Bacterial transformation
This “transforming principle” was identified as DNA by Avery, Macleod, and McCarty in 1944. They defined DNA as the chemical substance responsible for heredity.It is the method of introducing of foreign DNA into bacterial cells (e.g E. Coli).A short fragment of naked DNA isolated from one type of bacterial cell is incorporated into another type of bacterial cell. As a result, a recombinant or haploid DNA is formed. This phenomenon is called transformation
Summary
This “transforming principle” was identified as DNA by Avery, Macleod, and McCarty in 1944. They defined DNA as the chemical substance responsible for heredity.It is the method of introducing of foreign DNA into bacterial cells (e.g E. Coli).A short fragment of naked DNA isolated from one type of bacterial cell is incorporated into another type of bacterial cell. As a result, a recombinant or haploid DNA is formed. This phenomenon is called transformation
Things to Remember
- In 1928, an English health officer Griffith injected mice a mixture consisting of a few rough (noncapsulated and nonpathogenic) pneumococci and a large number of heat-killed smooth (capsulated and pathogenic) cells. (living smooth pneumococci cause pneumonia in humans and other animals.
- Thus transformation is the process whereby cell-free or “naked”, DNA containing a limited amount of genetic information is transferred from one bacterial cell to another.
- The DNA is taken in through the cell wall and cell membrane of the recipient cell. The molecular size of the DNA affects transformation. Molecular weights of DNA in the range of 3,00,000 to 8 million daltons have been shown the result with increasing concentration of DNA.
- Bacterial species that have been transformed include, besides Streptococcus pneumonia (pneumococcus), those in the genera Bacillus, Haemophilus, Neisseria, and Rhizobium.
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Bacterial transformation
Bacterial transformation
In 1928, an English health officer Griffith injected mice a mixture consisting of a few rough (noncapsulated and nonpathogenic) pneumococci and a large number of heat-killed smooth (capsulated and pathogenic) cells. (living smooth pneumococci cause pneumonia in humans and other animals. ”rough” and “smooth” refer to the surface texture of the colonies of the respective cells.) The mice subsequently died of pneumonia and live smooth cells were isolated from their blood. Apparently, some factor responsible for the pathogenicity of the smooth bacteria (even though they were dead) had been transferred to the living rough bacteria and had transformed them into pathogenic smooth ones. Griffith also showed that the transforming factor could be passed from the transformed cells to their progeny and thus had the characteristics of a gene.
This “transforming principle” was identified as DNA by Avery, Macleod, and McCarty in 1944. They defined DNA as the chemical substance responsible for heredity. It is the method of introducing of foreign DNA into bacterial cells (e.g E. Coli). A short fragment of naked DNA isolated from one type of bacterial cell is incorporated into another type of bacterial cell. As a result, a recombinant or haploid DNA is formed. This phenomenon is called transformation. In this way, DNA of donor cell expresses some of its properties into the recipient cell. Such cell is called transformation. Griffith confirmed it with experiments on Diplococcus pneumonia.Transformation takes place to a limited extent in many bacteria. But laboratory techniques have been developed that increase the rate DNA uptake.NormallyE. Coli does not pick up foreign DNA but can be made competent in the presence of calcium chloride. The competent bacterium picks the up the foreign DNA. The uptake of foreign DNA byE. Coli is carried out in ice-cold CaCl2 (0-5) with a subsequent heat shock.
Thus transformation is the process whereby cell-free or “naked”, DNA containing a limited amount of genetic information is transferred from one bacterial cell to another. The DNA is obtained from the donor cell by natural cell lysis or by chemical extraction. Once the DNA is taken up by the recipient cell, recombination occurs. Bacteria that have inherited markers (specific characteristics) from the donor cells are said to be transformed. Thus certain bacteria, when grown in the presence of dead cells, culture filtrates, or cell extracts of a closely related strain, will acquire and subsequently transmit a characteristics (s) of the related strain.
The DNA is taken in through the cell wall and cell membrane of the recipient cell. The molecular size of the DNA affects transformation. Molecular weights of DNA in the range of 3,00,000 to 8 million daltons have been shown the result with increasing concentration of DNA. However, each transformation results from the transfer of a single DNA molecule of double-stranded DNA.
After DNA entry into a cell, one strand is immediately degraded by deoxyribonucleases, while the other strand undergoes base pairing with a homologous portion of the recipient cell chromosome; it then becomes integrated into the recipient DNA. Since complementary base pairing takes place between one strand of the donor DNA fragment and a specific region of the recipient chromosome, only closely related strains of bacteria can be transformed.
Bacterial species that have been transformed include, besides Streptococcus pneumonia (pneumococcus), those in the genera Bacillus, Haemophilus, Neisseria, and Rhizobium.
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Properties of recipient cells
Conditions suitable for uptake of donor DNA-recipient cells occur only during the late logarithmic phase of growth. During this period, the transformable bacteria are said to be competent to take up and incorporate donor DNA. Competent cultures probably produce an extracellular protein factor that apparently acts by binding or trapping donor DNA fragments at specific sites on the bacterial surface. The uptake process has been found to be an energy requiring mechanism because it can be inhibited by agents that interfere with energy metabolism.
The significance of transformation as a natural mechanism of genetic change is questionable. It probably occurs following the lysis of a microbe and the release of its DNA into the environment. It is considerable that transformation between bacterial strains of low virulence (disease-producing power of a micro-organism) can give rise to transformed cells of high virulence. In any case, the phenomenon of transformation has proved to be extremely useful in genetic studies of bacteria in the laboratory, particularly in mapping the bacterial chromosome. This is because when DNA enters a recipient cell during transformation, the entering fragments of DNA are not unlike the DNA fragments transferred in a mating between Hfr and F- cells of E. Coli. They will undergo crossing – over (exchange of portions of homologous chromosomes) with the homologous DNA segments of the recipient cells, and recombinants will be formed. As in conjugation, the frequency of transformation of two genes at the same time is an indication of the distance between these genes in the chromosome.
References
Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.
Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.
Powar. and Daginawala. General Microbiology.
Rangaswami and Bagyaraj D.J. Agricultural Microbiology.
Lesson
Microbial genetics
Subject
Microbiology
Grade
Bachelor of Science
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