Bacterial Genetics
Key Requirements:
1,Understand how genetic mapping is
achieved in bacteria,Co-transformation,F
factor,Hfr strain,co-transduction and some
understanding of phage genetics
2,Read the textbook and do the problem set at
the back.
General Introduction about E.coli
E,coli is widely used in genetic studies.
E,coli grows quickly,grows on a simple
media and there are many auxotrophic
mutant strains.
Many different bacterial mutants types have
been identified,
Mutants unable to utilize a nutrient for growth,gal-,lac-,xyl-,etc
Mutants dependent upon a nutrient for growth---
an auxotroph,ala-,phe-,leu-,etc
Mutants sensitive or resistant to a drug or phage,tetR
Conditional mutations,grow only under certain conditions dnaAts
An experiment to isolate auxotroph
The bacterial chromosome and plasmids
Most or all bacteria possess only one chromosome,
and it is circular
The size of bacterial chromosomes vary from 1.5
mega-base pairs to 8 mega-base pairs,E,coli is 4.8
mega-base pairs (4,800,000),Several have had their
DNA sequence determined.
Nearly all bacteria possess plasmids.
Plasmids are small (usually between 2,000 to 100,000 bp)
extrachromosomal circular DNAs that replicates
autonomously from the chromosome)
Many plasmids possess specialized genes,antibiotic
resistance,ability to degrade specialized chemicals,or
genes for specific purposes,nitrogen fixation,toxin
production,etc,
Some possess genes for transfer to other bacteria
Bacterial Plasmid
Three kinds of genetic transfer
occur in bacteria
Transformation,donor cell releases DNA by lysis
and it is taken up by the recipient cell
Conjugation,physical contact between two
bacterial cells and transfer of DNA
Transduction,bacterial virus (phage) transfers
the DNA from donor cell to recipient cell
Gene Transfer in Bacteria
Bacterial transformation
Transformation involves uptake of DNA,followed by
either recombination of the DNA with the host
chromosome or self-replication of DNA (plasmids).
Genes that are close together on DNA can cotransform,
That is,alleles of both genes can be inserted on the
same piece of DNA.
Gene order can be determined by cotransformation.
Natural Transformation
Transformation of bacterial cells
Transformation does not naturally occur in most strains
of bacteria,including E,coli.
E,coli and other bacteria can be forced to take up DNA
by special chemical and temperature treatments
DNA can also be put into bacterial cells by first treating
the cells with a brief high voltage pulse (electroporation),
This causes a temporary production of pores in the cell
membrane,allowing the DNA to pass into the cell,
The sex factor,F
DNA transfer is mediated by F,the fertility factor,
F is a plasmid,a small self-replicating circle of
DNA.
F carries genes that direct the formation of the
conjugation tube and its replication and transfer
to the recipient
The F-factor replicates and a new copy is sent into
the F- recipient cell,The F-factor is an example of
a conjugative plasmid
Generation of an Hfr Strain
a single crossover between the F-factor and the
bacterial chromosome
This generates a high frequency recombination cell
(Hfr).
Recombination occurs at site of insertion sequence
The Hfr strain transfers a copy of the bacterial
chromosome along with the F -factor.
If mating is interrupted,only a portion of the
chromosome is transferred.
The location of a gene can be determined by
determining when it is transferred,
Genes are mapped in units of minutes of mating.
The gene closest to the inserted F-factor is
transferred first.
Many Hfr strains exist with F inserted in different
locations,
Any gene can be mapped by using several Hfr
strains
Entry times of genes depend upon their
distance from the entry point of the Hfr
Key Requirements:
1,Understand how genetic mapping is
achieved in bacteria,Co-transformation,F
factor,Hfr strain,co-transduction and some
understanding of phage genetics
2,Read the textbook and do the problem set at
the back.
General Introduction about E.coli
E,coli is widely used in genetic studies.
E,coli grows quickly,grows on a simple
media and there are many auxotrophic
mutant strains.
Many different bacterial mutants types have
been identified,
Mutants unable to utilize a nutrient for growth,gal-,lac-,xyl-,etc
Mutants dependent upon a nutrient for growth---
an auxotroph,ala-,phe-,leu-,etc
Mutants sensitive or resistant to a drug or phage,tetR
Conditional mutations,grow only under certain conditions dnaAts
An experiment to isolate auxotroph
The bacterial chromosome and plasmids
Most or all bacteria possess only one chromosome,
and it is circular
The size of bacterial chromosomes vary from 1.5
mega-base pairs to 8 mega-base pairs,E,coli is 4.8
mega-base pairs (4,800,000),Several have had their
DNA sequence determined.
Nearly all bacteria possess plasmids.
Plasmids are small (usually between 2,000 to 100,000 bp)
extrachromosomal circular DNAs that replicates
autonomously from the chromosome)
Many plasmids possess specialized genes,antibiotic
resistance,ability to degrade specialized chemicals,or
genes for specific purposes,nitrogen fixation,toxin
production,etc,
Some possess genes for transfer to other bacteria
Bacterial Plasmid
Three kinds of genetic transfer
occur in bacteria
Transformation,donor cell releases DNA by lysis
and it is taken up by the recipient cell
Conjugation,physical contact between two
bacterial cells and transfer of DNA
Transduction,bacterial virus (phage) transfers
the DNA from donor cell to recipient cell
Gene Transfer in Bacteria
Bacterial transformation
Transformation involves uptake of DNA,followed by
either recombination of the DNA with the host
chromosome or self-replication of DNA (plasmids).
Genes that are close together on DNA can cotransform,
That is,alleles of both genes can be inserted on the
same piece of DNA.
Gene order can be determined by cotransformation.
Natural Transformation
Transformation of bacterial cells
Transformation does not naturally occur in most strains
of bacteria,including E,coli.
E,coli and other bacteria can be forced to take up DNA
by special chemical and temperature treatments
DNA can also be put into bacterial cells by first treating
the cells with a brief high voltage pulse (electroporation),
This causes a temporary production of pores in the cell
membrane,allowing the DNA to pass into the cell,
The sex factor,F
DNA transfer is mediated by F,the fertility factor,
F is a plasmid,a small self-replicating circle of
DNA.
F carries genes that direct the formation of the
conjugation tube and its replication and transfer
to the recipient
The F-factor replicates and a new copy is sent into
the F- recipient cell,The F-factor is an example of
a conjugative plasmid
Generation of an Hfr Strain
a single crossover between the F-factor and the
bacterial chromosome
This generates a high frequency recombination cell
(Hfr).
Recombination occurs at site of insertion sequence
The Hfr strain transfers a copy of the bacterial
chromosome along with the F -factor.
If mating is interrupted,only a portion of the
chromosome is transferred.
The location of a gene can be determined by
determining when it is transferred,
Genes are mapped in units of minutes of mating.
The gene closest to the inserted F-factor is
transferred first.
Many Hfr strains exist with F inserted in different
locations,
Any gene can be mapped by using several Hfr
strains
Entry times of genes depend upon their
distance from the entry point of the Hfr
