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Restriction enzyme

A restriction enzyme cuts DNA where it recognises a particular sequence. How often that sequence appears depends on how long it is. With four bases that are equally common, the probability of one particular run of six bases is one divided by four to the power of six. The enzyme then cuts on average at that spacing. The real sequences are not perfectly even, so the distance varies. Gel electrophoresis later separates such fragments. CRISPR-Cas9 also cuts, but there a guide chooses where the cut should be.

46=40964^{6} = 4096average spacing for a six-base cut

Symbols

44bases, equally common
66bases in the recognition sequence

Example

An enzyme recognises 6 bases, and the bases are equally common.

Average gap between cuts: 46=40964^{6} = 4096 bases.

If the bases are not equally common, or the sequence is shorter, the spacing is different.
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← PCR · Gel electrophoresis →

Part of Biology 2: Biotechnology.