Bacterial Genetics

BACTERIAL GENETICS

MAPPING GENES IN BACTERIA

Introduction to Bacterial Genetics

  • Escherichia coli (E. coli)

    • Extensively used in genetics and molecular analysis.

    • Can be grown on a simple, defined medium and handled with straightforward microbiological techniques.

    • Commonly found in the large intestines of animals, including humans.

  • Bacterial Cell Structure

    • Bacterial cells are relatively small compared to eukaryotic cells.

    • Cytoplasm is rich in ribosomes.

    • Central nucleoid region contains genetic material:

    • Single circular chromosome consisting of DNA.

    • Absence of a nuclear membrane.

Mechanisms of Gene Transfer in Bacteria

  • Genetic material can be transferred between bacteria via three main processes:

    1. Transformation

    2. Conjugation

    3. Transduction

  • Characteristics of Gene Transfer:

    • Transfer is unidirectional.

    • No true diploid zygote is formed.

    • Only genes included in the circular chromosome are stably inherited.

    • These factors differentiate bacterial genetic transfer from eukaryotes.

Bacterial Transformation

  • Involves the uptake of DNA from a donor strain:

    1. DNA from a donor strain is extracted and purified.

    2. DNA is broken into small linear double-stranded fragments.

    3. Genetic material is added to a suspension of recipient bacteria with a different genotype.

    4. Recipients whose phenotypes change through recombination are termed transformants.

    • Any bacterial strain can act as either a donor or a recipient.

    • Transformants are produced only when there are genetic differences between the donor and the recipient strains.

  • Transformation Types:

    1. Transformation with DNA Fragments:

    • Exemplified by DNA fragments being incorporated into recipient bacteria.

    1. Transformation with a Plasmid:

    • Involves a plasmid integrating into the bacterial chromosome through nonreciprocal recombination leading to stable transformation.

  • Diagrammatic representations:

    • Successful transformation leads to either:

    • Stable transformation

    • Unsuccessful transformation or degradation of DNA/plasmids.

Competence in Bacteria

  • Certain bacteria, like Bacillus subtilis, can be transformed easily in vitro (in test tubes).

  • Wild-type E. coli is not readily transformable due to enzymes that rapidly degrade incoming DNA.

  • Cells can be treated chemically to enhance their permeability to DNA, termed competent cells.

  • Types of Bacterial Transformation:

    1. Natural Transformation:

    • Certain bacteria are naturally capable of incorporating DNA from their environment (example: Bacillus subtilis).

    1. Engineered Transformation:

    • Bacteria, such as E. coli, are genetically modified to enhance their ability to uptake DNA.

  • Applications of Transformation:

    • To determine gene linkage, gene order, and map distance.

    • Co-transformation occurring significantly above the expected frequency suggests genes are closely linked.

Inter-Kingdom Gene Transfer

  • Ti plasmid of Agrobacterium tumefaciens and Ri plasmid of A. rhizogenes:

    • Contains genes capable of transferring to plant cells.

    • Expression of these genes transforms plant cells into opine-producing factories.

    • Opines are low molecular weight compounds utilized by bacteria as nitrogen and energy sources.

    • Resultant infected cells form crown gall or root tumors respectively.

Conjugation in Bacteria

  • Conjugation is characterized as:

    • The transfer of genetic material through direct cell-to-cell contact or via a bridge-like connection between two cells.

    • A mechanism of horizontal gene transfer distinct from transformation and transduction, which do not involve direct contact.

  • Genetic exchange dynamics:

    • The process is unidirectional and not reciprocal.

    • Mediated by a fertility factor called F-factor:

    • Found in donor cell (F+).

    • Absent in recipient cell (F-).

    • The F factor is a type of plasmid—self-replicating genetic elements located in bacterial cytoplasm encompassing a DNA region referred to as the origin (O).

  • Conjugation Process:

    • Mixing of F+ and F- cells leads to conjugation.

    • Only between different mating types; no conjugation affects cells of the same type.

    • Some cells may integrate their plasmids, known as Hfr (high frequency of recombination) strains, facilitating efficient chromosomal gene transfers.

Transduction in Bacteria

  • Transduction defines the transfer of genetic material between bacterial strains via bacteriophages (viruses that infect bacteria).

    • Structure of bacteriophages:

    • Composed of genetic material (either DNA or RNA) enclosed by a protective protein coat.

  • Transduction Mechanism**:

    1. Phage injects its DNA into the host bacterial cell.

    2. Phage enzymes degrade the host's DNA.

    3. The host cell synthesizes new phages incorporating both phage DNA and segments of host DNA.

    4. The transducing phage injects donor DNA into a recipient host.

    5. Donor DNA is incorporated into the recipient's chromosome through recombination.

  • Types of Transduction:

    1. Generalized Transduction:

    • Any gene may be transferred between bacteria during this process.

    1. Specialized Transduction:

    • Only specific genes are transferred to the host during this mechanism, allowing for precise genetic modifications.