bacterial genetics. bacterial genome chromosome: plasmid: plasmids are extrachromosomal genetic...
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Bacterial Genetics
Bacterial Genome
Chromosome: Plasmid: Plasmids are extrachromosomal g
enetic 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 recombinati
on 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- crossesF- 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- crossesF- 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
DNA transferOrigin of
transferRolling circle
replication
• Pair formation
– Conjugation bridge
F+ F- F+ F-
F+ F+F+ F+
Mechanism of Hfr x F- Crosses
DNA transferOrigin of transferRolling circle
replication
Homologous recombination
• Pair formation
– Conjugation bridge
Hfr F- Hfr F-
Hfr F-Hfr F-
Mechanism of F’ x F- Crosses
DNA transferOrigin of
transferRolling circle
replication
• Pair formation
– Conjugation bridge
F’ F’F’ F’
F’ F- F’ F-
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 generalizedtransduction
Generalized transduction
1. It is relatively easy.
2. It is rather efficient (10-3 per recipient with P22HT, 10-6 with P22 or P1), using the correct phage.
3. 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.
4. 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. Very efficient transfer of a small region--can be useful for fine-structure mapping
2. Excellent source of DNA for the chromosomal region carried by the phage, since every phage carries the same DNA.
3. Can often be used to select for deletions of some of the chromosomal genes carried on the phage.
4. 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