Transformation is Gene Transfer by Naked DNA

The simplest way to transfer genetic information is for one cell to release DNA into the medium and for another cell to import it. The transfer of "pure" or "naked" DNA from one cell to another is known as transformation (Fig. 18.03). By "naked", is meant that no other biological macromolecules, such as protein, are present to enclose or protect the DNA. No actual cell-to-cell contact is involved in transformation, nor is the DNA packaged inside a virus particle. Bacterial cells can often take up naked DNA molecules and may incorporate the genetic information they carry.

In practice, transformation is mostly a laboratory technique. The DNA is extracted from one organism by the experimenter and offered to other cells in culture. Cells able to take up DNA are said to be "competent." Some species of bacteria readily take up external DNA without any pre-treatment. Probably they use this ability to take up DNA under natural conditions. From time to time, bacteria in natural habitats die and disintegrate. In doing so they release DNA that nearby cells may import.

competent cell Cell that is capable of taking up DNA from the surrounding medium

Original Fragments Transformed Recombinant


FIGURE 18.03 Gene Transfer by Transformation

Under the right conditions, bacteria can take up pieces of naked DNA from the external environment. The fragment of DNA may pass through the outer cell layers without the aid of a protein or virus. Once inside the bacteria, the fragment of DNA must recombine with the chromosome to prevent degradation by exonucleases or restriction enzymes.

Cells that have cell walls usually need some sort of treatment before they can take up DNA.

Other bacteria must first be treated in the laboratory to make them competent. There are two ways of doing this. The older method is to chill the bacterial cells in the presence of metal ions, especially high concentrations of Ca2+, that damage their cell walls and then to heat shock them briefly. This loosens the structure of the cell walls and allows DNA, a huge molecule, to enter. A more modern method is electroshock treatment. Bacteria are placed in an "electroporator" and zapped with a high voltage discharge. After genes or other useful segments of DNA have been cloned in the test tube, it is almost always necessary to put them into some bacterial cell for analysis or manipulation. Thus, laboratory transformation techniques are an essential tool in genetic engineering. E. coli is normally treated by some variant of the Ca2+/cold-shock treatment and does not require electroshock. Yeast cells may also be transformed. Since yeast has a very thick cell wall, electroshock is used. Conversely, animal cells, which lack cell walls, often take up DNA readily without any pretreatment, both when grown in culture and in the body.

The transfer of inherited characteristics due to the uptake of pure DNA was part of the original proof that DNA was the genetic information.

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