Restriction enzymes are specialized proteins that play a fundamental role in molecular biology. These enzymes function by precisely cutting DNA molecules at specific sites. Their ability to recognize and cleave DNA at defined sequences makes them invaluable tools in various scientific fields.
Bacterial Origins
Restriction enzymes are primarily found in bacteria and archaea, where they naturally occur. Each specific enzyme is named according to the microorganism from which it was first isolated. For example, the enzyme EcoRI derives its name from Escherichia coli strain RY13, with “E” for the genus, “co” for the species, “R” for the strain, and “I” indicating it was the first enzyme discovered from that strain.
Over 3,600 restriction endonucleases are known, with more than 800 commercially available. These diverse enzymes each recognize and cut a unique, short sequence of 4 to 8 base pairs. Many recognition sequences are palindromic, reading the same forwards and backwards on opposite DNA strands.
Natural Role in Microorganisms
In their natural environment, restriction enzymes defend bacteria and archaea against foreign DNA, especially from invading bacteriophages. When a bacteriophage injects its DNA to hijack a bacterial cell, restriction enzymes act as molecular scissors, cleaving the foreign viral DNA into inactive fragments and preventing infection.
Bacteria protect their own DNA from these enzymes using a complementary “restriction-modification system.” This system employs a modifying enzyme, typically a methylase, to add methyl groups to specific bases within the bacterium’s own DNA recognition sequences. This methylation acts as a protective tag, distinguishing host DNA from foreign DNA and preventing cleavage.
Applications in Genetic Engineering
Scientists harness the precise cutting ability of restriction enzymes for numerous applications in genetic engineering and biotechnology. This capability is fundamental to creating recombinant DNA, which involves combining DNA from different sources. For example, a target gene and a plasmid vector can be cut with the same restriction enzyme, allowing the gene to be inserted into the plasmid.
When restriction enzymes cut DNA, they produce two types of ends: “sticky ends” or “blunt ends.” Sticky ends are staggered cuts leaving short, single-stranded overhangs that readily pair with complementary sequences. Blunt ends result from straight cuts with no overhangs. Sticky ends are often preferred in genetic manipulation due to their complementary nature, which facilitates easier joining of DNA fragments using DNA ligase.
This process is crucial for gene cloning, where specific genes are isolated and replicated. Restriction enzymes are also used in DNA fingerprinting, a technique analyzing DNA fragment length variations to identify individuals.