(73)
3. Forklare hvad restriktonsenzymer er, og hvad de kan bruges til
Devlin, s.72
Stryer, s.144-5, fig - 6.1.
Stryer, s. 245
Issinger, 3.13

Restriction enzymes (restriction endonucleases) - enzymes that recognize specific base sequences in double-helical DNA and cleave, at specific places, both strands of a duplex containing the recognized sequences.

They are normally found in a wide variety of prokaryotes. Their biological role is to cleave foreign DNA molecules.

Many viruses inject their DNA genomes into cells, and once inside, the viral DNA hijacks the cells machinery to drive the production of viral proteins. Often, a viral infection results in the death of the host. Restriction endonucleases act as a protective mechanism and cleave the viral DNA.

The cells own DNA is not degraded, because the sites recognized by its own restriction enzymes are methylated.

Bacteriofages DNA is not methylated, so it can be cleaved by the bacterial restriction enzymes.

With few exceptions, restriction enzymes recognize sequences four to six nucleotides long. The recognized sequences are completely symmetrical inverted repeats, known as palindromes. The order of the bases is completely the same when the two complementary strands of the palindrome are each red in the 5`-3` direction.

The cleavage sites are symmetrically positioned.

The biochemistry of restriction enzymes cleavage is breaking of the phosphodiester bond between two specific nucleotides in each strand.

Restriction enzymes recognize specific sequences that occur along large DNAs with relatively low frequencies.

More then 400 restriction enzymes have been purified and characterized.

To biochemists, restriction enzymes are indispensable for analyzing chromosome structure, sequencing of very long DNA molecules, isolating genes and creating new DNA molecules that can be cloned.

• Restriction enzymes are used to cleave DNA molecules into specific fragments of different length that are more readily analyzed and manipulated then the entire DNA molecule.

• Restriction fragments can then be separated by gel electrophoresis and visualized.

A piece of DNA produced by action of one restriction enzyme can be specifically cleaved into smaller fragments by another restriction enzyme.

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