Question: What is the function of the enzymes called "restriction endonucleases"?
About the article "Virus", section "Bacterial viruses".

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Homing endonucleases differ from Type II restriction enzymes in the several respects:  Whereas Type II restriction enzymes bind short, usually symmetric, recognition sequences of 4 to 8 bp, homing endonucleases bind very long and in many cases asymmetric recognition sequences spanning 12 to 40 bp.
Homing endonucleases are generally more tolerant of substitutions in the recognition sequence.
Minor variations in the recognition sequence usually decrease the activity of homing endonucleases, but often do not completely abolish it as often occurs with restriction enzymes.
Homing endonucleases share structural motifs that suggest there are four families, whereas it has not been possible to determine simply recognisable and distinguishable families of Type II restriction enzymes.
Homing endonucleases act as monomers or homodimers, and often require associated proteins to regulate their activity or form ribonucleoprotein complexes, wherein RNA is an integral component of the catalytic apparatus.
Type II restriction enzymes can also function alone, as monomers or homodimers, or with additional protein subunits, but the accessory subunits differ from those of the homing endonucleases.
Thus, they can require restriction, modification, and specificity subunits for their action.
Finally, homing endonucleases have a broader phylogenetic distribution, occurring in all three biological domains—the archaea, bacteria and eukarya.
Type II restriction enzymes occur only in archaea, bacteria and certain viruses.
Homing endonucleases are also expressed in all three compartments of the eukaryotic cell: nuclei, mitochondria and chloroplasts.
Open reading frames encoding homing endonucleases have been found in introns, inteins, and in freestanding form between genes, whereas genes encoding Type II restriction enzyme genes have been found only in freestanding form, almost always in close association with genes encoding cognate DNA modifying enzymes.
Thus, while the Type II restriction enzymes and homing endonucleases share the function of cleaving double-stranded DNA, they appear to have evolved independently.  
Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain.
There is a small number of significant classes of endonucleases that cleave only at the specific nucleotide sequences (such as the "restriction endonucleases" which are vital in biotechnology).
At the extreme ends of a sequence there are restriction endonucleases, usually called restriction enzymes.
These are endonucleases from eubacteria and archea that recognize a specific DNA sequence.
The nucleotide sequence recognized for cleavage by a restriction enzyme is called the restriction site.
Typically, a restriction site will be a palindromic sequence of about four to six nucleotides long.
Most restriction endonucleases cleave the DNA strand unevenly, leaving complementary single-stranded ends.
These ends can reconnect through hybridization and are termed "sticky ends".
Once paired, the phosphodiester bonds of the fragments can be joined by DNA ligase.
There are hundreds of restriction endonucleases known, each attacking a different restriction site.
The DNA fragments cleaved by the same endonuclease can be joined together regardless of the origin of the DNA.
Such DNA is called recombinant DNA; DNA formed by the joining of genes into new combinations.
"Restriction endonucleases" (restriction enzymes) are divided into three categories, Type I, Type II, and Type III, according to their mechanism of action.
These enzymes are often used in genetic engineering to make recombinant DNA for introduction into bacterial, plant, or animal cells, as well as in synthetic biology. 
The major way bacteria defend themselves from bacteriophages is by producing enzymes which destroy foreign DNA.
These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells. 
"Hin"dIII (pronounced "Hin D Three") is a type II site-specific deoxyribonuclease restriction enzyme isolated from "Haemophilus influenzae" that cleaves the palindromic DNA sequence AAGCTT in the presence of the cofactor Mg2+ via hydrolysis.
The cleavage of this sequence between the AA's results in 5' overhangs on the DNA called sticky ends:5'-A |A G C T T-3'3'-T T C G A| A-5'Restriction endonucleases are used as defense mechanisms in prokaryotic organisms in the restriction modification system.
Their primary function is to protect the host genome against invasion by foreign DNA, primarily bacteriophage DNA.
There is also evidence that suggests the restriction enzymes may act alongside modification enzymes as selfish elements, or may be involved in genetic recombination and transposition. 
In molecular biology, the BsuBI/PstI restriction endonuclease family is a family of type II restriction endonucleases.
It includes BsuBI and PstI.
The enzymes of the BsuBI restriction/modification (R/M) system recognise the target sequence 5'CTGCAG and are functionally identical with those of the PstI R/M system.