As a general rule, a recipient bacterium will incorporate DNA by transformation, conjugation, or transduction only if the donor DNA is from the same species of bacteria. If DNA from another species termed foreign DNA, enters the cell, a deoxyri-bonuclease in the recipient cell will degrade it. This mechanism allows cells to maintain stability of their genome against invasion by foreign DNA.
The enzymes that degrade entering DNA are called restriction enzymes because they "restrict'' the entry of foreign DNA into cells. The enzymes recognize specific short sequences of 4 to 8 nucleotides termed the recognition sequence in foreign DNA and then cleave one strand of DNA at these sites. One cleavage site is on one strand, and the other is on the opposite strand (figure 8.25). Enzymes in different organisms recognize and cleave different nucleotide sequences (see figure 8.25a, b). Other deoxyribonucleases that degrade DNA without regard to its nucleotide sequence then break down the DNA into individual nucleotides. Restriction enzymes have been found in virtually all species of prokaryotes, but not in any eukaryotes. Thus, eukary-otes lack this barrier to gene entry. Restriction enzymes, which have proved to be extremely important in the genetic engineering of microorganisms, will be considered further in chapters 9 and 13. ■ restriction enzyme, pp. 219,231
How do bacteria prevent destruction of their own DNA by the restriction enzyme they produce? The partial answer to this question is that bacteria that produce a restriction enzyme also make another enzyme, a modification enzyme. The modification enzyme adds methyl (—CH3) groups to adenine and cyto-sine in the recognition sequence. These bases are also methylated in the recipient strain. Once the DNA is methylated, the restriction enzymes do not recognize the recognition sequence
Site of cleavage
Restriction enzyme 1
Restriction enzyme 1
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