Transcript F + Hfr

Bacterial Genetics
Bacterial Genome
 Chromosome:
 Plasmid: Plasmids are extrachromosomal
genetic elements capable of autonomous
replication. An episome is a plasmid that
can integrate into the bacterial chromosome.
 IS or Tn
Mechanism of bacterial variation
 Gene mutation
 Gene transfer and recombination
 Transformation
 Conjugation
 Transduction
 Lysogenic conversion
 Protoplast fusion
Types of mutation
 Base substitution
 Frame shefit
 Insertion sequences
What can cause mutation?
 Chemicals:
nitrous acid; alkylating agents
5-bromouracil
benzpyrene
 Radiation: X-rays and Ultraviolet light
 Viruses
Bacterial mutation
 Mutation rate
 Mutation and selectivity
 Backward mutation
Transformation
 Transformation is gene transfer resulting
from the uptake by a recipient cell of naked
DNA from a donor cell. Certain bacteria (e.g.
Bacillus, Haemophilus, Neisseria,
Pneumococcus) can take up DNA from the
environment and the DNA that is taken up
can be incorporated into the recipient's
chromosome.
Conjugation
 Transfer of DNA from a donor to a recipient
by direct physical contact between the cells.
In bacteria there are two mating types a
donor (male) and a recipient (female) and
the direction of transfer of genetic material is
one way; DNA is transferred from a donor to
a recipient.
Physiological States of F Factor
 Autonomous (F+)
 Characteristics of F+ x F- crosses
 F- becomes F+ while F+ remains F+
 Low transfer of donor chromosomal
genes
F+
Physiological States of F Factor
 Integrated (Hfr)
 Characteristics of
Hfr x F- crosses
 F- rarely becomes
Hfr while Hfr
remains Hfr
 High transfer of
certain donor
chromosomal
genes
F+
Hfr
Physiological States of F Factor
 Autonomous with
donor genes (F’)
 Characteristics of F’
x F- crosses
 F- becomes F’
while F’ remains
F’
 High transfer of
donor genes on
F’ and low
transfer of other
donor
chromosomal
genes
Hfr
F’
Mechanism of F+ x F- Crosses
• Pair formation
– Conjugation
bridge
 DNA transfer
F+
F-
F+
F-
F+
F+
F+
F+
 Origin of
transfer
 Rolling
circle
replication
Mechanism of Hfr x F- Crosses
• Pair formation
– Conjugation
bridge
 DNA transfer
 Origin of transfer
 Rolling circle
replication
 Homologous
recombination
Hfr
Hfr
F-
F-
Hfr
Hfr
F-
F-
Mechanism of F’ x F- Crosses
• Pair formation
– Conjugation
bridge
 DNA transfer
F’
F-
F’
F-
F’
F’
F’
F’
 Origin of
transfer
 Rolling circle
replication
R Plasmid
Transduction:
 Transduction is defined as the transfer of
genetic information between cells through
the mediation of a virus (phage) particle. It
therefore does not require cell to cell contact
and is DNase resistant.
Generalized Transduction
 Generalized transduction is transduction in
which potentially any bacterial gene from the
donor can be transferred to the recipient.
The mechanism of generalized
transduction
Generalized transduction
1.
2.
3.
4.
It is relatively easy.
It is rather efficient (10-3 per recipient with
P22HT, 10-6 with P22 or P1), using the
correct phage.
It moves only a small part of the
chromosome which allows you to change
part of a strain's genotype without affecting
the rest of the chromosome.
The high frequency of transfer and the small
region transferred allows fine-structure
mapping
Specialized transduction
 Specialized transduction is transduction in which
only certain donor genes can be transferred to
the recipient.
 Different phages may transfer different genes but
an individual phage can only transfer certain
genes
 Specialized transduction is mediated by lysogenic
or temperate phage and the genes that get
transferred will depend on where the prophage
has inserted in the chromosome.
The mechanism of specialized
transduction
Specialized transduction
1.
2.
3.
4.
Very efficient transfer of a small region--can be
useful for fine-structure mapping
Excellent source of DNA for the chromosomal
region carried by the phage, since every phage
carries the same DNA.
Can often be used to select for deletions of some
of the chromosomal genes carried on the phage.
Merodiploids generated using specialized phage
can be quite useful in complementation analyses.
Lysogenic conversion
 The prophage DNA as a gene
recombined with chromosome of
host cell.
Protoplast Fusion
 Fusion
of two protoplasts treated with
lysozyme and penicillin.
Application of bacterial variation
 Use in medical clinic: Diagnosis, Treatment,
Prophylaxis.
 Use in Genetic Engineering