Genetics of Prokaryotic Cells

Many bacteria contain extrachromosomal or extragenomic material known as plasmids.3   Plasmids often carry genes that impart some benefit to the bacterium, such as antibiotic resistance. Plasmids may also carry additional virulence factors, or traits that increase how pathogenic a bacterium is, such as toxin production, projections that allow the bacterium to attach to certain kinds of cells, or evasion of the host’s immune system. A subset of plasmids called episomes are capable of integrating into the genome of the bacterium.3  

Transformation results from the integration of foreign genetic material into the host genome. This occurs when another bacteria undergoes lysis and spill their foreign genetic material in the vicinity of a bacterium capable of transformation. Many species of gram-negative rods are capable of transformation.

Conjugation is the bacterial form of mating (sexual reproduction). It involves two cells forming a conjugation bridge between them that allows for the transfer of genetic material. The transfer is unidirectional, from the donor male (+) to the recipient female (–). The bridge is made from appendages called sex pili that are found on the donor male. To form the pili, bacteria must contain plasmids known as sex factors that contain the necessary genes.3 The best-studied sex factor is the F (fertility) factor in E. coli. Bacteria possessing this plasmid are termed F+ cells; those without are called F cells. During conjugation between an F+ and an F cell, the F+ cell replicates its F factor and donates the copy to the recipient, converting it to an F+ cell. This enables the cell obtaining the new plasmid to then transfer copies to other cells. Genetic recombination in this manner allows for quick antibiotic resistance capabilities throughout a colony since other plasmids can also transfer through the conjugation bridge. Through processes such as transformation, the plasmid can become integrated into the host genome, despite it being a sex factor. As a result, the entire genome replicates when conjugation occurs as it now contains the sex factor. The donor cell will try to move an entire copy of its genome into the recipient but the bridge usually breaks before the full DNA sequence can be transferred. Cells that have undergone this change are referred to by the abbreviation Hfr for high frequency of recombination.3

Conjugation
Conjugation

 

Transposons are genetic elements capable of inserting and removing themselves from the genome. This phenomenon is not limited to prokaryotes; it has been seen in eukaryotes as well. If a transposon is inserted within a coding region of a gene, that gene may be disrupted.

 

References

1) Villarreal, L. P. (2008, August 8). Are Viruses Alive. Scientific American.

2) Campbell, N. (2003). Biology:Concepts & Connections. San Francisco: Pearson Education.

3) Sinkovics, J., J, H., & A, H. (1998). he Origin and evolution of viruses (a review). Acta Microbiologica et Immunologica Hungarica, 349 – 390.

4) Crick FH, W. J. (1956). Structure of small viruses. Nature, 473- 475.

5) Boevink P, O. K. (2005). Virus-host interactions during movement processes. Plant Physiology, 1815–21.

6)  NC State University. “Prokaryotes: Single-celled Organisms

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