Bacterial Genetics

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  • Created by: Jess
  • Created on: 17-05-13 13:26

Bacterial Genetics

Prokaryotes lack the physically separated and functionally specialised compartments of a eukaryotic cell so transcription and translation occur simultaneously.

Bacteria are essentially haploid and contain a single double-stranded chromosome that is usually circular.

Often genes encoding related functions are clustered and co-transcribed as a single mRNA molecule, this arrangement is termed an operon.

Bacteria replicate semi-conservatively but have no equivalents to mitosis and meiosis. During vegetative growth they double in size and then divide by binary fission to produce two equal daughter cells.

As bacteria undergo clonal growth all cells in a colony are gentically identical.

Genetic exchange occurs by three distinct mechanisms: transformation, transduction and conjugation.

Recipient cells can become merodiploids (partial diploids) or merozygotes (partial zygotes).

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Plasmids are autonomous, independently replicating genetic elements that usually confer additional phenotypes onto the bacteria that harbour them.

They are found in virtually every bacterial species and mot are circular dsDNA molecules. They vary in size and encode a wide range of phenotypes. The most important phenotype asssociated with plasmids is antibiotic resistance.

Most plasmids above 40kb are conjugative as they promote the synthesis of pili, the formation of mating pair or groups, and the transfer of DNA to recipient cells.

Smaller plasmids are non-conjugative. Most can still be transferred during conjugation by a process called mobilisation provided they are in a donor bacterium that also carries a conjugative plasmid to initiate the cell-to-cell contact. The small plasmids then undergo rolling circle transfer of their DNA to a recipient cell.

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Types of Plasmid

- Bacteriocin-producing plasmids: Bacteriocins are antibacterial proteins made by a particular bacterium and active against bacteria of the same species.

- Virulence plasmids: Various plasmid-encoded phenotypes contribte to the pathogenicity of some bacteria.

- Degradation plasmids of Pseudomonas: Specific plasmids enable Pseudomonas species to feed on extremely diverse and unusual substrates.

- Symbiosis of Rhizobium: Their functions include host plant specificity, stimulation of root nodule formation and fixation of atmospheric nitrogen.

- Tumour induction plasmids of Agrobacterium: Most functions required for gall formation in crown gall disease are encoded by a large conjugative plasmid called Ti.

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Bacterial viruses (phages) are the smallest and simplest self replicating entities known and they comprise a genome encased in a protein coat called a capsid. Phages reproduce at the expense of the bacterial host which usually does not survive the process.

They attach to specific receptors on the cell surface and inject their nucleic acid into the cytoplasm leaving the empty capsid on the surface.

Linear dsDNA molecules form a circle and ssDNA molecules are made ds by the synthesis of a comlementary strand. The phage genes are then expressed. The infected cell fills with new capsid proteins and copies of the phage genome.

20-30 minutes after infection the cell lyses releasing more progeny phages. The presence of phages is recognised by the formation of plaques (a small area in which all the bacteria have been lysed).

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