Restriction enzymes are short nucleotide sequences isolated from bacteria cells that protect them from virus.

When a viral DNA enters the bacterial cell, the restriction enzyme is able to recognize a specific sequence (restriction site) on the DNA molecule, which is usually 4-8 nucleotides long. The restriction enzyme will cut the viral DNA at these sites and hence restrict the growth of the virus.

Several hundreds of these enzymes have been isolated from various organisms and most are available commercially. These enzymes are used to cut a segment of gene from a human DNA molecule.

Restriction enzymes can either produce sticky ends or blunt ends. Sticky ends are produced by cutting the DNA in a staggered manner within the recognition site producing single-stranded DNA ends. These ends have identical nucleotide sequence and are sticky because they can hydrogen-bond to complementary tails of other DNA fragments cut by the same restriction enzyme. 

Sticky ends will tend to recombine to form a single strand of DNA.

Ligases

When two sticky ends meet, hydrogen bonds will form between the complementary bases at the sticky ends, but the covalent bonds that hold the sugar-phosphate backbone together have not. These breaks have to be sealed to produce a stable molecule. Ligases are enzymes that catalyse the formation of phosphodiester linkages between adjacent nucleotides. This process is known as Ligation.