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