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Mechanism of natural competence by Jaimin Maheta

  1.  Introduction  History  Mechanism Of DNA uptake  Regulation Of Competence  Evolutionary functions and consequences of competence  Horizontal Gene Transfer
  2.  Competence is the ability of a Bacterial cell to alter its genetics material by taking up extracellular DNA from its environment trough plasmamembren in the process called transformation.  Competence may be differentiated between natural competence, a genetically specified ability of bacteria which is thought to occur under natural conditions as well as in the laboratory, and induced or artificial competence, which arises when cells in laboratory cultures are treated to make them transiently permeable to DNA.  Competence allows for rapid adaptation and DNA repair of the cell.
  3.  Natural competence was discovered by Frederick Griffith in 1928, when he showed that a preparation of killed cells of a pathogenic bacterium contained something that could transform related non - pathogenic cells into the pathogenic type.  In 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty demonstrated that this 'transforming factor' was pure DNA.  This was the first History compelling evidence that DNA carries the genetic information of the cell.
  4.  Since then, natural competence has been studied in a number of different bacteria, particularly Bacillus subtilis, Streptococcus pneumoniae (Griffith's "pneumococcus"), staphylococcus aureus and Haemophilus influenzae.
  5.  In the laboratory, DNA is provided by the researcher, often as a genetically engineered fragment or plasmid.  During uptake, DNA is transported across the cell membrane(s), and the cell wall if one is present.  Once the DNA is inside the cell it may be degraded to nucleotides, which are reused for DNA replication and other metabolic functions.  Alternatively it may be recombined into the cell's genome by its DNA repair enzymes.  If this recombination changes the cell's genomtype the cell is said to have been transformed.
  6.  Artificial competence and transformation are used as research tools in many organisms.  In almost all naturally competent bacteria components of extracellular filaments called type IV pili (a type of fimbria) bind extracellular double stranded DNA.  The DNA is then translocated across the membrane (or membranes for gram negative bacteria) through multicomponent protein complexes driven by the degradation of one strand of the DNA.
  7.  Single stranded DNA in the cell is bound by a well-conserved protein, DprA(Direct Peptide Reactivity Assay), which loads the DNA onto RecA (Bacterial DNA Recombination Protein), which mediates homologous recombination through the classic DNA repair pathway.  ( RecA is a 38 kilodalton Protein essential for the repair and maintenance of DNA ).
  8.  In laboratory cultures, natural competence is usually tightly regulated and often triggered by nutritional shortages or adverse conditions.  However the specific inducing signals and regulatory machinery are much more variable than the uptake machinery, and little is known about the regulation of competence in the natural environments of these bacteria.  Transcription factors have been discovered which regulate competence; an Regulation of competence example is sxy (also known as tfoX) which has been found to be regulated in turn by a 5' non-coding RNA element.
  9.  In bacteria capable of forming spores, conditions inducing sporulation often overlap with those inducing competence.  Thus cultures or colonies containing sporulating cells often also contain competent cells.  Recent research by Süel et al. has identified an excitable core module of genes which can explain entry into and exit from competence when cellular noise is taken into account.
  10.  Most competent bacteria are thought to take up all DNA molecules with roughly equal efficiencies, but bacteria in the families Neisseriaceae and Pasteurellaceae preferentially take up DNA fragments containing short DNA sequences, termed DNA uptake sequence (DUS) in Neisseriaceae and uptake signal sequence (USS) in Pasteurellaceae, that are very frequent in their own genomes.  Neisserial genomes contain thousands of copies of the preferred sequence GCCGTCTGAA, and Pasteurellacean genomes contain either AAGTGCGGT or ACAAGCGGT.
  11.  Most proposals made for the primary evolutionary function of natural competence as a part of natural bacterial transformation fall into three categories:  (1) the selective advantage of genetic diversity  (2) DNA uptake as a source of nucleotides (DNA as “food”) and  (3) the selective advantage of a new strand of DNA to promote homologous recombinational repair of damaged DNA (DNA repair)  A secondary suggestion has also been made, noting the occasional advantage of horizontal gene transfer.
  12.  Hypothesis of genetic diversity  Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species. Diversifying selection is the hypothesis that two subpopulations of a species live in different environments that select for different alleles at a particular locus.  Hypothesis of DNA as food  Researchers are demonstrated that the diets of organisms can affect the DNA sequences of their genes. Our hypothesis was that the composition of this food could alter an organism's DNA.
  13.  Hypothesis of repair of DNA damage  The DNA repair hypothesis for the maintenance of sex states that recombination is necessary for the repair of double-strand DNA damage. In a closed (mitotic) genetic system crossing-over generates homozygosity. Thus, outcrossing is required to restore heterozygosity destroyed by recombination.
  14.  Horizontal Gene Transfer means transfer gene to same generation.  A long-term advantage may occasionally be conferred by occasional instances of horizontal gene transfer also called lateral gene transfer, (which might result from non-homologous recombination after competence is induced), that could provide for antibiotic resistance or other advantages.  Regardless of the nature of selection for competence, the composite nature of bacterial genomes provides abundant evidence that the horizontal gene transfer caused by competence contributes to the genetic diversity that makes evolution possible.