Restriction enzymes cleave the sugar-phosphate backbone ofdouble-stranded DNA. They recognize a specific site of double-stranded DNA and cleave it within, or adjacent to, their recognition site. Restriction enzymes are a very important tool in molecular biology. They allow us to cut DNA strands in a highly predictable manner.

The resulting ends are divided into:

• 粘性的结束:一个链较长,resulting in either a 3' or 5' overhang.

• Blunt ends: Both strands are cut at the same base pair, resulting in an end without an overhang.

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Following are two examples of restriction enzymes

XbaI(pronounced Xba-one): produces sticky ends. It recognizes the following restriction site:

5'---T CTAGA---3'

3'---AGATC T---5'

I-SceI(pronounced Sce-one) is a homing endonuclease and has a very unique 18-base pair-long restriction site that does not naturally occur in mice or human genomes. Hence, it is very useful for highly specific restrictions. SceI recognizes the following restriction site:

5'...TAGGGATAA CAGGGTAAT...3'

3'...ATCCC TATTGTCCCATTA...5'

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• Types of restriction enzymes

• Restriction enzyme reaction conditons