3).
? Transposons (Tn),larger than IS,carry genes,
such as drug resistance markers and other
selectable genes.
? Some special viruses (such as Mu)
Both IS and Tn have short
inverted terminal repeats (IR)
at the ends of their DNA,IR are
involved in the transposition
process
Insertion of a transposable
element generates a duplication
How is the targeted
sequence duplicated?
Transposon mutagenesis
? Insertion of transposon within a gene leads to mutation.
? Transposon with antibiotic-resistant marker can be used for
selection purposes.
? Two tranposons widely used,Tn 5 (neomycin and kanamycin
resistance),Tn10 (tetracycline resistance).
Invertible DNA and the phenomenon of phase variation
When a DNA segment is oriented in one direction,a particular gene
is expressed,Whereas when it is oriented in the opposite direction,
a different gene is expressed.
Salmonella
flagella
synthesis
Mapping of the bacterial genome
? Genetic exchanges are used to map the
location of various genes on the
chromosome.
– Conjugation,map the particular region of the
chromosome
– Transduction,map more fine structure of the
chromosome
E,coli K-12
Genome
Questions for Microbial Genetics
? Describe as much as you know about plasmids.
? What is the difference between a plasmid and an episome?
? What are Hfr strain? F+ or F-,or F’ strain?
? Draw the F plasmid and describe functions of various DNA regions.
? Why is it said that conjugative plasmid contributes to evolution?
? How many types of plasmids and their functions you have learned?
? Schematically describe R100 plasmid and its functions.
? How do R plasmids inactivate antibiotics?
? What is an engineered plasmid?
? What points do F plasmid provide to its host?
? How to detect genetic recombination,Please cite one example.
? How is bacterial genome mapped? What are the three types of
transposable elements?
? Explain transposon mutagenesis and its possible application.
? Give an example to explain conversible DNA and phase variation.
? What techniques are used to map bacterial DNA genome?
Plasmids
? 1-1000 KB in size
? Typical plasmid 1/20 of chromosome
? Most are circular double-stranded
DNA,some linear ds DNA
? Transmitted from cell to cell via
conjugation process
? Some can integrated into
chromosome
? Can carry a variety of genes for
production of toxin,resistance to
antibiotics and heavy metals et al.
Circular genetic elements that
reproduce autonomously and have
an extra-chromosomal existence:
Plasmid
? Conjugative,plasmids which govern their own transfer by
cell-to-cell contact are called conjugative
? Tra region,a set of genes within the plasmid that control the
transmissability by conjugation
? Hfr (high frequency of recombination),strains of bacteria
that transfer large amounts of chromosomal DNA during
conjugation
? Supercoil,plasmids isolated from the cells are in supercoiled
configuration
? Plasmid separation,by ultracentrifugation or electrophoresis
? Curing of plasmids,elimination of plasmids from host cells
by various treatments.
Plasmids
? Replication,Most plasmids of gram-positive
bacteria replicate by a rolling circle
mechanism.
? Copy number,The number of plasmids in a
cell,can range from only 1-3 copies to 100
copies.
? Incompatibility,Two different types of
plasmids can not coexist in a cell.
? Episomes,Plasmids having the ability to
integrate into host chromosome
F-Plasmid-Fertility Plasmids
? 100 KB
? Can be cured with
acridine orange
? Incompatibility (inc)
? Origin of replication
(oriS)
? Transposable elements
(Tn)
? tra region
? phi,phage inhibition
? IS (insertion sequence)
? rep,replication
functions
Cell to Cell Transfer of Plasmids
? Conjugative,Plasmids that govern their own transfer
by cell-to-cell contact are called conjugative (not all
plasmids are conjugative)
? Some conjugative plasmids can transfer genetic
information between distintly related organisms
(between gram-positive and gram-negative bacteria,
between bacteria and plant cells,and between bacteria
and fungi),it is important for evolution.
Conjugation (接合作用）
Plasmid Biology
Episomes
Plasmid Biology
Types of Plasmids and Their
Biological Significance
? The presence of plasmids in a cell can have a
profound influence on the cell’s phenotype:
– the ability of conjugation
– the ability of Rhizobium to interact with plants
– the resistance to antibiotics and heavy metals
– the degradation of octane,camphor et al
– the production of enterotoxin
– the applications in genetic engineering
Types of Plasmids and Their Functions
Resistance Plasmids (R-Plasmids),
the most well studied plasmids
? The emergence of bacteria
resistant to several antibiotics is
medically significant
? Resistance can be transferred via
cell-to-cell contact
? This could be one of the reasons
for the rapid rise of multiply
resistant strains
? Plasmid recombination is one
mean by which multiply
resistant organisms might have
first arisen
? Infectious nature of the R
plasmids permits rapid spread of
the characteristic through
populations
? Typical example,plasmid R100
The presence of multiple antibiotic resistance is due to the fact that a
single R plasmid contains a variety of genes coding for different
antibiotic inactiviation enzymes
Biochemical
mechanism
of resistance
mediated by
R plasmids
Engineered
plasmids-Artificial
plasmids
? Incorporation
into artificial
plasmids of
genes from a
wide variety of
sources has
made possible
the transfer of
genetic
material across
virtually any
species barrier
Conjugation and Chromosome Mobilization
? Conjugative plasmids possess genetic
information to code for sex pili and for some
proteins needed for DNA transfer
? Rolling circle replication occurs for DNA
transfer during conjugation
F plasmid of E,coli has the
special property of being able
to mobilize the chromosome
so that it can be transferred
during cell-to-cell contact.
Transfer of
plasmid DNA
by conjugation
? The F plasmid of
an F+ cell is
being transferred
to a F- recipient
cell
Details of the replication
and transfer process
Conjugation and Chromosome Mobilization:
F+ and F- strains
? F+ strains,cells possessing an unintegrated F plasmid.
? F- strains,cells which can act as recipients for F+ or
Hfr,F- strains lack F plasmid.
? F plasmid provides its host cell with:
– ability to synthesize the F pilus
– mobilization of DNA for transfer to another cell
– alteration of surface receptors so that the cell is no
longer able to behave as a recipient in conjugation
Detection of
Genetic
Conjugation
Integration of an F plasmid
into the chromosome with the
formation of an Hfr,IS elements
are the sites of insertion.
homology
Hfr strain
? Hfr strains
arise as a
result of the
integration
of the F
plasmid into
the
chromosome
Manner of formation
of different Hfr strains
? The direction in
which the F factor is
inserted determines
which of the
chromosomal genes
will be inserted first
into the recipient
Important Concept,F’ plasmids
? Integrated F plasmids may be occasionally
excised from the chromosome and bring
some chromosomal genes with itself into
the liberated F plasmid.
? F’-mediated transfer resembles specialized
transduction in that only a restricted group
of chromosomal genes can be transferred.
Interrupted
Mating
? Mixing Hfr
and F- cells.
? Shake the
mixture
violently at
various time.
? Plate the
samples on
selective
medium for
recombinant
to grow.Mapping the
order of genes
Transposons and Insertion Sequences
? Transposition,the process by which gene moves
from one place to another in the genome.
? Transposable elements,transposition of genes is
linked to the presence of special genetic elements
called transposable elements.
? Three types of transposable elements in bacteria:
– Insertion sequences (IS)
– Transposons (Tn)
– Some special viruses (such as Mu)
Three types of transposable elements in bacteria
? Insertion sequences (IS),about 1000 nucleotides,
carry only information to move them to new
location (IS1,IS2 and IS3).
? Transposons (Tn),larger than IS,carry genes,
such as drug resistance markers and other
selectable genes.
? Some special viruses (such as Mu)
Both IS and Tn have short
inverted terminal repeats (IR)
at the ends of their DNA,IR are
involved in the transposition
process
Insertion of a transposable
element generates a duplication
How is the targeted
sequence duplicated?
Transposon mutagenesis
? Insertion of transposon within a gene leads to mutation.
? Transposon with antibiotic-resistant marker can be used for
selection purposes.
? Two tranposons widely used,Tn 5 (neomycin and kanamycin
resistance),Tn10 (tetracycline resistance).
Invertible DNA and the phenomenon of phase variation
When a DNA segment is oriented in one direction,a particular gene
is expressed,Whereas when it is oriented in the opposite direction,
a different gene is expressed.
Salmonella
flagella
synthesis
Mapping of the bacterial genome
? Genetic exchanges are used to map the
location of various genes on the
chromosome.
– Conjugation,map the particular region of the
chromosome
– Transduction,map more fine structure of the
chromosome
E,coli K-12
Genome
Questions for Microbial Genetics
? Describe as much as you know about plasmids.
? What is the difference between a plasmid and an episome?
? What are Hfr strain? F+ or F-,or F’ strain?
? Draw the F plasmid and describe functions of various DNA regions.
? Why is it said that conjugative plasmid contributes to evolution?
? How many types of plasmids and their functions you have learned?
? Schematically describe R100 plasmid and its functions.
? How do R plasmids inactivate antibiotics?
? What is an engineered plasmid?
? What points do F plasmid provide to its host?
? How to detect genetic recombination,Please cite one example.
? How is bacterial genome mapped? What are the three types of
transposable elements?
? Explain transposon mutagenesis and its possible application.
? Give an example to explain conversible DNA and phase variation.
? What techniques are used to map bacterial DNA genome?
