Restriction Enzymes-BIOL 202

How do bacteria protect themselves against phage and other invading DNA molecules?

One protection system involves restriction-modification or rm systems Unmethylated DNA is cut or “restricted”

Phage “restriction” discovered in 1950s Enzymology difficult to work out 1970-first major report of purification and study of a restriction enzyme Quickly became workhorse enzymes Revolutionized molecular biology in mid-1970s

Two-part system: restriction enzyme cleaves DNA, modification enzyme methylates (protects) DNA

How do rm systems know where to cut or methylate? They recognize specific sequences in the DNA called restriction sites They cut or methylate at these sites The sequences are 4, 6 or 8 bases in length

Restriction enzymes always cut a given piece of DNA at the same site(s) generating a characteristic set of fragments

When you separate the fragments by electrophoresis through an agarose gel and then stain the DNA the result is called a DNA fingerprint or a restriction enzyme fingerprint

No two different DNA samples will produce the same set of fragments upon restriction enzyme digestion.

This means that restriction enzyme fingerprinting can be used to match or exclude DNA samples, for example in forensic analyses