GRIFFITH EXPERIMENT - TRANSFORMATION

                       TRANSFORMATION 

Transformation was the first horizontal gene transfer mechanism discovered in bacteria. It was discovered  by Fred Griffith in 1928. Transformation is the uptake of a naked DNA molecule or fragment from the surrounding environment   by a cell and the incorporation of this molecule into the chromosome of the recipient. Transformation may be natural or artificial 

Natural  transformation is a very rare event and has been observed in both Gram-positive as well as gram-negative. The ability of a recipient bacterium to take up DNA molecules from the surrounding environment and become transformed is known as Competence. Competent bacteria that can take up DNA encode proteins called Competence factors.  

The mechanism of transformation has been intensively studied in Streptococcus pneumonia, a gram-positive bacteria. It becomes competent during the exponential growth phase. Competent bacterial cells bind dsDNA fragments. The uptake of DNA into the cytoplasm is random and any portion of the donors may be entered. During uptake one strand is hydrolyzed by a membrane-associated exonuclease; the other strand moves into the cytoplasm. 

 

PROCESS OF TRANSFORMATION 

in the process of transformation, naturally competent bacteria actively pull DNA fragments from their environment into their cells. DNA can be naturally released into the environment when bacterial DNA die and subsequent lysis. Once the DNA comes in the contact with competent bacteria, linear, dsDNA enters into the cell. One strand translocated across the membrane int6 the cytoplasm, with its 3' end leading.

The strand not translocated, is degraded by exonuclease. Translocated strand integrates with chromosome by homologous recombination. Recipient cells that undergo this process and acquire a new phenotype as a result are said to be transformed.   

Single-stranded exogenote are unstable and will usually be degraded unless they are integrated into the endogenote. By the process of Homologous recombination the transforming DNA integrates into the bacterial chromosome. If the exogenote contains an allele an allele of the endogenotes, the resulting recombinant double helix would contain one ore more mismatched base pairs, and is referred to as a Heteroduplex. These mismatches are repaired by DNA repair enzyme that scan the DNA and repair. The enzyme systems preferentially replace the base in the new strand, thus restoring the genotype of the recipient. If the region is replicated before it is repaired, however, one of the two daughter chromosomes will be recombinant and other will be identical to the recipient. 

MAPPING OF TRANSFORMATION 

Transformation is used for Gene mapping. Mapping by transformation experiments is based on the principle that two genes transform together if they are near enough to be carried on the same DNA  fragment. Let us consider two genes A and B present on the bacterial chromosome. If two genes A and B are widely separated in the chromosome that they are always carried on two different DNA fragments, the probability of simultaneous transformations of the A-B- recipient is very less. If the two genes, however, are so near one another that they are often present on a single DNA fragment, he frequency of simultaneous transformations is same as the frequency of single gene transformation. Thus, the frequency of co-transformation is inversely proportional to the distance between two genes.