8.11

Restriction modification systems

were discovered by scientists studying why certain bacterial strains are relatively resistant to phage infection. research showed that those strains degrade foreign DNA through the combined action of two types of enzymes: a restriction enzyme and a modification enzyme.

Restriction enzymes

recognizes a specific short nucleotide sequence within the cell and then cuts the DNA molecule at that sequence.

Modification enzyme

protects that cell's own DNA from the action of the restriction enzyme by adding methyl groups to certain nucleobases recognized by the restriction enzyme. Because restriction enzymes cannot degrade methylated DNA, a restriction enzyme will destroy incoming foreign DNA but not the host DNA. Occasionally, the modification enzyme will methylate the incoming foreign DNA before the restriction enzyme has acted so that the invading DNA will not be degraded. Different bacteria have different versions of restriction and modification enzymes so many variations exist each recognizing different sequences in DNA.

CRISPR system (clusters of regularly interspersed short palindromic repeats)

discovered when scientists recognized that certain bacterial genomes include very small pieces of phage DNA. The bacteria had survived certain phage infections and retained small segments of that invader's DNA, incorporating them into the bacterial genome. There, the segments are used by the bacterial cell to recognize and destroy that specific invading DNA, providing the cell with a form of adaptive immunity. At least one type of CRISPR system recognizes and cuts foreign RNA.