Natural Competence

The process of DNA transformation involves recipient cells, termed competent with the unusual ability to take up and integrate donor DNA into their chromosome. The process occurs in a number of stages. First, DNA passes through the cell wall and cytoplasmic membrane. This is one of the most interesting aspects of the DNA transformation process. As discussed in chapter 3, usually only molecules whose molecular weight is no greater than a few hundred can pass through the cell envelope. It is still a mystery how DNA molecules, which are 100,000 times larger, can pass through. It is clear, however, that only under certain growth conditions can bacteria take up such large molecules of DNA. This often occurs near the end of the log phase of growth. Some bacteria take up DNA from any source; others take up only their own DNA. The mechanism by which a cell becomes competent is poorly understood, but apparently the cell undergoes a series of changes. These include modification of the cell wall and the synthesis of protein receptors on the cell surface that can bind DNA. Other competence proteins inside the cell are also synthesized, including those involved in the integration of donor DNA. Cells also can be made competent through artificial means in the laboratory, as will be discussed shortly. ■ log phase, p. 102

Entry of DNA

Although double-stranded DNA molecules bind to competent cells, single-stranded DNA enters. As one strand enters, nucleases at the cell surface degrade the other (figure 8.14a, b).

Integration of Donor DNA

Once inside the recipient cell, the single-stranded donor DNA becomes positioned, by hydrogen bonding, next to the complementary region of the recipient DNA (see figure 8.14c). The two regions are said to be homologous. The donor and recipient strands are held together by hydrogen bonds between complementary bases. Then, a nuclease cleaves the recipient cell's single-stranded DNA on either side of the donor DNA. This fragment of DNA is released into the cytoplasm, where it is degraded by nucleases. The donor DNA then replaces the recipient DNA precisely, by the process of homologous recombination (see figure 8.14d). This mechanism of recombination in which the homologous region of the recipient DNA is replaced by donor DNA is termed breakage and reunion. The same mechanism also accounts for the integration of donor DNA into recipient cells in both conjugation and transduction.

204 Chapter 8 Bacterial Genetics Gene conferring Strr Gene conferring Str

Degradation of one donor stranc by nucleases

Degradation of one donor stranc by nucleases

Recipient DNA

Single strand of donor DNA enters

Transforming DNA (double-stranded) attaches to recipient cell surface

Recipient DNA

Single strand of donor DNA enters

Pairing of donor DNA with homologous region of recipient chromosome

Gene conferring StrR

Gene conferring StrR

Integration of single-stranded donor DNA by breakage and reunion

Integration of single-stranded donor DNA by breakage and reunion

Figure 8.14 DNA-Mediated Transformation The donor DNA comes from a cell that is streptomycin resistant (StrR).The recipient cell is streptomycin sensitive (Str5).

Double-

Cell stranded Cytoplasmic wall DNA membrane

Double-

Cell stranded Cytoplasmic wall DNA membrane

Recipient cell

Fragments of donor DNA

Figure 8.15 Electroporation The electric current makes holes in both the cell wall and the cytoplasmic membrane through which the double-stranded DNA can pass.These holes are then repaired by the cell, and the DNA becomes incorporated into the chromosome of the cell.

Multiplication of Transformed Cells

The transformed cells multiply under selective conditions in which the non-transformed cells cannot grow and form colonies (see figure 8.14f). For example, if the donor cells are StrRand the recipient cells are StrS, then only cells transformed to StrR will grow on medium that contains streptomycin. Since only one strand of the recipient cells DNA is transformed initially to streptomycin resistance, only one half of the daughter cells will be streptomycin resistant. The other half will be streptomycin sensitive and will die on streptomycin-containing medium (see figures 8.14e and 8.14f). Although many other donor genes besides StrR will be transferred and integrated into the chromosome of the recipient cells, these transformants will go undetected since the donor and recipient cells are identical in these other genes.

How To Get Rid Of Yeast Infections Once And For All

How To Get Rid Of Yeast Infections Once And For All

No more itching, odor or pain or your money is refunded! Safe and DRUG FREE Natural Yeast Infection Solutions Are you looking for a safe, fast and permanent cure for your chronic yeast infection? Get Rid of that Yeast Infection Right Now and For Good!

Get My Free Ebook


Post a comment