Restriction enzymes

These are enzymes, mainly of bacterial origin, capable of cleaving double-stranded DNA (and whatever its origin) to specific locations (nucleotide sequences) called restriction sites usually 4 to 6 pairs of bases.

DNA sequences recognized by the restriction enzymes :

Nucleotide sequences recognized by restriction enzymes are usually called palindromic sequences. The palindromic sequences are sequences where the succession of nucleotides read in the sense 5'à3 '(left-right) for the first strand is the same as the sequence read in the right-left direction for the second strand (sense 5'à3 '). These palindromic sequences are most often consist of 4, 5 or 6 pairs.

Nomenclature of restriction enzymes :

Restriction enzymes have a specific nomenclature. Their name has several letters (3 or 4). The first letter in the name of the enzyme is capitalized, it corresponds to the gender of the bacterium from which the enzyme was derived. The second letter and the third letter (lowercase) correspond to the species of the bacteria from which the enzyme is extracted. There may be a fourth letter written in capital corresponding to the bacterial strain. Finally to complete, a Roman numeral indicating the order of characterization of these enzymes.

Examples:

Eco RI Extracted from Escherichia coli RYB, site recognized: G / AATTC

Sma I Extracted from Serratia marcescens, site recognized: CCC / GGG

PstI Extracted from Providencia stuartii, site recognized: CTGCA / G

Types of cuts made ​​by restriction enzymes :

Restriction enzymes can provide two types of cuts: cut with a blunt ends and cut with a sticky ends.

The blunt ends result in a break in the middle of the palindromic sequence. The cut of sticky ends  is a cut which is on either side of the center of symmetry. The restriction sites are identified in the DNA by restriction enzyme that cuts DNA in principle as many times as there are restriction sites. This is valid for a given restriction enzyme, for another enzyme, the cut will be in a different position on the DNA. The DNA is cut into variable fragments and this both bacteria circular DNA or plasmids that linear DNA.

 

Examples of blunt and sticky ends of 

Methylation of the restriction site and restriction enzyme inactivation :

Bacterial DNA contains restriction sites may be identified by restriction enzymes that the bacteria has. To avoid self-destruction, modification enzymes of bacterial DNA involve. These modification enzymes are bacterial methylases ( or Methylation enzymes). Methylation of cytosine (at 5 carbon) or adenine (at nitrogen 6) belonging to the restriction sites leads to inactivation of the corresponding restriction enzyme. This methylation can be carried on a base or several bases belonging to the restriction site. Bacterial methylases are very specific.