Explanation of Restriction Enzymes

Restriction enzymes are used for cutting DNA molecules.

From time to time, a scientist may need to sequence a particular part of a strand of DNA. Another scientist may need to remove a section of DNA from the cell of one organism and place it in the cell of a different organism. In either case, the tools that make this possible are restriction enzymes, also known as restriction endonucleases.

Scans DNA

A restriction enzyme is coded for a particular sequence of DNA nucleotides. The nucleotides of DNA are taurine, adenine, cytosine and guanine (TACG.) The enzyme will scan the entire length of the strand of DNA until it finds the particular sequence of nucleotides it is supposed to find. Once the particular sequence of nucleotides is discovered, then the enzyme cuts the strand of DNA.

Blunt Ends

A strand of DNA is shaped in a double helix, with each nucleotide on one strand paired with another nucleotide on the opposite strand. This is called a base paring. Taurine always pairs with adenine, and cytosine always pairs with guanine. Now, certain restriction enzymes are able to cut straight across the double helix. When this occurs, the DNA strand is left with what scientists refer to as a blunt end.

Sticky Ends

Not all restriction enzymes cut straight across the double helix. When they don't, the DNA sequence is left with a lone nucleotide at each end of the strand. DNA strands that are cut in this way are referred to as sticky ends. Because there is a lone nucleotide available, it can be inserted into any other strand of DNA that has an available, complementary nucleotide.

Specific Enzymes

Over 900 restriction enzymes have been identified since the 1970s. For example, HaeIII and AluI both produce blunt ends. HaeIII codes for GGCC, and Alul codes for AGCT. Some restriction enzymes that produce sticky ends are BamHI (codes for GATC), HindIII (codes for AGCTT) and EcoRI (codes for AATT.)

Derivations

Many restriction enzymes are derived from certain bacteria. For example, Hemophilus aegypticus is the bacterium that produces the enzyme HaeIII. Because bacteria can be easily grown and because they often contain small circular pieces of DNA called plasmids, they are often a plentiful source of specific nucleotide sequences.

References

• Users: Restriction Enzymes
• Access Excellence: Restriction Enzymes
• Ask a Biologist: Restriction Enzymes