Restriction Enzymes

Theoretical Basis Using Restriction EnzymesThe activity of restriction enzymes is dependent upon precise environmental condtions:

PHTemperatureSalt ConcentrationIons

An Enzymatic Unit (u) is defined as the amount of enzyme required to digest 1 ug of DNA under optimal conditions:

3-5 u/ug of genomic DNA 1 u/ug of plasmid DNAStocks typically at 10 u/ul

Restriction Endonucleases: Type II

Restriction EnzymesHundreds of restriction enzymes have been identified. Most recognize and cut palindromic sequencesMany leave staggered (sticky) endsby choosing correct enzymes can cut DNA very preciselyImportant for molecular biologists because restriction enzymes create unpaired "sticky ends" which anneal with any complementary sequence

bacterial" immune system": destroy any "non-self" DNA methylase recognizes same sequence in host DNA and protects it by methylating it; restriction enzyme destroys unprotected = non-self DNA (restriction/modification systems)

As an example, consider a 5000 base pair, circular plasmid DNA containing single recognition sites for enzymes A, B, and C. Any one of these enzymes will cleave the DNA once to produce a linear molecule of 5000 base pairs.

Differently paired combinations of enzymes in the same reaction mixture (double-digests) will produce the following DNA fragments (sizes in base pairs):

Arbitrarily placing one of the cleavage sites at the top of a circle. This site acts as a reference point.

The closest cleavage site to this point can be placed in a clockwise orcounterclockwise direction.

The triple digest, A + B + C is a confirmatory testGenerally, a restriction enzyme map is constructed by first determining the number of fragments each individual enzyme produces. The size and number of fragments is determined by electrophoresis.

If a DNA molecule contains several recognition sites for a restriction enzyme, then under certain experimental conditions, it is possible that certain sites are cleaved but not others. These incompletely cleaved fragments of DNA are called partial digests(partials). Partials can arise if an insuffi cient amount of enzyme is used or the reaction is stopped after a short time (Figure 5). Reactions containing partials may also contain some molecules that have been completely cleaved.

Restriction Enzyme MappingTwo possible maps inferred from the observations

Restriction Enzyme Mapping

PCR and Restriction enzymes