1. Restriction Enzymes-BIOL 202

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

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

4. 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

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

6. 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

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

8. 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

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

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